Soil & soil fertility

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Soil & soil fertility. Africa Soil Health Consortium 2014. Lecture 2: Introduction to soil and soil fertility. Objectives. Gain knowlegde on the principles underpinning ISFM practises Introduction to soil Soil texture Porosity Mineral fraction Organic matter Introduction to nutrients - PowerPoint PPT Presentation

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  • Soil & soil fertilityAfrica Soil Health Consortium2014Lecture 2: Introduction to soil and soil fertility

  • ObjectivesGain knowlegde on the principles underpinning ISFM practises

    Introduction to soilSoil texturePorosityMineral fractionOrganic matterIntroduction to nutrientsUnderstanding the function of nutrients in plant growthRecognizing nutrient deficienciesSoil fertilityUnderstanding the concept of soil fertilityIntroduction to soil fertility managementConservation agriculture & organic agricultureMinimizing losses of added nutrients

  • SoilSoil solidsPore space+ soil fauna and floraPore space: -space for roots and micro-organisms-air for micro-organisms-water storageMineral fraction:Provides support to plant rootsSlowly releases nutrients into the soil solutionOrganic fraction:Soil organic matter (SOM)Key issue in soil fertility management

  • Pore spacePorosity: volume of the soil occupied by air and the soil solution

    Porosity inWell-drained moist soil: sufficient moisture for plant growth and sufficient aeration for proper root functionDry soil: all pores are filled with air drought stressFlooded soil: pores are saturated with water roots cannot breathe and plants may die

    Illustration adapted from Brady 1984, The nature and properties of soils, 9th edition.

  • Mineral fractionSand: 0.05 - 2.0 mmSilt: 0.002 - 0.05 mmClay: < 0.002 mmIllustration adapted from: www.iconn.orgSiltClaySand

  • Mineral fraction

  • Mineral fractionThe finger test

  • Mineral fraction & PorositySoil texture affectsPorosityWater holding capacityNutrient retention and supplyDrainageNutrient leachingIllustrations adapted from: http://wegc203116.uni-graz.at/meted/hydro/basic/Runoff/print_version/04-soilproperties.htm Infiltration Variations by Soil TextureSandSiltClay

  • Mineral fraction & CECCations: positively charged ions (e.g. K+, NH4+)Cation exchange capacity (CEC): the maximum quantity of total cations that a soil is capable of holding. Clay fraction and SOM: Small particle size Large negatively charged surface area More positions to hold cations High CEC

    Illistration adapted from: http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm

  • Mineral fraction & CECCEC depends onClay contentType of clay mineralSOM contentSoil pH

    Clay minerals differ in structure1:1 clay minerals CEC varies with soil pH Found in most upland soils in SSA2:1 clay minerals Large inherent CEC capacity Found in fertile lowland soils

    Illustration adapted from Lory Structure of Clays www.soilsurveys.org

  • Organic fraction: SOMSOM: plant and animal residues, in various stages of decompisition Picture: http://www.guiadejardineria.com/jardineria/suelos-y-abonos/page/7/

  • Organic fraction: SOMContains essential plant nutrientsImproves the soils Cation Exchange Capacity Improves the soils water-holding capacity (SOM can hold up to five times its own weight in water!)Improves water infiltration Buffers soil pHBinds with toxic elements in the soilImproves soil structure by stimulating activity of soil flora and faunaRegulates the rates and amounts of nutrients released for plant uptake

    SOM is a key issue in soil fertility management!

    Illustration adapted from: http://www.tekura.school.nz/departments/horticulture/ht106_p4.html

  • Soil analysisSoil test: chemical method for estimating the nutrient-supplying power of a soilLaboratory needs a representative composite sample of 0.5 kg

    Be aware of heterogeneity within fields when sampling!

  • Guidelines for soil samplingTake a representative sample!!!

    Check the area to be sampled for notable features (e.g. slope, soil types, vegetation, drainage).Draw a sketch map, and identify and mark the location of sampling sites.Take soil samples with a soil auger at the sampling depth (0-20 cm or 20-40 cm).Take 10-35 sub-samples per site, the number depending on the size and heterogeneity of the field. Combine the sub-samples to one composite per site and mix thoroughly. If necessary, reduce sample weight by sub-dividingLabel the sample of soil properly.Air-dry the sample and when dry, store it, properly labelled, in a plastic bag or a glass bottle for further analyses.

  • NutrientsMacronutrients: at least 0.1% of plant dry matter per macronutrient

    Nitrogen (N): Amino acid/Protein formationPhotosynthesis

    Phosphorus (P):Energy storage/transferRoot growthCrop maturityStraw strengthDisease resistanceNeeded in large amounts during plant growthRequired for N2-fixation by legumes

    Potassium (K):Plant turgor pressure maintenanceAccumulation and transport of the products of plant metabolismDisease resistanceRequired for N2-fixation by legumes

    Sulphur (S):Part of amino acids (protein formation)Synthesis of chlorophyll and some vitaminsRequired for N2-fixation by legumes

    Magnesium (Mg):PhotosynthesisActivates enzymesCarbohydrate transport

    Calcium (Ca):Cell growth and walls Activates enzymes (protein formation and carbohydrate transfer)Essential in calcicole plants (e.g. Groundnut) for seed production.Influences water movement, cell growth and divisionRequired for uptake of N and other minerals

    Poor mobilityVery mobileVery mobileVery mobileVery mobileQuite poor mobilityVery mobileVery mobilePoor mobilityQuite mobileQuite poor mobilityMedium mobility

  • Nutrients

    Micronutrients: less than 0.1% of plant dry matter

    Iron (Fe):PhotosyntheissRespiration

    Manganese (Mn):PhotosynthesisEnzyme function

    Boron (B):Development/growth of new cells

    Zinc (Zn):Nucleic acid synthesis and enzyme activationCopper (Cu):Chlorophyll formationSeed formationProtein synthesis

    Molybdenum (Mo):Protein synthesis and N uptakeN2-fixation by legumes

    Chlorine (Cl):Movement of water and solutesNutrient uptakePhotosynthesisEarly crop maturityDisease control

    Cobalt (Co):N2-fixation by legumes

    Nickel (Ni):Required for enzyme urease

    Sodium (Na):Water movement and balance of minerals

    Silicon (Si)Cell wallsProtection against piercing by sucking insectsLeaf presentationHeat and drought tolerance

  • Nutrient deficiencyHealthyN-deficientP-deficientK-deficientDiseased

  • Nutrient deficiencies

  • Nutrient deficiency: exercise

  • Nutrient deficiency: exerciseP-deficientStunted growthPurplish colouringK-deficientBrowning of leaf edges

  • Nutrient uptake

    NutrientPlants take upN NO3-, NH4+PH2PO4- , HPO42-KK+SSO42-MgMg2+CaCa2+FeFe2+ and Fe3+MnMn2+ and Mn3+B(BO3)3-ZnZn2+CuCu2+MoMo42+ClCl-CoCo2+NiNi2+NaNa+Si(SiO4)4-

  • Nutrient availabilityReadily available- Nutrients from soluble fertilizers (e.g. KCL), readily mineralized SOM, nutrients held on the edges of soil particles, and in the soil solution

    Slowly available- Nutrients in organic form, such as plant residues and organic manures (particularly with a high C/N ratio), slowly soluble mineral fertilizers (e.g. Phosphate rock) and the SOM fraction resistant to mineralization

    Not available- Nutrients contained in rocks, or adsorbed on soil particles

  • Soil fertilityThe capacity of soil to supply sufficient quantities and proportions of essential chemical elements (nutrients) and water required for optimal growth of specified plants as governed by the soils chemical, physical and biological attributes.

    Chemical elements for plant nutritionAdequate soil volume for plant root developmentWater and air for root development and growthAnchorage for the plant structure

    Inherent DynamicSoil textureSoil organic matter (SOM)DepthNutrient- and water-holding capacityParent material Soil structure

  • Soil fertility management practicesNutrient deficiencies prevent a good harvestNutrient deficiencies can be expressed during plant growth

    Use mineral (fertilizer) or organic (manure, crop residues) to supply nutrientsUse special fertilizer blends containing micronutrients or manure in case of micronutrient deficiencies

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercroppingHealthyN-deficientP-deficientK-deficient

  • Soil fertility management practicesAcidity is caused byinherent soil propertiesacidity inducing management (e.g. long-term use of ammonium based fertilizer)Acid soils have high exchangeable Al (Al toxicity)

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercroppingLimeIncreases pHPrevents Al and Mn toxicity in acidic soils (pH

  • Soil fertility management practicesCompaction sub-surface soil barrier to root growthBreak hardpans by ploughing or chisel ploughing to 30 cm depth

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercroppingIllustration adapted from: http://locallygerminated.wordpress.com/Surface crust

  • Soil fertility management practicesCapture more rainfall in areas that are prone to droughtHarvesting additional water (e.g. Za)Promoting infiltration by coversing the soil surface with mulchLabour intensiveCorrecting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercroppingZa pits in NigerMulching of bananas, western UgandaPictures: fao.org

    Lydia Wairegi - revised this from 'are' to 'include' in notes

  • Soil fertility management practicesProne to erosion: fields on steep slopes, or on gentle slopes with course-textured top soilMeasures: live barriers (e.g. grass strips), teracces, surface mulch

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercroppingBunds on sloping land in Burundi

  • Soil fertility management practicesGood seedbed preparation improves germination and reduces the chance for diseases

    A delay in planting date often affects yield negativelyPlanting time is important especially when the growing season is short

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercropping

  • Soil fertility management practicesCrops compete for nutrients, water and lightUse a correct planting density, adjusted to crop type and the environment. Consider the distance between rows, between plants within rows and the number of plants per planting hole.

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercropping

    CropOptimal rainfallPoor rainfallDensityBetween rowsWithin rowsDensityBetween rowsWithin rows000 Plants/hacmcm000 Plants/hacmCmBeans (common)20050101335015Maize447530379030Soybean444455333605

  • Soil fertility management practicesUse viable seed (at least 80% germination)Plant seeds at the correct depth and insert cuttings at correct anglePlant more seeds than required for optimal plant density.

    Weeds compete with crops for nutrients, water and light.Timely removal of weeds is essentialWeed before top dressing crop with fertilizer

    Control pests and diseases at specific growth stages

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercroppingDelayed weeding reduces the crop response to fertilizer

  • Soil fertility management practicesIntercropping arrangements: take into account specific growth features and needs of individual crops to minimize intercrop competition.Examples: delayed planting of one intercrop, adjusting spacing, strip intercropping

    Correcting nutrient deficienciesSoil acidity correctionBreaking hardpansWater harvestingErosion controlLand preparationPlanting dateSpacingPlanting practicesWeedingPest and disease managementIntercroppingMaize-pigeonpeaMaize-cassavaCassava-soybean

  • Conservation agriculture (CA)Basic principlesSoil disturbance is minimized by reduced or zero-tillageUse of at least 30% soil cover (mulch or cover crops)Use of crop rotations/associations

    AdvantagesRapid planting of large areasReduction of soil erosionPitfallsCompeting uses of crop residues needed for mulchYields may decrease on the short-term (the increase often comes on the longer-term)Increased weed pressure caused by reduced tillageFull CA requires a fundamental change in the farming system. This may not be practical or enomic for the farmerPossible decrease in agronomic efficiency of fertilizer use

    Lydia Wairegi - revised 'agriculture'Lydia Wairegi - 'thus is underpinned' to 'this is underpinned'

  • Organic agricultureReliance on organic resources to provide nutrients to sustain soil fertility and produce economic crop yields

    However, mineral fertilizers are an essential component in sustainable agriculture in SSASoil nutrients stocks in large parts of SSA have already become depleted and require replenishmentOrganic resources are not available in large enough quantities to replenish and sustain nutrient stocks in the soilLarge and economic responses to mineral fertilizer are obtained in many parts of SSAOrganic resources are bulky and their management is labour intensive

    ISFM: use of mineral fertilizer in combination with organic resources. The combination provides the greatest benefits!

  • Minimizing losses of added nutrientsLosses of nutrients into the environmentDepletion of nutrients in farming systemsEutrophication in case of excessive mineral fertilizer use (not common in SSA)

    Losses throughHarvesting crops recyclingWater and wind erosionLeachingVolatilization

    Nitrogen is the most susceptible to lossesVery mobile, can be lost through different waysNO3- is susceptible to leaching.

  • Losses: Water and wind erosion10 kg N/ha, 2 kg P/ha and 6 kg K/ha lost in low-input production systems in SSA

    Measures: grass strips, stone rows, mulch layer, soil preparation methods (e.g. Za), improving SOM

    Tied rigdesBunds on sloping land in Burundi

  • Losses: LeachingProblematic in high rainfall areas and coarse-textured sandy soils (>35% sand)Mainly NO3- and exchangeable bases (K and Mg) percolate beyond the reach of crop roots

    Measures: Improving soil structure to promote good root development for increased accessibility of nutrients Growing annual crops in association with trees, which can pump water and nutrients from deeper layers

  • Losses: VolatilizationDenitrification of NO3- NO3- N2O and N2 (gasses) Occurs under anaerobic conditions Measures: improved soil drainage and maintain a good soil structure to avoid anaerobic growing conditions

    Volatilization of NH3 in alkaline soils (high pH)Measures: deep placement of N-fertilizers

    Volatilization of NH3 during storage and handling of manureMeasures: use anaerobic storage pits

  • SummaryPorosityCECTextureSoil organic matterNutrientsFunctionsAvailabilityMobilityDeficienciesSoil fertility management optionsConservation agricultureOrganic agricultureMimimizing losses of added nutrientsErosionLeachingvolatilization

    The major objective is to gain knowlegde on the principles that underpin the practices contained in ISFM. If you want to understand the function of soil fertility practices, youll also need to have a basic understanding on what soil is, and the processes that take place related to nutrient release for plant growth.

    Well start this lecture by giving an introduction on what soil is. Soil consists of different elements and there are different types of soils, partly determined by their texture. We will thus talk about soil texture, porosity, the functions of the mineral fraction in soil and the function of the organic matter in soil.

    Plants grow in soil and extract nutrients from the soil. Nutrient management is a key element ISFM, and to understand certain nutrient management practices, you need to understand how nutrients behave in the soil, and what their function is for plant growth. Well also discuss nutrient deficiencies in plants and how to recognize them.

    Then, when you understand the soils functions, well talk about soil fertility. Well discuss the concept of soil fertility, and give an introduction to different soil fertility management practices. Well talk in more detail about conservation agricultre and organic agriculture, and we will conclude this lecture with how to minimize losses of added nutrients to the environment. *Soil is made up of four essential elements. The largest part of the soil comprises the mineral fraction, about a quarter of the soil comprises air, another quarter is the soil sulution and a small part is organic matter.

    -click - The mineral fraction and the soil solids together form the soil solids, and the air and soil solution from the pore space. In addition to those materials soil is made from, there are also fauna and flora in the soil.

    The pore space provides the space for these fauna and flora. The pore space also provides the air for micro-organisms and stores water. The mineral fraction provides support to plant roots. Through processes as mineralization, the mineral fraction releases nutrients into the soil solution. The organic fraction exists of soil organic matter and plays a key role in soil fertility management. Well discuss the roles of pore space, mineral fraction and organic fraction in more detail in the next couple of slides.*As we discussed in the previous slide, the porosity of the soil is the volume of the soil occupied by air and the soil solution.

    -click - The soils pore space is a space for the soil solution, which is mostly water, and air. When a soil is very dry, all the pores are filled with air instead of water, and there is drought stress. In flooded soils, all pores are saturated with water. This means that roots cannot breathe and plants may die. An exception here is rice, which is adapted to growing in flooded soils. In a well drained moist soil, there is sufficient soil solution for plant growth, and also sufficient aeration for proper root function.

    *The mineral fraction comprises sand, silt, or clay, or a combination of these three. Sand, silt and clay have different particle sizes. With particles between 0.05 and 2 mm, sand has the largest particles. You can see sand particles with the naked eye. Silt particles are much smaller, between 0.002 and 0.05 mm. Clay has the smallest particles, smaller than 0.0002 mm. Youll need a microscope to see individual clay and silt particles.

    *The proportion of sand, silt and clay determines the soil texture.

    -click - In this diagram you can see how to classify soil texture based on the proportions of clay, sand and silt.

    -click - A sandy loam soil for example contains much sand, between 50 and 70%.

    -click - A silty clay loam contains mainly silt and clay. Each of the texture classes shown in the diagram has advantages and disadvantages in terms of its use in agriculture. Soils containing a large proportion of clay (so-called heavy-textured soils) are difficult to work. Soils containing a large proportion of sand are referred to as light- or coarse-textured soils and are more drought-prone than soils containing more clay.

    In the next couple of slides well discuss how you can determine the soil texture when you are in the field, the relationship between soil texture and porosity and the effect of the mineral fraction on nutrient availability. *You can determine the soil texture in the laboratory, but you can also determine soil texture in the field by rubbing a small amount of wet soil between finger and thumb. This is called the finger test. You can have a look at this method later yourself. *http://wegc203116.uni-graz.at/meted/hydro/basic/Runoff/print_version/04-soilproperties.htm

    Soil texture is a very important feature because it determines to a large extent the dynamics of water flow in the soil.

    -click - Soil texture affects the behaviour of soils in terms of PorosityWater holding capacityNutrient retention and supplyDrainageNutrient leaching

    A sandy soil with large particles will have large pores. A clay soil with very small particles has small pores. However, the total pore volume is larger in a soil with smaller particles than in a soil with larger particles. A sandy soil thus has less porosity than a clay soil.

    The infiltration of water depends on the porosity, and thus on the soil texture. In general, the vertical flow of water in soil is much higher in sandy soils than in clayey soils. In sandy soils, water infiltrates very quickly. This means that the nutrients which are contained in the percolating water may be transported below the reach of plant roots. In addition, a sandy soil has a low water holding capacity. In clay soils, infiltration rates are much smaller. This means that nutrients will not be transported out of reach of plants very quickly, but there is an increased for surface water runoff and erosion. *Cations are the positively charged ions such as potassium, K+, and ammonium NH4+. Cations are retained in the soil by binding to soil particles, either from the mineral or from the organic fraction. Not every soil can hold the same amount of cations. We have a measure that expresses the maximum amount of cations that a soil can hold. This is called the Cation Exchange Capacity.

    As you now know, clay minerals, but also organic minerals from soil organic matter have small particles. Therefore, they have a large surface area relative to their weight. In addition, some of the clay and SOM particles have a negatively charged surface. The combination of the relatively large surface area and the negatively charged surface areas means that clayey soils or soils rich in organic matter have more positions to hold cations than soils with large particles and a smaller total surface area, such as sandy soils. Because less cations can be retained in sandy soils, the cations can more easily leach out of the soils. In general, the higher the CEC, the more fertile the soil.

    The size of CEC thus depends on clay content, the type of clay mineral, the amount of SOM, and the soil pH (i.e. a measure of soil acidity, see below).The main difference between clay minerals is in their structure:2:1 clay minerals contain two silicate layers for every aluminium oxide or hydroxide layer and have a large CECcapacity (e.g. illite). 2:1 minerals are more common in fertile lowland soils (e.g. rice paddy fields).1:1 clay minerals contain one silicate layer per aluminium oxide/hydroxide layer and have low CEC capacity that is dependent on soil pH (e.g. kaolinite): if the soil is acid, the CEC is small. Most upland soils in SSA contain mainly 1:1 clay minerals that have a low and pH-dependent CEC, and their capacity to retain or supply nutrients to support crop growth is therefore inherently poor.

    Picture adapted from: http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm*However, the size of the CEC also depends on the type of clay mineral, and the soil pH Well now discuss the difference between the different types of clay minerals, but we wont go into very much detail here. The main difference between clay minerals is in their structure: there are 2:1 clay minerals and 1:1 clay minerals. -click - 1:1 clay minerals contain one silicate layer per aluminium oxide/hydroxide layer. 2:1 clay minerals contain two silicate layers for every aluminium oxide or hydroxide layer. The cations bind on the silicon sheet (green). In 1:1 clay minerals the only places for cations to bind to the clay are thus the outsides: (click).

    -click - 2:1 clay minerals contain two silicate layers for every aluminium oxide or hydroxide layer and cations can bind also in between two clay minerals (click). 2:1 clay minerals thus have a larger surface on which cations can be retained. 2:1 clay minerals are mainly found in the fertile lowland soils, for example rice paddy fields.

    The CEC of 1:1 clay minerals depends on the soil pH. If the soil is acid, the CEC is small. Most upland soils in SSA contain mainly 1:1 clay minerals that have a low and pH-dependent CEC, and their capacity to retain or supply nutrients to support crop growth is therefore inherently poor.

    Picture adapted from Structure of Clays created by Josh Lory for www.soilsurveys.org*Well now shift to the last constituent of soil: The organic matter fraction. The organic fraction in soil is often called Soil Organic Matter, or SOM. SOM consists of plant and animal residues. Often, these residues are in various stages of decomposition, ranging from freshly added crop residues or farmyard manure to soil organic materials that have been modified by biological activity to form humus.

    Organic matter has several important soil functions.

    In the first place it contains significant amounts of essential plant nutrients. Organic matter is an important source of N for plant growth, but it also contains other essential plant elements, such as P, Mg, Ca, S and micronutrients. In tropical soils, SOM content also plays a role in determining the soils CEC. Depending on the pH of the soil solution, SOM releases H+ ions. About 55% of SOM is carbon and an increase of 1 g/kg in the amount of soil organic carbon provides an additional 0.4 cmol (+)/kg of CEC (at pH 7). In addition to supplying nutrients and improving the CEC, SOM provides the following benefits: It improves the soils water-holding capacity, because it can hold up to five times its own weight in water. It improves water infiltration into the soil and therefore indirectly improves soil moisture storage and reduces surface water runoff. It functions as a buffer for soil pH. It binds with Mn and Al, thereby reducing their concentration (and toxicity) in the soil solution. It improves soil structure by stimulating activity by soil flora and fauna that produce soil aggregates and therefore indirectly reduces susceptibility to erosion.

    Organic matter thus plays many roles. Therefore, it is a key issue in soil fertility management. In most tropical soils, the concentration of organic matter in soil decreases sharply with increasing depth. A small loss of top soil, for example by erosion, therefore results in the loss of a disproportionately large amount of a soils SOM. Currently, declining contents of SOM constitute a threat to the sustainability of many agricultural systems.

    The SOM content is related to the soils clay content. Clay particles can protect SOM from decomposition and therefore help to increase the amount of SOM that accumulates in the soil. It is difficult to increase the amount of SOM in coarse-textured soils containing little clay and in soils where the clays capacity to protect SOM is already saturated. That is why ISFM places more emphasis on the replenishment of SOM. In the context of ISFM, the importance of organic materials is in their potential to improve the agronomic efficiency of fertilizer use.

    Pictures: black and white: http://www.tekura.school.nz/departments/horticulture/ht106_p4.html

    *Well now shift to the last constituent of soil: The organic matter fraction. The organic fraction in soil is often called Soil Organic Matter, or SOM. SOM consists of plant and animal residues. Often, these residues are in various stages of decomposition, ranging from freshly added crop residues or farmyard manure to soil organic materials that have been modified by biological activity to form humus.

    Organic matter has several important soil functions.

    In the first place it contains significant amounts of essential plant nutrients. Organic matter is an important source of N for plant growth, but it also contains other essential plant elements, such as P, Mg, Ca, S and micronutrients. In tropical soils, SOM content also plays a role in determining the soils CEC. Depending on the pH of the soil solution, SOM releases H+ ions. About 55% of SOM is carbon and an increase of 1 g/kg in the amount of soil organic carbon provides an additional 0.4 cmol (+)/kg of CEC (at pH 7). In addition to supplying nutrients and improving the CEC, SOM provides the following benefits: It improves the soils water-holding capacity, because it can hold up to five times its own weight in water. It improves water infiltration into the soil and therefore indirectly improves soil moisture storage and reduces surface water runoff. It functions as a buffer for soil pH. It binds with Mn and Al, thereby reducing their concentration (and toxicity) in the soil solution. It improves soil structure by stimulating activity by soil flora and fauna that produce soil aggregates and therefore indirectly reduces susceptibility to erosion.

    Organic matter thus plays many roles. Therefore, it is a key issue in soil fertility management.

    -click - In most tropical soils, the concentration of organic matter in soil decreases sharply with increasing depth. A small loss of top soil, for example by erosion, therefore results in the loss of a disproportionately large amount of a soils SOM. Currently, declining contents of SOM constitute a threat to the sustainability of many agricultural systems.

    The SOM content is related to the soils clay content. Clay particles can protect SOM from decomposition and therefore help to increase the amount of SOM that accumulates in the soil. It is difficult to increase the amount of SOM in coarse-textured soils containing little clay and in soils where the clays capacity to protect SOM is already saturated. That is why ISFM places more emphasis on the replenishment of SOM. In the context of ISFM, the importance of organic materials is in their potential to improve the agronomic efficiency of fertilizer use.

    *A soil test in a laboratory can estimate the nutrient supplying power of a soil. Laboratories require a sample of about 0.5 kg for their analyses.

    -click - Soils are normally heterogenous and wide variability can occur even in fields that are apparently uniform. Unless the field sampling procedure is implemented properly, there is a real chance that the soil analytical data will not be representative of the field. Youll see some guidelines for collecting a representative soil sample in the next slide.

    *Avoid sampling across different soil types and land uses and in distinctive spots (e.g. ash and manure piles,threshing places, wet spots).*The first part of this lecture mainly concerned the soil functions. The second part of this lecture well discuss nutrients.

    Well start with the macronutrients. In the slide you see all the macronutrients and their main function in the plant. Nitrogen is for example essential for amino acid and protein formation as well as for photosynthesis. Each nutrients has its speficic functions in the plant, but when one nutrient is not sufficient, this will also affect the functioning of the other nutrients.

    The primary nutrients important for plant growth are N, P and K. Secondary nutrients are Ca, Mg and S.

    These nutrients are all very different. Not only in their function, but also how they move in the soil and in the plant. It is imporatant to know how nutrients behave, because this affects how they should be applied to the soil. Nutrient mobility in the plant is important for recognizing nutrient deficiences.

    -click- This shows the mobility of nutrients in the soil (blue). So nitrogen is very mobile, which means it is easily leached. Phosphorus on the other hand has poor mobility. This means that phosphorus can stay in the soil for a long time when it is not taken up by plants. However, it also means that when you apply phosphorus, you should apply it there where the roots can access it.

    -click- This shows the mobility of nutrients in the plants. The mobility in the plant is not always similar to the mobility in the soil. When nutrient uptake from the soil is limited, the nutrients which are less mobile are not moved from the older leaves to support growth in the younger leaves. When nutrient are less mobile, defiency symptoms thus first appear on younger leaves. When nutrients are more mobile, it is the other way around. To support growth in younger leaves, mobile nutrients move from older to younger leaves. Nutrients deficiencies in the nutrients which are mobile in the plant are thus first visible on older leaves.

    *Here we have the micronutrients and the functions.(Up to lecturer if he/she wants to discuss in more detail or have this slide as quick reference)*You can do a soil test to determine the capacity of the soil to supply nutrients, but you can also observe plants to check for nutrient deficiencies. Nutrient deficiency symptoms are often visible on crop plants. -click - A healthy leaf (1), -click - compared with N-deficient (2), -click - P-deficient (3), -click - K-deficient (4) and -click - diseased (5) maize leaves.

    *Different nutrient deficiencies thus show different symptoms on plants. When nutrients are less mobile within plants and nutrient uptake is not sufficient, the nutrients are not transported from the older to the younger leaves. When the nutrients are more mobile in the plant, the deficiency symptoms first occur on the older leaves, because the nutrients are quickly transported to the younger leaves. This means that the deficiency symptoms first occur on the younger leaves. This is the first point on which you can start distinguishing different nutrient deficiencies.

    *Which nutrient deficiencies do we observe in the plants in the pictures? Use the diagram to diagnose the deficiencies**The nutrients which plants use often dont occur in their pure elemental form. Often nutrients are part of larger molecules. Here you see a cholorofyll molecule, which occurs in plants.

    -click - You see 4 N atoms here in the middle. When this plant material is used again as organic fertilizer, the nitrogen is not yet available. It is locked up in the larger molecules.

    -click- In this table here you see the forms in which plants take up nutrients. When nutrients are contained in larger molecules, these molecules have to be mineralized or dissolved first before plants can take up the nutrients. Organic nitrogen for example first need to be converted into ammonium to make the nitrogen available for plants. Well discuss mineralization of organic materials in more detail in the next lecture. *From the last slide we learned that nutrients are available for plant uptake only in their specific forms. Mineral fertilizers often contain the nutrients in a readily available form. These mineral fertilizers contain nutrient complexes which dissolve in the soil solution such that the nutrients become immediately available for plants. Soil organic matter which is already or easily mineralized also supplies nutrients which are readily available.

    When nutrients are locked up in their organic form, as we saw on the previous slide, they can become available though the process of mineralization. This is often a gradual process and nutrients become available slowly. Some organic materials however mineralize very fast and they can supply nutrients which are readily available. In the next lecture we will distinguish between different types or organic materials and theit capacity to supply nutrients in more detail. Although most mineral fertilizers dissolve easily in the soil solution and supply nutrients immediately, there are also mineral fertilizers that dissolve very slowly and give away their nutrients at a slow rate. Phosphate rock is one of those fertilizers which slowly releases its nutrients.

    Nutrients can also be locked up and unavaible to plants. For example when nutrients are contained in rocks, or when adsorbed on soil particles, they are not avaible to plants. Over time, rocks wheather and do release some nutrients, but we do not consider this a nutrient supply for plants.*When we need to give a definition of soil fertility, we could use this one: The capacity of soil to supply sufficient quantities and proportions of essential chemical elements (nutrients) and water required for optimal growth of specified plants as governed by the soils chemical, physical and biological attributes.

    Soil fertility thus refers to the capacity of a soil to support the production of crops. This support, or soil fertility, is not provided by the provision of nutrients weve just talked about, but also byan adequate soil volume for plant root development;water and air for root development and growth;anchorage for the resultant plant structure.We use these four attributes to describe the overall productive quality of an agricultural soil. So when we talk about soil fertility, it is also about soil quality.

    In this regard, we can also distinguish between inherent and dynamic soil quality indicators:Inherent soil quality indicators refer to those attributes of soil in its natural state that enable it to function properly and include soil texture, depth and parent material (mineralogy). While soil texture does not change over time, soil depth may be reduced as a result of erosion leading to a change in the texture of top soils. We generally adapt agricultural practices to accommodate inherent soil properties.Dynamic soil quality indicators concern those attributes dependent on how the soil is managed and include soil organic matter (SOM) content, nutrient- and water-holding capacity, and soil structure. Soil phosphorus (P) and potassium (K) stocks may be increased over time through the application of fertilizers and animal manure. Top soil texture can be regarded as a dynamic property because it is affected by erosion. These indicators change over time and are affected directly by farming practices.Because it is difficult or even impossible to manipulate inherent soil attributes in plant production, the maintenance and improvement of dynamic soil parameters is the major focus for soil management in agriculture. For example, there is scope for farmers to manage SOM and associated soil biological properties to influence the productivity of agricultural soils.

    In the next set of slides well give an overview of those management practices with which you can use control the dynamic soil parameters.

    *There are many ways in which you can alter or control the dynamic soil properties. Well quickly discuss each practice from the list on the right. This lists consist of practices to control the dynamic soil properties soil organic matter, nutrient and water holding capacity and soil structure. There are also a number of other practises that are not directly related to controlling these dyanmic soil properties, but they are part of the integrated soil fertility management approach, so well discuss them here as well.

    The first thing than you can control is the capacity of the soil to supply nutrients. Nutrient deficiences prevent a good harvest, and as we discussed, nutrient deficiences can be expressed and observed during plant growth.

    To correct nutrient deficiencies, you can use mineral fertilizers to quickly supply nutrients which plants can use. You can also use organic materials for a slower nutrient supply. When the soil does not contain sufficient micronutrients or when those micronutrient are not sufficiently available for plants you can use special fertilizer blends which contain micronutrients or manure. Manure often contains several of the micronutrients. *When a soil is very acidic, this will compromises crop growth, even when you have applied all the necessary nutrients.

    Soils can be acid, either because of inherent soil properties or because of long- term acidity-inducing management practices. The long-term use of ammonium-based fertilizer for example causes acidity. Acidity in itself is often not the major problem, unless the pH is very low (e.g.

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