Soil Fertility and Soil Fertility Management Practises

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Soil & soil fertilityAfrica Soil Health Consortium 2014Lecture 2: Introduction to soil and soil fertility


ObjectivesGain knowlegde on the principles underpinning ISFM practises Introduction to soil Soil texture Porosity Mineral fraction Organic matter Introduction to nutrients Understanding the function of nutrients in plant growth Recognizing nutrient deficiencies Soil fertility Understanding the concept of soil fertility Introduction to soil fertility management Conservation agriculture & organic agriculture Minimizing 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 roots Slowly 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 in Well-drained moist soil: sufficient moisture for plant growth and sufficient aeration for proper root function Dry soil: all pores are filled with air  drought stress Flooded 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 mm Silt: 0.002 - 0.05 mm Clay: < 0.002 mmIllustration adapted from: www.iconn.orgSiltClaySand


Mineral fraction


Mineral fractionThe finger test


Mineral fraction & PorositySoil texture affects Porosity Water holding capacity Nutrient retention and supply Drainage Nutrient 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 on Clay content Type of clay mineral SOM content Soil pH Clay minerals differ in structure 1:1 clay minerals CEC varies with soil pH Found in most upland soils in SSA 2:1 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 nutrients Improves the soil’s Cation Exchange Capacity Improves the soil’s water-holding capacity (SOM can hold up to five times its own weight in water!) Improves water infiltration Buffers soil pH Binds with toxic elements in the soil Improves soil structure by stimulating activity of soil flora and fauna Regulates 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 soil Laboratory 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-dividing Label 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 formation Photosynthesis Phosphorus (P): Energy storage/transfer Root growth Crop maturity Straw strength Disease resistance Needed in large amounts during plant growth Required for N2-fixation by legumes Potassium (K): Plant turgor pressure maintenance Accumulation and transport of the products of plant metabolism Disease resistance Required for N2-fixation by legumes Sulphur (S): Part of amino acids (protein formation) Synthesis of chlorophyll and some vitamins Required for N2-fixation by legumes Magnesium (Mg): Photosynthesis Activates enzymes Carbohydrate 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 division Required 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): Photosyntheiss Respiration Manganese (Mn): Photosynthesis Enzyme function Boron (B): Development/growth of new cells Zinc (Zn): Nucleic acid synthesis and enzyme activationCopper (Cu): Chlorophyll formation Seed formation Protein synthesis Molybdenum (Mo): Protein synthesis and N uptake N2-fixation by legumes Chlorine (Cl): Movement of water and solutes Nutrient uptake Photosynthesis Early crop maturity Disease control Cobalt (Co): N2-fixation by legumes Nickel (Ni): Required for enzyme urease Sodium (Na): Water movement and balance of minerals Silicon (Si) Cell walls Protection against piercing by sucking insects Leaf presentation Heat and drought tolerance


Nutrient deficiencyHealthyN-deficientP-deficientK-deficientDiseased


Nutrient deficiencies


Nutrient deficiency: exercise


Nutrient deficiency: exerciseP-deficient Stunted growth Purplish colouringK-deficient Browning of leaf edges


Nutrient uptake


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 soil’s chemical, physical and biological attributes. Chemical elements for plant nutrition Adequate soil volume for plant root development Water and air for root development and growth Anchorage for the plant structure


Soil fertility management practicesNutrient deficiencies prevent a good harvest Nutrient deficiencies can be expressed during plant growth Use mineral (fertilizer) or organic (manure, crop residues) to supply nutrients Use special fertilizer blends containing micronutrients or manure in case of micronutrient deficiencies Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management IntercroppingHealthyN-deficientP-deficientK-deficient


Soil fertility management practicesAcidity is caused by inherent soil properties acidity inducing management (e.g. long-term use of ammonium based fertilizer) Acid soils have high exchangeable Al (Al toxicity) Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management IntercroppingLime Increases pH Prevents Al and Mn toxicity in acidic soils (pH <5.5) Supplies Ca Increases P and Mo availability Can increase microbiological activity Apply lime to reduce exchangeable Al to +/- 15%


Soil fertility management practicesCompaction  sub-surface soil barrier to root growth Break hardpans by ploughing or chisel ploughing to 30 cm depth Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management IntercroppingIllustration adapted from: crust

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Last Updated: 8th March 2018

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