FUNDAMENTALS OFSOIL SCIENCE

 Definition and importance of soilDefinition: Is a naturally occurring unconsolidated material on the earth surface that has been influenced by parent material, climate, organisms and relief, all acting over a period of time to produce a soil that may differ from the material from which it was derived from in terms of physical, chemical, mineralogical, biological and morphological properties.Importance:Filter waterRegulate the flow of riversProvide habitat for millions of species of organismsProvide water, nutrients and support for plantsSequester carbon from the atmosphereServe as a “compost bin” for the EarthModerate and regulate distributions of solar energyProvide a historical record of the influences of climate and living organisms upon the parentmaterial from/in which the soil is formedSoil formation/pedogenesisSoil is formed from rocks through the process of weathering. Weathering is the chemical alteration and physical breakdown of rocks during exposure to the atmosphere (air surrounding the earth), hydrosphere (water systems), and biosphere (entire earth- water, air and organisms) to their basic constituents. During this process, rocks and minerals are broken down, modified or destroyed physically and chemically and their soluble products carried away. At the same time, it synthesizes new minerals in the soils.Three types of weathering exist:Physical weatheringFreezing and thawing: The expansion force of water as it freezes is sufficient to split any mineral or rock. Freezing and thawing can occur on a daily cycle.Heating and cooling: Differences in temperature in a rock or soil mass give rise to differential expansion and contraction. The resulting stresses can fracture the minerals. Temperature changes also can bring about exfoliation, where a thin layer of an entire rock is removed often making the rock round.Wetting and drying: The disruption of soil by wetting and drying results in the swelling and contracting of soil peds and particles. Abrasion among particles within the soil makes the particles finer. The soil shrinks when dry, and cracks develop, creating an irregular boundary between horizons.Grinding or rubbing: Grinding action, or the rubbing of moving rock or soil particles against each other, also results in the disintegration of the rock or soil particles. In soils high in clay (Vertisols), during the dry season the soil cracks and fills with soil particles from above. During the wet season the soil swells shut and the expanding forces causing the soil peds to have slick, smooth surfaces called Slickensides which are evidence of soil movement below the surface of the soil.Organisms: Action of organisms, including animals and plants reduces the size of rocks and minerals. Plant roots are capable of splitting the hardest rock. Digging by animals or plowing by humans result in a slow breaking of rocks into finer particles.Unloading: Unloading is the removal of thick layers of sediments overlying deeply buried rocks by erosion or uplift. The response of the rock to this reduced pressure is to expand, and cracks and fissures are created. Unloading is a physical process which can also result in chemical weathering, because the temperatures are less in the soil environment than where the rock was formed, and exothermic chemical reactions occur among minerals, water, oxygen, and carbon dioxide in the soil. Tree roots are often able to penetrate the rock fractures that resulted from unloading.Chemical WeatheringThis is the decomposition of rocks and minerals as chemical reactions transform them into new chemical combinations that are stable at or near the Earth's surface. Chemical weathering occurs because minerals are made more soluble and are changed in structure, causing easy fragmentation. Forms of chemical weathering are:Dissolution: Is the dissolving of a solid in liquid, changing solid materials into separate ions (for instance, sodium chloride (NaCl) dissolves into Na+ and Cl- ions).This permits more independent and greater chemical changes than in a non-ionized (usually solid) state. Water is capable of dissolving many minerals by hydrating the cations and anions until they become dissociated from each other and surrounded by water molecules. (e.g. dissolution of gypsum):CaSO4.2H2O + 2H2O                                     Ca2+ + S042+ + 4H2OHydrolysis: a process where minerals react with water to form hydroxides, which usually are more soluble than the original mineral. Hydrolysis is one of the most important weathering processes causing soil profile changes. Water molecules split into their hydrogen and hydroxyl components and hydrogen replaces a cation from the mineral structure (e.g. transformation of feldspar to kaolinite):KAlSi3O8 + H2O                                        HAlSi3O8 + K+ + OH-Acidification: Weathering is accelerated by the presence of the hydrogen ion in water, such as that provided by carbonic and organic acids. Carbonic acid, a weak acid produced when gaseous carbon dioxide is dissolved in water. Thus acidification is a form of dissolution. The carbon dioxide comes partially from the atmosphere, but mostly from biological respiration and from the decomposition of plants. Carbonic acid dissolves minerals more readily than water alone and forms the more soluble bicarbonates.    CO2 + H2O                      H2CO3Hydration: Is the combination of a solid mineral or element with water. When the water molecules are chemically bonded to the mineral, the size of the chemical structure is increased, thereby making a softer, more stressed, and more easily decomposed mineral. Intact water molecules bind to a mineral transforming hematite into ferrihydrate eg;5Fe2O3 + 9H2O                        Fe10O15.9H2OOxidation: Oxidation, as used in mineral weathering, is both the chemical combination of oxygen with a compound and the change in oxidation number of some chemical element (electrons are lost in oxidation). Oxidized minerals have a volume increase and are usually softer. If an element's oxidation number is changed, this can also unbalance the mineral's electrical neutrality, making it more easily weathered by water and carbonic acid. Oxidation is most evident in the weathering of iron-bearing minerals.4Fe + 3O2                               2Fe2O32Fe2O3 + 2Al3+                                   2Fe 2+ + Al2O3Reduction: Is a chemical process in which electrons are gained. In soils, reduction usually takes place when oxygen is scarce, as in stagnant water conditions. Reduction in minerals may result in electrically unstable compounds, more soluble ones, or more internally stressed ones, which eventually decompose more rapidly.           MgO + Ca2+                                    CaO + Mg2+Biological Weathering:It is caused by living organisms. It is done: Physically through roots cracking Production of CO2 that forms weak carbonic acid and by production of organic acids that decompose some minerals. It also decomposes through chelation (chele: Greek for claw). This is the process of clasping metallic ions in a claw-like manner. Factors influencing soil formationSoil formation occurs as a result of environmental processes. This process was first postulated by Dokuchaev, a Russian scientist who is referred to as the father of soil science in the late 1800s. Hans Jenny, an American, further developed a model of soil formation processes, which is widely cited today, mainly due to its simplicity:S= ƒ (Cl,T,P,O,T…)Where Cl= Climate, T=Topography or Relief, P= Parent material, O= Organisms and t= Time. These factors influence the rate of soil weathering. They are divided into two; active and passive factors. Climate and organisms are active factors since they provide energy that acts upon this mass for soil formation while topography (relief) parent material; and time are passive factors.Active factors Climate: Temperature, humidity, evapotranspiration, type, amount and duration of rainfall are the major sub-factors. Temperature is significant particularly on weathering. The table below shows how temperature and rainfall in terms of relative dissociation of water affects weathering in the Arctic, temperate and tropical areas of the world: In the tropics, weathering is high with high organic matter decomposition and the soils also have higher clay content.Organisms: They include flora and fauna. They are responsible for organic matter accumulation, profile mixing and structural activity. Bacteria and fungi initiate breakdown of plant tissues. Arthropods e.g termites and mites are involved more in plant tissue breakdown while earth worms mix organic matter with mineral part of the soil.Vegetation: These determine a specific soil type e.g grassland and forest soil. Vegetation cover also contributes to nutrient cycling in the following ways: Trees take in nutrients preferentially and on decomposition, they return them to the soil. Coniferous forests are lower in metallic ions (Ca, Mg and K) than deciduous ones. They also extract nutrients from the deep layers of the soil and deposit them onto the surface.Effects of man:Clears forests or burn grass modifies soil forming factorsIrrigation of arid landsAddition of fertilizers and herbicides alters original environmentCultivation- mixing horizon when deeply ploughed.Passive factorsThese provide the source of soil mass.Relief /Topography/SlopeMay hasten or delay climatic forces i.e. temperature and rainfall. Higher elevation depresses temperature but raises rainfall.More clouds are less warming in highlands-it encourages thicker organic matter horizons in the environment.The compass direction that a slope faces is called aspect. Sun facing slopes (aspect) will have soil warming effects hence different soils that are different from those facing away from the sun.Steep slopes encourage runoff or natural soil erosion therefore no deep soil profile forms The long term effect of slope/ relief/topography is to develop a sequence of soils that are linked by a progression of pedogeomorphic activities, each being unique to slope position. This sequence of soils is called a catena.Parent MaterialThis is the unconsolidated or consolidated material of organic or inorganic origin that is little affected by the current processes from which soil has formed. It is also called Initial Soil. Parent Material can be classified as a mineral or a rock.Parent material can either be in situ or transported.The single main parent material is the rocks and a rock is an assembly of mineralsThe parent material influences soil characteristics eg sandy texture originates from coarse grained quartz-rich parent material such as granite or sandstone. Chemical and mineralogical composition of the parent material influences both chemical weathering and the natural vegetation. It also influences quantity and type of clay minerals. Inorganic parent materials can be formed in a place where they weather from the parent material and are referred to as residual material, or they can be transported from one location and be deposited to another, where they are referred to as transported material. In wet environments, eg swamps and marshes, incomplete decomposition may allow organic parent materials to accumulate from residues of many generations of vegetation.TimeYoung soils have properties similar to parent material. This is shown in areas that have undergone glaciations and landslides. These areas have soils closer to the parent material due to time factor. In alluvial (river) and lacustrine (lake) areas, the soils have feather-like parent material. In recently uplifted areas of coastal plain soils are not mature. Other examples where young soils are found include open-pit mining areas, and glaciated areas. The term young or old soil refers to the degree of weathering and profile development rather than the age of the soil, since profile development takes place very slowly. Carbon dating or presence of fossils and human artifacts are indirect way of time required for the different aspects of soil development to occur. Time effect on soil development can be studied by use of Chronosequence: a set of soils that share a common community of organisms, climate, parent material and slope, but differ with the regard to the length of time that the materials have been subjected to weathering and soil formation.Processes of Soil Formation/GenesisDuring the process of soil formation (genesis) from a parent material, the regolith undergoes many profound changes, often referred to as soil forming or pedogenic processes:• Transformations: Occur when soil constituents are chemically or physically altered or destroyed as others are re-synthesized from the precursor materials.• Translocations: The movement or organic and inorganic materials laterally within a horizon or vertically from one horizon up or down to another. Water, either percolating down with gravity or rising up by capillary action, is the most important translocation agent. Materials moved include fine clay particles, dissolved salts and organic substances.• Additions: Inputs of materials to the developing soil profile from outside sources are in this category e.g. organic matter from fallen leaves, and sloughed-off roots (the carbon having originated from the atmosphere).• Losses: This occurs when material is lost from the profile through leaching by ground water, erosion of surface materials.The above processes give the regolith specific characteristics distinguishing one parent material from the other. Specific processes however include the following:• Podzolization: The chemical migration of Fe and Al and/or organic matter resulting in consolidation of silica (Silication) in the layer eluviated. Sesquioxides are translocated to the lower horizons. This is typical in high altitude, high rainfall and low temperature areas. The high rainfall percolates and carries intermediate organic matter products which complex Fe and Al and/or organic matter to lower horizons, forming a layer of eluviation leaving silica that is grey or bleached (Al and Fe are preferentially removed). In the silica region, the pH is strongly acidic. It forms an Albic horizon.• Calcification: The accumulation of CaCO3 in the C horizon and probably in other horizons. This is typical in low rainfall areas where the wetting depth is 1-15cm. as water losses speed, it deposits CaCO3. Its characteristics include:• Laterization (Desilication, Ferritization, Altization or Ferrallitization): This is the removal of silica (SiO) from the soil preferentially to Fe and Al due to the pH. It is favoured by high rainfall, temperatures and extreme leaching. This is the situation in humid tropics. It is the reverse of Podsolization. There are high rates of decomposition and high exchange rates of ions between plants and soil. In lowlands of such areas where rainfall is less, bases concentrate forming smectite soils.• Salinization: The accumulation of soluble salts such as SO42- or Cl- of Na, Mg and K in salic (salty) horizon. This operates mainly in sub-humid, semi-arid and arid regions and coastal regions (though not much). Rocks A rock can be described as:An assembly of minerals. any mineral or aggregate of minerals that form an essential part of the earthas extensive mineral bodies, composed of one or more minerals in varying proportionsA mineral:  Are the building stones of the earth's crust. They are stony mixtures of one or more of the ninety-two relatively stable elements that man has found in the earth's surface and its rocks.A naturally occurring chemical element or compound formed as a product of inorganic process with known crystal structure and regular geometric arrangement of constituents. In soil science, it is an inorganic amorphous (non crystalline) component formed in the soil. Described by colour, streak, hardness, cleavage, acid reaction or weatheringClassification of mineralsClassification of rocks:According to silica content:According to structure & mode of formationIgneous: Formed by action of heat.Formed from molten magma and may be the providing most of the components of the other rocks (sedimentary and metamorphic). Made up of largely silicates together with some oxides and sulphides along with considerable quantities of water and other gases in solution under great pressure.Igneous rocks can be:• Extrusive: molten mass cooled on the surface of the earth then consolidated.• Intrusive: molten mass cooled in shallow depth beneath the surface of the earth• Plutonic: molten mass cooled in greater depth beneath the surface of the earthSedimentary Rocks: Formed through agency of water. Also known as stratified rocks. Formed from accumulation and ultimate consolidation of products of weathering derived from pre-existing rocks and organic debris. Materials responsible for formation are transported from their place of origin and deposited of sedimentary rock development. They may be transported as discreet particles by wind, water or ice while others are transported as solution or colloidal state. Due to the great sources of these rocks, great variations occur in composition. Processes that convert sediments into sedimentary rocks are referred to as Diagenesis and include:• Compaction• Recrystallization• CementationExamples of sedimentary rocks are: limestone, Chalk, halite, Diatomite Shelly coral and coal.Metamorphic: Formed from subsequent transformation of either Igneous or Sedimentary. This is the transformation of rocks into new types through the process of re-crystallization of their elements. The rocks under mineralogical and textural natural changes that do not involve any change in bulk chemical composition of the rock. Rocks involved are igneous, sedimentary or metamorphic. Heat and pressure are the key agents. It involves a gradual change of a rock from one form to another e.g.:• Shale Chlorite schist• Limestone marble• Granite GneissMetamorphic rocks and laminated or banded.Soil Profile: Vertical section of the soil showing the various layers from the surface to the unweathered rock. The layers are called horizons.  A soil horizon is a layer of soil or soil material that lies approximately parallel to the land surface as observed in road cuts. It differs from adjacent genetically related layers in properties such as color, structure, texture, consistence, and chemical, biological, and mineralogical composition.Two types of horizons (a) organic (b) mineralOrganic horizons form above mineral horizons.  Result from litter from dead plants & animals and common in forests. Designated as O-horizon. May be designated as H-horizon if saturated with water for long periods.Mineral horizons are designated as A,B,C & REach master horizon is identified by capital letters O, A, B, C, and E and lowercase letters for distinctions of these horizons. Most soils have three major horizons -- the surface horizon (A), the subsoil (B), and the substratum (C). Some soils have an organic horizon (O) on the surface, but this horizon can also be buried. The master horizon, E, is used for subsurface horizons that have a significant loss of minerals (eluviation). Hard bedrock, which is not soil, uses the letter R. There are three major processes involved in horizon development:• Accumulation of organic matter in the surface layers• Leaching of the profile, sometimes considered the same as eluviation• The deposition of leached constituents (illuviation)Master Horizons.O: An organic horizon usually at the soil surface but may be buried. It consists of unconsolidated organic material (leaf litter, roots, leaves etc) not saturated with water. It may also be denoted by L for litter, F for fermentation or H for humusA: A mineral horizon formed at or near the surface where humified organic matter is associated with mineral materials. Humus is defined as stable dark coloured organic material that accumulates as a by-product of decomposition of plant and animal residues added to the soil. B: Sub-surface mineral horizon resulting from the change in situ of soil material, i.e., the obliteration of the original rock structure or the washing in of material from overlying horizons, i.e. accumulation of silicate clay, organic matter, aluminum, or iron in a process called Illuviation.E: Mineral horizon just below the soil surface that has lost its silicate clay, organic matter, aluminum or iron by downward movement leaving a concentration of resistant sand and silt particles. “E” stands for “Eluviation Horizon” a soil layer formed by the removal of constituents such as clay or iron. Eluviation describes the process whereby constituents of soil are removed in suspension.C: Unconsolidated or weakly consolidated mineral horizon that retains evidence of rock structure, but lacks diagnostic properties of the overlying A, E and B horizons. This horizon is little affected by pedogenic (soil forming) processes. Examples include beach sand, wind blown silt (Loess), alluvium deposited by rivers and glacial till deposited by glacial ice.R: Continuous (consolidated) hard or very hard rock. These horizons however have transitions into each other thus forming sub-horizons as a result of the various soil forming factors. These transitions are denoted by lower case letters as shown below:Ah: uncultivated A horizon, or an A horizon with humus accumulationAp: cultivated (ploughed) A horizonEg: poorly drained E horizonBg: poorly drained B horizonBh: B horizon with accumulation of humusBs: B horizon with accumulation of organic matter and iron and aluminum oxidesBt: B horizon with accumulation of clayBw: B horizon with changes of colour or structureBx: B horizon with fragipan, a compact, slowly permeable subsurface horizon that is brittle when moist and hard when dry.By: B horizon with accumulation of gypsum (calcium sulphate)Bz: B horizon with accumulation of salts more soluble than gypsumCg: Poorly drained C horizonCk: C horizon enriched with CaCO3Cm: C horizon with cemented materialCx: C horizon with fragipanCy: C horizon with accumulation of gypsumCz: C horizon with accumulation of salts more soluble than gypsum.The above lower case letters next to the upper case ones may be substituted by numbers eg A1, A2, B1, B2 etc. Horizons A and B are collectively called the Solum. It is important to note that not all the above horizons may be obvious in every soil.Soil classification and taxonomyDifferent systems of soil classification have been developed to group soils of similar properties in one class, allowing exchange of information on soils found in different areas. Soil classification also helps in determining the best possible use and management of soils. Although many soil classification systems exist; however, two systems are widely used: The USDA Soil Taxonomy and the FAO/UNESCO legend. The French system (ORSTROM) is also commonly used in France and in Francophone Africa. Early type of classification systems were based on:Texture- classified as loamy, clayey, sandy, organic soils. Parent material- soils called limestone soils, alluvial, granitic, sandstone soils etc.Dokuchaev, a Russian scientist lead a team in the 1880s in making a first attempt in modern classification based on observable soil feature by using three levels:• Zonal: based on climate• Intrazonal: based on overriding soil formation process• Azonal: based on parent material.Soil Classificationa. Old Classification or GeologicalSoils grouped depending upon the rocks from which they were derived:i. Sedentary or transportedii. Red, black, lateritic, delta, desert.b. Modern Classification:i. Physical: based on texture of the plough layerii. Genetical: based on genesis of the soil and its subsequent developmentSoils are divided into well-defined categories:USDA soil taxonomy FAO-UNESCO soil classification commonly used in KenyaSoil taxonomyThe classification of soils starts with examination of soil profiles. Morphologically, soils are composed of a series of horizons. Soil horizons are layers of different appearance, thickness, and properties which have arisen by the action of various soil-forming processes. The horizons are normally parallel to the surface. Collectively, the horizons make up what is called the soil profile or soil "pedon". A soil profile is defined as a vertical section of the soil to expose layering. In soil classification, the item to be classified is the soil profile. The classification or study of the entire profile consists of recognizing and naming the horizons which make up the profile. In the study of soil profiles, sub-soil horizons are given greater emphasis than surface horizons which are frequently changed by human activity to such an extent that they bear hardly any relationship with genetic process.Soil classification recognizes a number of diagnostic horizons based on morphology and chemical characteristics. Two types of diagnostic horizons emerged based on location on top of or within the profile: diagnostic surface horizons called epipedons-on top of and diagnostic sub surface horizons.Several classification systems exist in the world eg, The USDA system, FAO/UNESCO, ORSTROM French system commonly used in France and in Francophone Africa etc.Diagnostic Surface horizons. The epipedons include the following:• Mollic: dark coloured, thick horizon, typical of grassland regions with over 50% of the exchange capacity dominated by base cations.• Umbric: similar to mollic, except that the base saturation is less than 50%• Histic: a peaty surface horizon, saturated with water part or all of the year, having a large amount of organic carbon• Anthropic: similar to a mollicepipedon, but man-made with large amount of phosphate accumulated by long-continued farming• Plaggen: a man-made epipedon more than 50 cm thick raised above the original soil surface with properties that depend on the original soil.• Mellanic: a black, thick epipedon occurring in soils developed in volcanic ash. Usually has a low bulk density.Ochric: epipedons that are too light in colour, too low in organic carbon, or too thin to belong to mollic, umbric, anthropic, plaggen or histicepipedons. This is the most common form of epipedon.Diagnostic Subsurface Horizons include:• Agric (ager=field): a compact horizon formed immediately below the plough layer by cultivation and contains significant amounts of alluvial silt, clay and humus• Albic (albus=white): bleached, light coloured horizon from which the clay and free iron oxides have been removed.• Argillic (argilla=white grey): an alluvial horizon enriched with clay to a significant extent.• Calcic (calcic=lime): a horizon enriched with calcium carbonate or calcium and magnesium carbonate in the form of powdery lime or secondary concretions, more than 15 cm thick.• Cambic (cambiare= to change): an altered horizon in which the parent material has been changed into soil by formation of soil structure, liberation of iron oxide, clay formation and obliteration of the original rock structure. “The definition of a cambic horizon is complex because the soil material must show evidence of change by pedogenesis, but not so much change that an argillic, spodic,”… or other horizon is evident. Does not occur in extremely sandy soils, must be in very fine sands or finer.• Gypsic (=gypsum): a horizon enriched with calcium sulphate, more than 15 cm thick.• Kandic: new term derived from the word kandite, used to describe kaolinite clay minerals. A low activity clay sub-surface horizon similar to an oxic horizon but with more clay in the overlaying surface horizon and an abrupt change of texture between the surface and the lower horizons.• Nitric (=natrum=sodium): a clay enriched illuvial horizon, with the cation exchange complex dominated by a high sodium content. Similar in all aspects to an argillic horizon.• Oxic (=oxides): a horizon with a very low content of weatherable minerals (meaning that it is the most highly weathered), in which clay is composed largely of kaolinite, contains accessory highly insoluble minerals such as quartz sand, low exchange capacity, and clays are poorly dispersed.• Placic (=plax=flat): thin black or reddish-brown brittle pan, cemented with iron, iron and manganese, or an iron-organic complex. Forms barrier to roots.A thin iron pan.• Salic (=sal=salt): a horizon enriched with salts more soluble than gypsum, more than 15cm thick.• Sombric (=somber=dark): a freely drained dark sub-surface horizon containing illuvial humus with a low CEC and low base saturation. Sometimes mistaken for a buried A horizon.• Spodic (=spodos=wood ash): an illuvial horizon enriched with organic matter, iron and aluminum.• Sulfuric (=sulfur): mineral or organic horizon more than 15 cm thick which has a pH of 3.5 or less and contains the mineral jarosite or more than 0.05%water soluble sulphate. Other diagnostic horizons:• Glossic: a horizon more than 5 cm thick in which an upper E horizon penetrates (tongues) down into a lower argillic, nitric or kandic horizon.• Duripan (=durus=hard +pan): a sub-surface horizon cemented by silica or aluminum silicate to the degree that fragments from the air-dry horizon do not slake during prolonged shaking.• Fragipan (fragilis=fragile+pan): a compact slowly permeable loamy sub-surface horizon with a high bulk density, brittle when moist, but hard when dries. Slakes or fractures when placed in water.• Petrogypsic: a cemented gypsic horizon• Petrocalcic: a cemented calcic horizon• Andic: a soil horizon composed of volcanic glass.• Permafrost: soil horizon where temperature is constantly below 0 °C with permanent ice.• Plinthite: material found in tropical regions, arising due to ferraltization (laterization) soil formation process. Vesicular, porous first then hardens into iron crusts (ironstones).Soil physical propertiesSoil texture: The degree of fines or roughness of sizes of soil separates (clay, loam and sand).Importance of soil texture: affects and is related to several soil properties such as soil structure, aeration, water holding capacity, nutrient storage, water movement, and bearing strength.There are two schemes of soil separate classification: USDA (United States Department of Agriculture) and International Society of Soil Science (ISSS).Soil Textural ClassesRarely do soils consist entirely of a single separate, but instead are a mixture. Textural classes are based on different combinations of sand, silt, and clay. The twelve basic textural classes in order of increasing proportions of the fine separates and with appropriate abbreviations are:1. Sand (s)                                                          7. Sandy clay loam (scl)2. Loamy sand (ls)                                              8. Clay loam (cl)3. Sandy loam (sl)                                               9. Silty clay loam (sicl)4. Loam (l)                                                        10. Sandy clay (sc)5. Silt loam (sil)                                                 11. Silty clay (sic)12. Silt (si)                                                           13. ClayThe texture of the soil is dependent on the mixture of the different particle size separates (soil separates). From largest to smallest the soil separates are: Stones and cobbles are bigger than 64 mm (diameter)Gravel is from 2 mm to 64 mm Sand is from 0.05 to 2 mm Silt is from 0.002 to .05 mm Clay is less than 0.002 mm.Textural triangle:The determination of soil texture is called particle size analysis or mechanical analysis. Particle size analysis can be done through:By feel.In the laboratory by hygrometer methodDetermining the texture in the laboratory uses a basic principle of sedimentation called "Stokes Law". Stokes Law states that the speed or velocity with which particles settle out of a liquid medium is dependent on a constant factor (K) and the radius of the particles. Or, the bigger the particle, the faster it will fall out of suspension hence sand will be at the bottom, silt and clay will be at the top.Soil structureRefers to the way soil separates are attached together. Soil separates do not act in the soil as individuals, but as partners, or aggregates. Aggregates are the clumps of soil separates. When aggregates are bound together into larger masses they are called PEDS.Types of soil structuresGranular Structure Granular structure is the most beneficial form of soil structure for plant growth. Granular structure aggregates are formed by the breaking apart of larger aggregates through the physical processes of wetting and drying, and freezing and thawing. These aggregates are then cemented together by the by-products of the microbial decomposition of organic matter, which are called microbial gums. The more microbial gums, the greater the aggregate stability. The way to obtain microbial gums is by adding organic matter to the soil; thus, plant residues contribute indirectly to better soil structure.Platy Structure Platy structure is often found in the E horizon (below the A) where water moves laterally through the soil. Platy structure can be detrimental because it restricts root and water penetration.3. Blocky Structure Blocky structural peds are found most frequently in the B horizons. They have been created by the wetting and drying and freezing and thawing cycle of the B horizon. The clay films also act as a binding agent for the blocky aggregates. The B horizon can often be determined in a profile by looking for the location of blocky peds which can be readily seen. Blocky can be either angular (sharp ped edges) or sub-angular (rounded ped edges).Prismatic or Columnar StructureWhere the blocky peds are longer than they are wide, the prismatic or columnar structure is identified (common only in B horizons). They are often the first structure formed in a soil, because their formation only requires vertical cracking in the soil.Structureless:C horizons generally lack any structural aggregation. Their lack of structure is termed "massive." Massive structure is hard to break apart and appears in very large clods. Where very sandy soils lack aggregation, or the soil particles don't stick together the structureless condition is termed "single-grained." Single grained always accompanies a loose consistence.Soil colorSoil color gives important information about the soil's characteristics. It is determined by comparing the color of the soil to the chips in the soil color charts. Munsell color book and the Earth Colors book are used to determine soil color. Soil color consists of 3 parts: hue, value, and chroma.Hue is the dominant spectral color of the rainbow - yellow, reds, orange. In looking at the page in color book, hue is given in the upper right hand corner of the Munsell page (10YR) and bottom of the Earth Colors page. Value is expressed as the numerator of the fraction and is along the left hand margin of the page. Value is the relative darkness or lightness of the soil color. Chroma is along the bottom, and is the denominator of the fraction. Chroma is the relative purity or strength of the color, low chromas have dull colors, while high chromas have bright colors. Soil ConsistenceSoil consistence is the soil's ability to cohere or stick together. The soil's consistence may be evaluated at three moisture conditions: air dry, moist, and wet. Moist consistence is evaluated by placing the soil between the thumb and forefinger and gently applying pressure. The ease with which a ped can be crushed determines the consistency. Terms commonly used to describe moist consistence are:Loose - Non-coherent when dry or moist; does not hold together in a mass.Friable - When moist, crushes easily under gentle pressure between thumb and forefinger and can be pressed together into a lump.Firm - When moist, crushes under moderate pressure between thumb and forefinger, but resistance is distinctly noticeable. Plastic - When wet, readily deformed by moderate pressure but can be pressed into a lump; will form a "wire" when rolled between thumb and forefinger.Sticky - When wet, adheres to other material and tends to stretch somewhat and pull apart rather than to pull free from other material.Hard - When dry, moderately resistant to pressure; can be broken with difficulty between thumb and forefinger.Soft - When dry, breaks into powder or individual grains under very slight pressure.Cemented - Hard; little affected by moistening.Soil compositionSoil is made up of three phases:Solid phase – 50%Liquid phase – 25%Gaseous phase -  25%NB: Soil water and air vary in composition in both time and space however, the three phases vary continuously and depend upon such variables as: weather, vegetation and management.Elements in the soil environment:Oxygen – 46.6%Silicon – 27.72%Aluminum – 8.13%Iron – 5.00%Calcium – 3.63%Sodium – 2.83%Potassium – 2.59%Magnesium – 2.09%Others – 1.41% Density of solids (Mean particle density) ρsMost mineral soils have mean particle density ranging between 2.6 – 2.7 gm/cm3, this is very close to the density of quartz which is prevalent in sandy soils.NB: 1. Presence of iron oxides and various heavy minerals increases the average value of ρs.Presence of organic matter lowers ρsDry bulk density ρbThis is the ratio of the mass of dried soil to its total volume (solids and pores together)ρbis always smaller than ρs. In sandy soils ρbcan be as high as 1.6 whereas in aggregated loams and clay soils it can be as low as 1.1 g/cm3.Bulk density is affected by:Soil structure; the soils degree of looseness or compaction The soils swelling and shrinkage characteristicsTotal/wet bulk density ρtThis is an expression of the total mass of a moist soil per unit volume.Dry specific volume νbIt is another index of the degree of looseness or compaction of the soil.Porosity ƒPorosity is the index of relative pore volume in the soil. The value generally lies in the range of 30 – 60%.Void ratio eIs the index of the fractional volume of soil spores. It varies between 0.3 and 2.0.Soil wetnessThis can be expressed in several ways:Mass wetness w                           This is the mass of water relative to mass of dry soil particles.It is often referred to as the gravimetric water content.Volume wetness θ                              Is often referred to as volumetric water content or volume fractional of soil water.Water volume ratio vw                                  It refers to volume of water present to volume of particles rather than the total volume.Degree of saturation s                                     It expresses the volume of water present in soil in relation to the volume of soil pores. The index can range from zero in very dry soils to 100% in highly saturated soils which is always rare.Air filled porosity/ fractional air content fa                                                                                                                   Is the measure of relative air content of the soil.Other relationshipsRelation between porosity and void ratio                                                                       Relation between volume wetness and degree of saturation                                                                  Relation between porosity and bulk density                                                                     Relation between mass wetness and volume wetness                                                                        ρw is the density of water (mass of water/volume of water) which is approximately equal to 1 g/cm3.Relation between volume wetness, fractional air content and degree of saturation                                                                         Soil chemical propertiesSoil pHpH is defined as the negative logarithm of the hydrogen ion (H+) concentration. When water ionizes to H+ and OH- (a neutral solution), both H+ and OH- ions are in equal concentrations of 0.0000001 moles per liter. That is a very small concentration.HOH <—> H+ + OH-[H+] = [OH-] =1 x 10-7 moles/liter. The H+ ion and OH- concentrations in water are very small. The pH scale has been devised for conveniently expressing these small concentrations by expressing pH= - Log [H+]  or Log 1/[H+]Soil pH influences many facets of crop production and soil chemistry, including availabilities of nutrients and toxic substances, activities and  nature of microbial populations, solubility of heavy metals, and activities of certain pesticides. Causes for Acid SoilsThe pH of a soil is dependent on the parent material, the climate, the native vegetation, the cropping history (for agricultural soils), and the fertilizer or liming practices. The pH range for most mineral soils would be from 5.5 to 7.5.Soils become acidic when precipitation leaches away basic cations (which provide the OH- ions). Eventually all the Ca++, Mg++ and other cations are are replaced by H+ ions.  Exchangeable hydrogen is the principal source of H+ until the pH of the soil goes below 6. Below 6, exchangeable aluminum becomes the source of hydrogen ions, due to the dissociation of Al from clay minerals.Soils tend to become acidic as a result of: (1) Rainwater leaching away basic ions (calcium, magnesium, potassium and sodium); (2) Carbon dioxide from decomposing organic matter and root respiration dissolving in soil water to form a weak organic acid; (3) Uptake of positive ions by plant roots and the resulting release of H+ by the root to balance internal charge; (4) Formation of strong organic and inorganic acids, such as nitric and sulfuric acid, from decaying organic matter and oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids. Sources of H+ ions in the soil: 1) Dissociation of carbonic acid (H2CO3), which forms readily in soils when CO2 is present; ---                    H2CO3             CO2 + H + + HCO3-2) Organic acids formed during the decomposition of organic matter3)the burning of coal in electrical power plants releases sulfur to the atmosphere which is added to soils during precipitation as sulfuric acid, and fertilizers containing sulfur, which adds H+4) The conversion of NH4+ to NO3- releases H+ during the nitrogen cycle or when nitrogen fertilizers are added to soils. 5) Uptake of positive ions by plant roots and the resulting release of H+ by the root to balance internal chargepH is < 4.0 = indicates that the soil contains free acids probably as a result of sulfide oxidation pH is < 5.5 = indicates that the soil's exchange complex is dominated by Al pH is < 7.8 = soil pH is controlled by a range of factors pH is > 7.8 = Indicates that the soil contains CaCO3Where leaching is minimal, the concentration of basic cations (Ca++, Mg++, K+, and Na+) on the exchange complex will be large. These basic cations will come from the weathering of rocks and minerals, from dust blown on soils, from irrigation water or runoff water. When basic cations dissociate in the soil solution, they will produce hydroxyl ions (OH-). This will raise the pH of the soil. NB: The "pH of the soil" refers to the concentration of hydrogen ions in the soil solution --not on the exchange complex.Soil pH Determination pH indicator dye. This is found in indicator dye pH kit called Poly D. It is easy to use and gives a suitable pH value for most soils. The indicator dye is added to the soil in the spot plate until it is saturated. The solution is stirred using a small spatula. The solution will change color depending on the soil pH. The solution color is compared to a color card that has been calibrated to various pH readings. (Be sure to clean the spot plates when you are through.)pH meter and glass electrode (this is the most accurate method). The electrical conductance of the solution is measured using the meter. The conductance is correlated in the machine to pH values which are read directly. Soil cation exchange capacitySoil Cation Exchange - The interchange between a cation in solution and another cation on the surface of any negatively charged material such as clay or organic matter. Cation Exchange Capacity (CEC) is the ability of the soil to hold onto nutrients and prevent them from leaching beyond the roots. The more cation exchange capacity a soil has, the more likely the soil will have a higher fertility level. When combined with other measures of soil fertility, CEC is a good indicator of soil quality and productivity. The cation exchange capacity of a soil is simply a measure of the quantity of sites on soil surfaces that can retain positively charged ions by electrostatic forces. Cations retained electrostatically are easily exchangeable with other cations in the soil solution and are thus readily available for plant uptake. Thus, CEC is important for maintaining adequate quantities of plant available calcium (Ca++), magnesium (Mg++) and potassium (K+) in soils. Other cations include Al+++( when pH < 5.5) , Na+, and H+. Cation exchange is influenced by: 1) strength of adsorption--->Strong adsorption » Al+3> Ca2+> Mg2+> K+ = NH4+> Na+ >H+ »Weak adsorption 2) the relative concentration of the cations in the soil solution. At any one time the quantity of ions on the exchange compared to what is in the soil solution is determined by the kind of ions present and the quantity of ions present in the soil. Cation Exchange Capacity can be expressed two ways:1) the number of cation adsorption sites per unit weight of soil or, 2) the sum total of exchangeable cations that a soil can adsorb. Soil CEC is normally expressed in units of charge per weight of soil.The units are: meq/100 g (milliequivalents of element per 100 g of dry soil) or cmolc/kg (centimoles of charge per kilogram of dry soil). The unit of milliequivalents (meq) per 100 g of oven dry soil is used to better reflect it is the charge in the soil that determines how many cations can be attracted. The notation of CEC for the international SI units is centimoles of positive charge per kilogram [cmol(+)/kg]. A soil with 10 meq/100g will also have 10 [cmol+/kg]. The equivalent weight of an element is the molecular or atomic wt (g) ÷ valence; or charges per formula milliequivalent (MEQ). One meq wt. of CEC has 6.02 x 10 20 adsorption sites. Cation exchange sites are found primarily on clay and organic matter (OM) surfaces.Normal CEC ranges in soils would be from < 1 meq/100 g, for sandy soils low in OM, to >25 meq/100 g for soils high in certain types of clay or OM.Soil OM will develop a greater CEC at near-neutral pH than under acidic conditions. Additions of an organic material will likely increase a soil's CEC. Soil CEC may also decrease with time through acidification and OM decomposition. Determining CEC The determination of CEC in a soil testing laboratory can be summarized with the following two procedures: 1) Sum of cations : remove all cations and total the amount of all the cations removed from the soil exchange sites.2) NH4+saturation:the soil is saturated with NH4+, then the NH4+ is replaced by Ca++, and lastly the NH4+ removed is measured to determine the number of exchange sites that were occupied by ammonium. Base saturationBase Saturation (B.S.) refers to the number of basic cations that are held on the soil exchange (CEC sites) in comparison to the total number of sites.Base saturation can also be defined as the amount of basic cations that occupy the cation exchange sites, divided by the total cation exchange capacity (CEC). Bases ÷ CEC × 100=Base Saturation %Saline SoilsSaline soils (mostly white patches on the soil with accumulation of soluble salts such as NaCl, CaCl2, and KCl):Are those that have accumulated soluble salts.The conductivity of the saturation extract is greater than 4 mmhos/cm, the exchangeable-sodium percentage is less than 15The pH is usually less than 8.5. Have sufficient soluble salts to impair plant growth, mainly by increasing the osmotic pressure of the soil solution which will restrict water uptake. Soluble salts may accumulate naturally in soils in arid regions. In some cases the lack of rainfall reduces leaching, and salts can build up in the profile. Other factors leading to salinity:Irrigation water can also be the supplier of soluble salts to soilsAddition of fertilizersThe symptoms of salt injury in the plant are:Chlorosis or burning of the leaf edge. On some plants, wilting will also occur due to the lack of water absorption. Seed germination can also be reduced in saline soils. Soils that are saline will have a neutral to slightly alkaline pH. Crops that are tolerant of a slightly saline environment are barley, cotton, and alfalfa. Saline soils can be reclaimed by leaching the soils to remove the soluble salts.Organic matterOrganic matter is the vast array of carbon compounds in soil. Originally created by plants, microbes, and other organisms, these compounds play a variety of roles in nutrient, water, and biological cycles. Organic matter can be divided into two major categories: Stabilized organic matter which is highly decomposed and stable.Active fraction which is being actively used and transformed by living plants, animals, and microbes. Two other categories of organic compounds are living organisms and fresh organic residue.The Changing Forms of Soil Organic MatterAdditions:  When roots and leaves die, they become part of the soil organic matter.Transformations: Soil organisms continually change organic compounds from one form to another. They consume plant residue and other organic matter, and then create by-products, wastes, and cell tissue.Microbes feed plants: Some of the wastes released by soil organisms are nutrients that can be used by plants. Organisms release other compounds that affect plant growth.Stabilization of organic matter: Eventually, soil organic compounds become stabilized and resistant to further changes.A figure showing the changing of organic matter: Factors for OM decomposition that would be ideal include: a) Soil temperatures near 25 – 30ºc b) Moisture of 50 to 70% of the soil's water holding capacity; c) Aeration--oxygen must be in adequate supply for aerobic decomposition; d) food supply or fresh organic matter additions. e) Presence of decomposing organisms Humus Is a by-product of organic matter decomposition. It is resistant to further decomposition and is the source of nutrient storage capacity. In the formation of soil humus, there is a rapid decomposition of the water soluble constituents: sugars, organic acids, amino acids, lipids, and nucleotides. Polysaccharides form the bulk of organic matter naturally added to the soil; the most abundant polysaccharide is cellulose, a linear polymer of the sugar glucose. Cellulose is relatively resistant to decay. Importance of soil humus:Reservoir of nitrogen, phosphorus and sulfurSource of negatively charged sites which can attract basic cations called cation exchange capacity or CECMaintenance of soil organic matter is usually accomplished by incorporating plant residues into the soil and the more mixing of soil and residue, the faster the decomposition. It is very difficult to increase the percentage of organic matter in cultivated soils over that which was in the virgin soil. This is especially the case for prairie soils because of the increase in temperature and aeration after cultivation.Importance of organic matter:Nutrient cyclingIncreases the nutrient holding capacity of soil (CEC).Is a pool of nutrients for plants.Chelates (binds) nutrients, preventing them from becoming permanently unavailable to plants.Is food for soil organisms from bacteria to worms. These organisms hold on to nutrients and release them in forms available to plants.Water dynamicsImproves water infiltration.Decreases evaporation.Increases water holding capacity, especially in sandy soils.StructureReduces crusting, especially in fine-textured soils.Encourages root development.Improves aggregation, preventing erosion.Prevents compaction.Other effects of soil organic matterPesticides break down more quickly and can be "tied-up" by organic matter (and clays).Dark, bare soil may warm more quickly than light-colored soils, but heavy residue may slow warming and drying in spring.Many of the effects of organic matter are related to the activity of soil organisms as they use soil organic matter. Plant residues and other organic material may support some diseases and pests, as well as predators and other beneficial organisms.C to N Ratio of Organic Materials & Composting If organic material is added to soil that has a wide carbon to nitrogen ratio, the nitrogen in the soil will be used by the organisms to decompose the organic matter. This can lead to nitrogen deficiencies for plants growing in the soil. In order to avoid N immobilization it is best to compost organic materials that have a C:N ration wider than 30:1 before they are added to the soil. Everything organic has a ratio of carbon to nitrogen (C:N) in its tissues. Manure (Fresh) C: N=15:1 Legumes (peas etc.) 15:1 Grass Clippings 20:1 Manure w/Weeds 23:1 Weeds (Fresh) 25:1 Hay (Dry) 40:1 Leaves (Fresh) 40:1 Leaves (Dry) 60:1 Weeds (Dry) 90:1 Straw, cornstalks 100:1 Pine Needles 110:1 Sawdust 500:1 Wood Chips 700:1 It is the combination of materials that creates the ideal climate for compost microbes which is around a C:N ratio of 30:1. This combination, along with moisture, oxygen and surface area, is what makes a fast, hot pile.NB: As a rule-of-thumb, materials with C:N ratios less than 30:1 will not trigger temporary nitrogen deficiency.Biological properties of soilSoil organisms span a wide range in size, from microscopic forms, such as bacteria, fungi, and protozoa, to large animals, such as insects, worms, and burrowing mammals. The larger organisms assist in decomposition by ingesting plant residues, breaking them into finer particles, and mixing them as waste throughout the moist soil environment. These wastes become food for the microorganisms, which digest the organic matter, releasing plant nutrients and gases, and producing glues that stick the soil mineral particles together to form aggregates.Macro soil organisms influence soils mainly by being mixers of soil materials. Ground squirrels, badgers, gophersand crawfish are some animals that mix soil horizons with their digging. RootsRoots absorb the water and nutrients that are needed by the plants for photosynthesis and respiration.Roots in the soil play an important role in the activity of organisms. Roots are often a little leaky, and the material that they leak is referred to as root exudates. The area immediately around the root is known as the rhizosphere. The rhizosphere environment has a lower pH, and the soil atmosphere has lower O2 and higher CO2 concentrations.The rhizosphere is higher in soil organism activity due to increased food supply leaked by the root for the organisms to use. This includes: amino acids, organic acids, carbohydrates, nucleic acids, growth factors, enzymes, and soughed-off tissue. Benefits for the plant of having a rhizosphere include: enhanced N mineralization, enhanced N2 fixation, and nutrient solubilization. The rhizosphere is defined as an intense zone of stimulated microbial activity around the root. Within the rhizosphere microbial numbers are much greater than in the bulk soil. Microbes in the rhizosphere can be arbitrarily subdivided into the following groups:Pathogenic (invades and kills plants). Beneficial (often symbiotic with plants) Harmful (normally non-pathogenic opportunists on plants) Saprophytic (live on dead plants) Neutral (no effect on plants) The microbes listed above are all competing for the some resources (space, nutrients and carbon) in the rhizosphere. The rhizosphere is a battlefield between pathogenic and non-pathogenic microorganisms. There are some micro-organisms present in the rhizosphere which are good for roots. An example of this is the actinomycete Streptomyces which secretes antibiotics and toxins into the soil which then inhibits the growth of other rhizosphere microorganisms. e.g. It prevents the spread of the pathogenic "damping-off" fungus. EarthwormsThey deposit 10 to 15 tons of castings per acre on the surface of the soil during a year. Research on the beneficial effects of worms generally does not show they will increase yields. However, there casts are high in bacteria, organic matter, and plant nutrients. Castings have an NPK ratio of 0.5:0.5:0.3 and are 50% organic matter with 11 trace minerals. Worm castings work like time-released fertilizer. Worms prefer a moist non-acid environment. They also need organic matter, which they use as a food source, and high amounts of available calcium. They leave numerous channels in the soil which may increase pesticides and nutrients movement on the subsoils at faster rates. These channels allow preferential flow of water, rather than the water moving only through the soil pores. BacteriaAre the most abundant organisms in the soil. They can be so numerous that a pinch of soil can contain millions of organisms. Soils often have between 1,000,000 to 10,000,000 bacteria per gram.Bacteria are common throughout the soil, but tend to be most abundant in or adjacent to plant roots (an important food source).Importance of bacteriaFree-living bacteria fix atmospheric nitrogen, adding it to the soil nitrogen pool..Other nitrogen-fixing bacteria form associations with the roots of leguminous plants such as lupine, clover, alfalfa, and milk vetch. Some bacteria exude a sticky substance that helps bind soil particles into small aggregates (hence they help improve water infiltration, water- holding capacity, soil stability, and aeration). Bacteria are becoming increasingly important in bioremediation, meaning that we (people) can use bacteria to help us clean up our messes. Bacteria are capable of filtering and degrading a large variety of human-made pollutants in the soil and groundwater so that they are no longer toxic. The list of materials they can detoxify includes herbicides, heavy metals, and petroleum products. Bacteria can be divided into 2 large groups based on their carbon source:Autotrophic: bacteria are independent of any carbon in the soil since they fix atmospheric CO2; and obtain energy from the reactions of nitrogen and sulfur compounds in the soil. Heterotrophic: bacteria require carbon compounds as a food source. They are extremely important in decomposing organic matter. In the carbon cycle, CO2 is absorbed by plants during photosynthesis. As these plants die and are incorporated into the soil, heterotrophic bacteria and other soil organisms decompose this organic matter and release CO2; into the soil atmosphere.Actinomycetes:Are a broad group of bacteria that form thread-like filaments in the soil.They are responsible for the distinctive scent of freshly exposed, moist soil. Importance of actinomycetes: Effective at breaking down tough substances like cellulose (which makes up the cell walls of plants) and chitin (which makes up the cell walls of fungi) even under harsh conditions, such as high soil pH.Form associations with some non-leguminous plants (important species are bitterbrush, mountain mahogany, cliff rose, and ceanothus) and fix nitrogen, which is then available to both the host and other plants in the near vicinity. Biological Fixation Nitrogen fixing organisms:Cyanobacteria (blue-green algae)These organisms used to be called blue green algae because it was thought they were more akin to algae than any other organism. It is now known that they are bacteria- like in cell structure and definitely prokaryotic organisms (algae are eukaryotic). No eukaryote fixes nitrogen. Cyanobacteria include nostoc and anabaena and both fix N both non-symbiotically (for example in a rice paddy) and symbiotically in water ferns and other plants. Nitrogen Fixing AssociationsFree living N fixers (they fix N2 on their own). Free living nitrogen fixers that generate ammonia for their own use (e.g. bacteria living in soil but not associated with a root) include the bacteria, Azospirillum, Azotobacter spp. and Clostridium spp. (fix 30 % of all N2 fixed in world)Symbiotic N fixers --example is bacteria (Rhizobium) and plant (soybean) (fix 70 % of all N2 fixed in world). Symbiotic nitrogen fixers are associated with plants and provide the plant with nitrogen in exchange for the plant's carbon and a protected home.Rhizobium-legume symbiosisThe gram negative bacteria Rhizobium, Bradyrhizobium and Azorhizobium associate with leguminous plants (members of the bean family).Gram positive bacteria Frankia associate with certain fast growing trees, and cyanobocteria associate with some aquatic ferns. In the Rhizobium-legume symbiotic process:The bacteria infect the roots of the plant and a structure known as a nodule is formed. Once the nodule is established, the differentiated bacteria (they become non-motile bacteroids) living in the infected plant cells, reduce atmospheric nitrogen to ammonia which is excreted to the plant cell and is, in turn, assimilated to organic nitrogen (proteins and amino acids) by the plant. The plant provides the bacteroid with carbon skeletons (photosynthate) which are required by Rhizobium, a strict aerobe, to provide the energy that is needed for nitrogen fixation. This symbiosis is a specific process, a certain species of Rhizobium can only nodulate a certain type of legume, for example: R. etlinodulates beans (Phaseolus), R. melilotinodulates alfalfa (Medicago sativa). The bacterial enzyme responsible for the reduction of gaseous N2 to ammonia is the nitrogenase enzyme complex which is formed from the joining together of three different polypeptides. Different nitrogenase enzyme systems have been found in different microorganisms. FungiFungi are not plants; Fungi are placed in their own Kingdom. The living body of the fungus is a mycelium made out of a web of tiny filaments called hyphae. Fungi are the most active decomposers of organic materials in a forested soil. This is mainly because they are tolerant of acid soil conditions. All of us have seen fungi. Fungi have evolved to use a lot of different items for food. Some are decomposers living on dead organic material like leaves. Some fungi cause diseases by using living organisms for food. These fungi infect plants, animals and even other fungi. MycorrhizaeAre fungi associated with the fine roots of most plants. The term itself means "fungus root". There are hundreds species of fungi which function as mycorrhiza; most are basidiomycetes, the class of fungi which form mushrooms.These fungi can benefit plants by enhancing the nutrient absorbing ability of roots (are especially important in facilitating uptake of phosphorous). This enhancement of nutrient uptake is a result of the extensive system of hyphae and mycelia (thread-like filaments of the mycorrhizal fungus) that pervade soils. They function like root hairs but are much more far reaching. The relationship of this fungus with plants is a mutually beneficial one, with the fungi receiving energy in the form of carbohydrates from the host plant. There are two types of mycorrhizae:Ectomycorrhizae: Filaments penetrate between cells of roots, but not into root cellsForm a thick cylindrical sheath around young lateral roots. The affected roots become short and thickened, and are deficient in root hairs. It is believed that this sheath may protect roots from invasion by plant pathogens. Most trees have this type of mycorrhizae. 2. Endomycorrhizae: Filaments penetrate into root cells and extend out from roots like root hairs to absorb nutrients from the soil solution. They do not form a sheath around roots nor do they alter the structure of roots. Endomycorrhizae are found on a greater range of plants than are ectomycorrhizae. Plant growth is enhanced by the presence of mycorrhizae. Mycorrhizae are particularly abundant in forest soils but are found in almost all soils, with the possible exception of grasslands where no trees have previously grown. Growth enhancement is especially significant for plants growing on infertile soils and dry soils. Interestingly, mycorrhizae development decreases following heavy fertilization of soil. NematodesAre microscopic worms that feed on organic matter and other soil animals or infect plant roots.Parasitic nematodes are the most important from an agricultural standpoint. Many plants are affected, such as tomatoes, carrots, potatoes, peas, alfalfa, turfgrass, and fruit trees. Nematodes can parasitize virtually all crops and ornamental plants and can cause significant economic damage by reducing both yield and quality. Properly taken samples from small field units can reduce production costs by allowing the grower to eliminate nematodes 1. Lance nematodes, Hoplolaimusspp: are large nematodes which are highly resistant to effects of temperature extremes and dry soil conditions. One species, H. columbus, causes severe damage to soybeans and cotton.Another more widely distributed species, H. galeatus, is primarily a pathogen on grasses. Lance nematodes feed externally along root surfaces but may also feed with at least part of the body embedded in the root. Larvae look similar to adults except that they are smaller. This group of nematodes is easily detected with soil sampling. 2. Root-knot nematodes,Meloidogyne spp., Are one of the important plant-parasitic nematodes because of their wide host range and widespread distribution.Root-knot larvae enter roots of host plants near root tips and remain inside the root at one location throughout their life. As larvae feed, the root cells divide rapidly near the nematode's head. This rapid cell division and enlargement cause the swelling or knots on roots. . Plant growth requirementsIn order for plants to grow well, they need: Proper temperatureLight quality, intensity and  durationMoistureCarbon dioxide (CO2)Oxygen for respirationHormonesSixteen essential nutrientsTemperaturePhotosynthesis increases as the temperature increases.The plant’s uptake of minerals and water are affected by temperature.Plants have an optimum temperature for growth and development.Plants have a minimum temperature below which they cannot live.High temperatures may dry out plants.Plants should be chosen for the climatic conditions of the area.LightQuality:  Plants require white light or sunlight. Chlorophyll absorbs red and blue light and reflects green light.Intensity:  Intense light provides sufficient energy for photosynthesis to occur.Duration:  Different plants require different exposure times to light in order to be productive. Photoperiod is the length of time that the plant needs light.MoistureSoil moisture carries minerals and nutrients from the soil into the plant via the xylem.Water is used in chemical reactions such as photosynthesis.Turgor is the pressure which allows the plant to stand erect, is provided by water within the cell.Water cools the plant via transpiration.Water moves (translocates) products that are produced by photosynthesis from the leaves to the rest of the plant via the phloem.Carbon dioxideCarbon Dioxide is absorbed through the stomata.It is used in photosynthesis to produce sugars for the plant.HormonesHormones are low molecular weight chemicals produced in the plant to regulate growthAuxin-promotes cell elongation, apical dominance, induces roots on cuttings, stimulates fruit development and stimulates ethylene synthesisABA-Stimulates stomatal closure, may be necessary for abscission and dormancy in some speciesCytokinin-Promotes apical dominance, shoot growth and fruit developmentGiberellins-GA-Flowering stimulation in long-day plants and biennials, shoot elongation and regulates production of seed enzymes in cereals Ethylene-Promotes fruit ripening, leaf and flower senescence and abscissionEssential nutrientsSixteen chemical elements are known to be important to a plant's growth and survival. The sixteen chemical elements are divided into two main groups: non-mineral and mineral.  Non – mineral nutrientsThe Non-Mineral Nutrients are hydrogen (H), oxygen (O), & carbon (C).These nutrients are found in the air and water.  Since plants get carbon, hydrogen, and oxygen from the air and water, there is little farmers can do to control  how much of these nutrients a plant can use.Mineral nutrientsThe 13 mineral nutrients, which come from the soil, are dissolved in water and absorbed through a plant's roots. There are not always enough of these nutrients in the soil for a plant to grow healthy. This is why many farmers and gardeners use fertilizers to add the nutrients to the soil.  Soil texture affects how well nutrients and water are retained in the soil. Clays and organic soils hold nutrients and water much better than sandy soils. As water drains from sandy soils, it often carries nutrients along with it.Soil pH is one of the most important soil properties that affects the availability of nutrients.    Macronutrients tend to be less available in soils with low pH.Micronutrients tend to be less available in soils with high pH.Lime can be added to the soil to make it less sour (acid) and also supplies calcium and magnesium for plants to use. Lime also raises the pH to the desired range of 6.0 to 6.5.  The mineral nutrients are divided into two groups:  macronutrients and micronutrients.  Macronutrients   Macronutrients are in two more groups:  primary and secondary nutrients.  The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients usually are lacking from the soil first because plants use large amounts for their growth and survival.  The secondary nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). There are usually enough of these nutrients in the soil so fertilization is not always needed. Also, large amounts of Calcium and Magnesium are added when lime is applied to acidic soils. Sulfur is usually found in sufficient amounts from the slow decomposition of soil organic matter, an important reason for not throwing out grass clippings and leaves. Nitrogen (N)Nitrogen is a part of all living cells and is a necessary part of all proteins, enzymes and metabolic processes involved in the synthesis and transfer of energy.Nitrogen is a part of chlorophyll, the green pigment of the plant that is responsible for photosynthesis. Helps plants with rapid growth, increasing seed and fruit production and improving the quality of leaf and forage crops. Nitrogen often comes from fertilizer application and from the air (legumes get their N from the atmosphere, water or rainfall contributes very little nitrogen)Phosphorus (P)Like nitrogen, phosphorus (P) is an essential part of the process of photosynthesis. Involved in the formation of all oils, sugars, starches, etc.Helps with the transformation of solar energy into chemical energy; proper plant maturation; withstanding stress.Effects rapid growth.Encourages blooming and root growth.Phosphorus often comes from fertilizer, bone meal, and superphosphate. Potassium (K)Potassium is absorbed by plants in larger amounts than any other mineral element except nitrogen and, in some cases, calcium. Helps in the building of protein, photosynthesis, fruit quality and reduction of diseases.Potassium is supplied to plants by soil minerals, organic materials, and fertilizer.Calcium (Ca)Calcium, an essential part of plant cell wall structure, provides for normal transport and retention of other elements as well as strength in the plant. It is also thought to counteract the effect of alkali salts and organic acids within a plant. Sources of calcium are dolomitic lime, gypsum, and superphosphate.Magnesium (Mg)Magnesium is part of the chlorophyll in all green plants and essential for photosynthesis. It also helps activate many plant enzymes needed for growth.Soil minerals, organic material, fertilizers, and dolomitic limestone are sources of magnesium for plantsSulfur (S)Essential plant food for production of protein.Promotes activity and development of enzymes and vitamins.Helps in chlorophyll formation.Improves root growth and seed production.Helps with vigorous plant growth and resistance to cold.Sulfur may be supplied to the soil from rainwater. It is also added in some fertilizers as an impurity, especially the lower grade fertilizers. The use of gypsum also increases soil sulfur levels. Micronutrients  Micronutrients are those elements essential for plant growth which are needed in only very small (micro) quantities. These elements are sometimes called minor elements or trace elements, but use of the term micronutrient is encouraged by the American Society of Agronomy and the Soil Science Society of America. The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Recycling organic matter such as grass clippings and tree leaves is an excellent way of providing micronutrients (as well as macronutrients) to growing plants.Boron (B)Helps in the use of nutrients and regulates other nutrients. Aids production of sugar and carbohydrates. Essential for seed and fruit development. Sources of boron are organic matter and boraxCopper (Cu)Important for reproductive growth.Aids in root metabolism and helps in the utilization of proteins. Chlorine (Cl)Aids plant metabolism. Chloride is found in the soil. Iron (Fe)Essential for formation of chlorophyll.Sources of iron are the soil, iron sulfate, iron chelate. Manganese (Mn)Functions with enzyme systems involved in breakdown of carbohydrates, and nitrogen metabolism. Soil is a source of manganese.Molybdenum (Mo)Helps in the use of nitrogenSoil is a source of molybdenum. Zinc (Zn)Essential for the transformation of carbohydrates.Regulates consumption of sugars.Part of the enzyme systems which regulate plant growth. Sources of zinc are soil, zinc oxide, zinc sulfate, zinc chelate.Plant nutrient disorders and corrective measuresMacronutrientsReplace macronutrients in soils regularly (at least once per growing season)Factors affecting nutrient availability and uptakeNatural supply of nutrients in the soil (which depends upon the nature of the parent material of that soil and vegetation previously grown)Soil pH Relative activity of microorganisms (which play a vital role in nutrient release and may directly function in nutrient uptake)Fertility addition in the form of commercial fertilizer, animal manure and green manureSoil temperatureMoisture Aeration. FertilizersA fertilizer is any material, organic or inorganic, natural or synthetic, that supplies plants with the necessary nutrients for plant growth and optimum yield.Fertilizer Classification: Fertilizers are classified into straight, complex or mixed fertilizers. Straight fertilizers supply only one primary plant nutrient, either Nitrogen or Phosphorus or Potassium. Urea, Ammonium Sulphate, Potassium Chloride and Potassium Sulphate are examples of straight fertilizers. Complex fertilizers contain two or three primary nutrients of which two primary nutrients are in chemical combination.These fertilizers are usually produced in granular form. Diammonium Phosphate, Nitrophosphates and Ammonium Phosphate are complex fertilizers. Mixed fertilizers are made by thoroughly mixing the ingredients either mechanically or manually. NB: Fertilizers are grouped on the basis of the nutrient present in the fertilizers, namely, Nitrogenous fertilizers, Phosphatic fertilizer, Potassic fertilizer, Boron fertilizers, etc.Methods of fertilizer application1. Broadcasting: Fertilizers are applied uniformly to the soil surface. This is done either before sowing or in the standing crop. This is the most widely practiced method of fertilizer application, mostly because it is easy. It is mostly done with solid fertilizers, but some farmers manage to broadcast liquid fertilizers as well by mixing it with sand.2. Band Placement: Application of fertilizers in narrow bands beneath and by the side of crop rows is known as band placement of fertilizers. Band placement is done under the following situations: When soil fertility is low. When crops need a good start. When fertilizer materials react with soil constituents leading to fixation. Where volatilization losses are high. For crops with tap root systems, bands can be 5cm below the seed. In cereals and millets with fibrous root systems, bands are placed 5cm away from the seed row and five cm deeper than the seed.3. Point Placement: This is the placement of fertilizers near the plant either in a hole or in a depression followed by closing or covering with soil. It is adopted for top dressing of Nitrogenous fertilizers in widely spaced crops. In Sugarcane, two or three holes are made around the plant and Nitrogenous fertilizers placed in them. Similarly, soil near tobacco plants is removed, and fertilizer placed and covered with soil. This method ensures high recovery of Nitrogen fertilizer.Subsoil Placement: In this method, fertilizers are placed in the subsoil with the help of high power machinery. This method is recommended in humid and sub humid regions where subsoils are acidic.4. Fertigation: Application of fertilizers with irrigation water is known as fertigation. It is generally used in drip-irrigation systems.5. Root Dipping: Roots of seedlings are dipped in nutrient solution before transplanting. For example, in soils deficient in Phosphorus, roots of Rice seedlings are dipped in Phosphorus slurry before planting.6. Foliar Spray: Application of fertilizers to the foliage of crops as a spray solution is known as foliar spray. This method is suitable for the application of small quantities of fertilizers, especially micronutrients. Foliar application can only supplement soil application and not substitute it (i.e., Zinc Sulphate in Rice).Time of Application 1. Basal Application Application of fertilizers before or at the time of sowing is known as basal application. A portion of a recommended dose of Nitrogen and the entire quantity of Phosphorus and Potassium are usually applied as the basal dose. 2. Split Application Application of recommended dose of fertilizers at two or three different times during the crop period is known as split application. Application of fertilizers to standing crop is known as top dressing. The number of split application has to be more in light soils than in heavy ones because there is more leaching in light soils.Biofertilizer Some microorganisms are capable of fixing Nitrogen, while others increase the availability of Nitrogen and Phosphorus. These Biofertilizer are often used in leguminous crops to enhance their Nitrogen fixing capability. Soybean, for example, is inoculated with Rhizobium japonicum bacterial culture at the rate of 500 gm per 75 kg of seed to facilitate nodulation and Nitrogen fixation.1. Saprophytes Microorganisms that are capable of decomposing organic matter at a faster rate can be used as fertilizers for quick release of nutrients. They accelerate the natural process of decomposition and composting time is reduced by 4-6 weeks. Examples are Aspergillus, Penicillium, and Trichoderma. 2. Rhizobium This bacteria is capable of fixing atmospheric Nitrogen in association with leguminous crops. They enter the roots of the host plant and form nodules. They take up carbohydrates and water from the host and supplies it with Nitrogen. 3. Free Living Organisms The important free living organisms that can fix atmospheric Nitrogen are Blue Green Algae (BGA), Azolla, Azotobacter and Rhizospirillum.Organic fertilizersSoil fertility on smallholder farms is almost entirely dependant on locally available resources. Cattle manure, cereal and legume stover, and woodland litter are the commonly used organic fertilizers, but these are rarely applied in sufficient quantities to impact on crop yields. The use of high quality organic fertilizers is rarely practiced.The main advantage of using organic fertilizers is that, compared to mineral fertilizers, they are usually available on or near the farm at very little or no cost other than labor costs of handling, transportation, or opportunity costs of land used for their production.Time of application: For organic materials, decomposition rate and timing of application influence the release of nutrients to the crop. Organic fertilizer application methods include broadcasting, banding, and spot application (or side-dressing). Broadcasting requires less labor and helps to evenly cover the field surface before incorporation into soil through plowing or hand-hoeing. Incorporation generally increases the fertility status of the field. If the quantity of organic fertilizer is limited, it may be banded along furrows or spot applied, but the seed needs to be placed away from the fertilizer. Side-dressed organic fertilizers are not likely to have much immediate effect due to delayed nutrient release.Effectiveness: Continued use of organic fertilizers results in: increased soil organic matter, (ii) reduced erosion, (iii) better water infiltration and aeration, (iv) higher soil biological activity as the materials decompose in soil, and (v) increased yields after the year of application (residual effects). Proper handling of organic fertilizers enhances their quality and effectiveness. For example, with the exception of green manures, there is significant crop response if organic fertilizers are combined with N-based mineral fertilizers or other N-rich organic materials.Inorganic (mineral) fertilizersMineral fertilizers need to be applied to crop at least two times within a growing season (split application), either basally at planting or top-dressed during vegetative growth. The amount of inorganic fertilizer used in most smallholder farming systems falls far below standard extension recommendations, due to poor purchasing power, risk aversion due to poor and unreliable rainfall, and lack of significant returns. When available, fertilizer use is not overly labor intensive, thus allowing time for other tasks (or for earning income elsewhere).Application: Mineral fertilizers can be applied by hand or with application equipment. When hand applied, it is essential to distribute the fertilizers uniformly and at the recommended rates to avoid over- or under-fertilization. Application equipment needs proper adjustment to ensure uniform spreading. Broadcast fertilizer should be incorporated after application to enhance effectiveness or to avoid evaporation losses of N. With banding or spot application, take care that no fertilizer is placed too close to either the seed or the germinating plant, to avoid damage to the seedling or roots.Effectiveness: Mineral fertilizers on the other hand immediately supply nutrients needed by crops. Basal fertilizers contain elements required for good crop establishment and early growth while top-dressing can be done through split applications depending on visible hunger signs and/or moisture availability. In risky environments, spot application of small amounts of N fertilizers improves fertilizer effectiveness. The best response to fertilizer use is obtained if the soil has a high inherent fertility level (high organic matter status). Building inherent fertility requires practices such as retaining crop residues on the field.Major limitations of organic and inorganic fertilizersOrganic fertilizersGenerally require large amounts to have desired effectsExtra investment in labor for harvesting (green manures) and preparation (cattle manure)Unavailability of seed for green manures is one of the major limitationsQuality for most has to be enhanced by combining with expensive mineral fertilizers Green manures must occupy land at a time when other food crops could be grown. Mineral/ inorganic fertilizersRequire high purchasing powerAvailability is an obstacle, especially in remote areas Need to be applied seasonallyHigh risk in low rainfall and very high rainfall areasFertilizer calculationsExamples of fertilizers normally used and there percentages:Urea – N:P:K:S = 46:0:0:0CAN – 20 – 30% CaCO3 and 70 – 80% ammonium nitrateDAP – N:P:K:S = 18:46:0:0SSP – N:P:K:S = 0:8.8:0:11TSP – N:P:K:S = 0:45:0:0MOP – 60% KNB: 1 unit = 1 kgCalculating the amount of fertilizer required: Example 1: (a) You need 20kg/ha of phosphorus (P) and you plan to use single super phosphate with 8.8% P. Calculate the amount of super phosphate required (kg/ha).Amount of fertilizer kg/ha = kg/ha nutrient ÷ % nutrient in fertilizer x 100.Amount of super phosphate required (kg/ha) = 20 kg/ha P÷ 8.8 P x 100 = 227 kg/haTry this: (b) what would be the amount of single super phosphate required in 0.75ha?Calculating the amount of nutrient applied:Example 2: You plan to apply 41.6kg/ha of TSP. Calculate the amount of P applied (kg/ha).Amount of nutrient (kg/ha) = Amount of fertilizer (kg/ha) x % nutrient in fertilizer ÷100Amount of P applied (kg/ha) = 41.6kg/ha x 45 P ÷ 100 = 18.72kg/ha PTry this: How much P would you apply in 0.5 ha?Calculating the cost per single nutrient:Example 3: (a) 100kg of DAP cost Ksh. 4560. Calculate the cost of P in it?Cost per nutrient (ksh) = cost of 100kg DAP x unit of nutrient in fertilizer ÷100kg of DAPCost per nutrient (ksh) = 4560 x 46 kg ÷100kg of DAP = Ksh 2097.60 for PTry this: (b) If the above 100kg DAP was to be applied on 1ha. Calculate the cost of P and N to be applied in 1.75ha.Integrated soil fertility managementIs the application of soil fertility management practices and knowledge to adapt to local conditions in order to optimize fertilizer and organic resource use efficiency and crop productivity. These practices necessarily include appropriate fertilizer and organic input management in combination with the utilization of improved germplasm.Soil fertilitySoil fertility can be defined simply as the capacity of the soil to supply nutrients to the plant. Using this definition, a fertile soil is one that contains an adequate supply of all the nutrients required for the successful production of plant life. This principle is probably best summed up by the “Law of the Minimum” propounded by Justus von Liebig in the mid-1800’s. This law states that if one of the nutritive elements is deficient or lacking, plant growth will be poor even when all the other elements are abundant. Any deficiency of a nutrient, no matter how small an amount is needed, will hold back plant development. If the deficient element is supplied, growth will be increased up to the point where the supply of that element is no longer the limiting factor. Increasing the supply beyond this point is not helpful, as some other element would then be in a minimum supply and become the limiting factor.Fertile soil is characterized by ongoing complex interactions involving decomposition of rocks, organic matter, animals, and microbes to form inorganic nutrient ions in soil water.Integrated soil fertility management and soil organic matterSoil organic matter (SOM) plays a critical role in soil processes and is a key element of integrated soil management (ISFM). Almost all ISFM technologies are SOM dependent for their full success. Furthermore, SOM content can be used to set critical values that can help to make decisions when implementing ISFM programs. The SOM and ISFM relation is tricky, SOM build up is ISFM dependent and ISFM efficacy is SOM dependent.ISFM and rhizobiumRhizobium is an important soil organism. This is a genus of soil bacteria that is responsible for symbiotic nitrogen fixation in legume plants. These organisms penetrate plant roots causing the formation of small nodules on the roots. They then live in symbiotic relation with the host plant. Actions/measures for improvement and maintenance of soil fertility: adding nutrients to replenish stocks and flows in the soilblocking nutrient flows leaving the farm (‘leaks in the system’)doing a better job in recycling nutrients that are not optimally used within the farmIncreasing the efficiency with which nutrients are used by the various production systems. 1. Adding Organic or Inorganic FertilizerIt is a simple fact that plants use soil nutrients to grow and reproduce. If whole plants or major portions of them are continuously removed from the field and no nutrients are added, the soil’s reserve of some or all of the elements will not be sufficient for economic agricultural production. This continuous removal of nutrients is known as nutrient mining.Adding nutrients is done to achieve two main objectives – short- and long-term: To achieve short term gains in soil fertility farmers may:grow a green manure cropadd mineral fertilizersApply farmyard manure. Long-term strategies include fallowing:application of organic matter with a high C/N contentApplication of one-time high doses of inorganic P or phosphate rock.2. Reducing Nutrient Losses - Blocking Nutrient Flows Leaving the FarmNutrient losses can be reduced by controlling erosion, run-off and leaching. Erosion losses can be reduced through the construction of bunds, terraces, or stone lines. Trees can be instrumental in re-capturing nutrients leached from the subsoil. Nitrogen losses through NH3 volatilization during storage and handling of manure limit its effectiveness as a nutrient source. Anaerobic storage in pits with or without addition of crop residues can significantly reduce N losses.3. Better Management of Available Resources - Managing Internal Flows of NutrientsBetter integration of crop and livestock management, use of household waste, composting and incorporating crop residues into the soil are promising ways to improve nutrient cycling within the farm. Bedding in stables absorbs urine and conserves nutrients.Composting is a process where material with a high C/N ratio (e.g. rice straw) is converted into material with a low C/N ratio. Farmers may improve the nutritional quality of compost by adding ashes, eggshells and droppings of small ruminants. During composting, about 50% of the carbon in the initial material is typically lost but mineral nutrients are mostly conserved. Finished compost is therefore generally more concentrated in nutrients than the initial combination of raw materials used and can serve as an effective means of building soil fertility. 4. Improving the Efficiency of Nutrient UptakeImproving input use efficiency is a key intervention as it results in reduced production costs and environmental risks. The more nutrients a crop converts to grain or fiber, the less opportunity for nutrients to reach streams, lakes or groundwater. Nutrient recovery may be enhanced in several ways:Improved crop management: It is important that nutrient addition be synchronized with plant demand for nutrients and fertilizer application may greatly enhance recovery. Better management of yield reducing factors like weeds, pests and diseases may greatly increase nutrient recovery from fertilizers. Placement of fertilizers: Plants take up nutrients more efficiently if fertilizers are applied close to the roots. It has been shown that micro-doses of growth-limiting nutrients placed near the roots may greatly enhance crop performance. Mulching (e.g. with rice straw or other plant residues) may conserve moisture and smother out weeds, enabling better crop establishment and nutrient uptake.It is also important to consider the value of balanced fertilization. When nutrient supply is unbalanced, yields and profits decline and, quite often, the quality of the crop are impaired.