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


  • 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: 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:

  • 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

  • Organic fraction: SOMSOM: plant and animal residues, in various stages of decompisition Picture:

  • 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:

  • 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: 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:

    Lydia Wairegi - revised this from


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