soil & soil fertility africa soil health consortium 2014 lecture 2: introduction to soil and soil...

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  • Slide 1
  • Soil & soil fertility Africa Soil Health Consortium 2014 Lecture 2: Introduction to soil and soil fertility
  • Slide 2
  • Objectives Gain 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
  • Slide 3
  • Soil Soil solids Pore space + soil fauna and flora Pore space: -space for roots and micro-organisms -air for micro-organisms -water storage Mineral fraction: -Provides support to plant roots -Slowly releases nutrients into the soil solution Organic fraction: -Soil organic matter (SOM) -Key issue in soil fertility management
  • Slide 4
  • Pore space Porosity: 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. Soil particle Water film Air space mm 0 0.5 1.0
  • Slide 5
  • Mineral fraction Sand: 0.05 - 2.0 mm Silt: 0.002 - 0.05 mm Clay: < 0.002 mm Illustration adapted from: Silt Clay mm 0 12 3 45 Sand
  • Slide 6
  • Clay % Silt % Sand % Mineral fraction
  • Slide 7
  • The finger test
  • Slide 8
  • Mineral fraction & Porosity Soil texture affects -Porosity -Water holding capacity -Nutrient retention and supply -Drainage -Nutrient leaching Illustrations adapted from: Pore Space in Sandy Soil vs. Clay Soil Sandy soil Clay soil Larger pores Smaller pores Less total pore volume = Less porosity Greater total pore volume = Greater porosity Infiltration Variations by Soil Texture Sand Silt Clay
  • Slide 9
  • Mineral fraction & CEC Cations: positively charged ions (e.g. K +, NH 4 + ) 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: H+H+ Ca 2+ Mg 2+ NH 4 + Na + K+K+ H+H+ K+K+ H+H+ H+H+ Sand Clay Clay Many positions to hold cations Sand Few positions to hold cations
  • Slide 10
  • Mineral fraction & CEC CEC 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
  • Slide 11
  • Organic fraction: SOM SOM: plant and animal residues, in various stages of decompisition Picture:
  • Slide 12
  • Organic fraction: SOM -Contains essential plant nutrients -Improves 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 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: Top soil Sub soil % Organic matter Litter layer 4 3 2 1 5 Organic matter
  • Slide 13
  • Soil analysis Soil 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!
  • Slide 14
  • Guidelines for soil sampling Take a representative sample!!! 1.Check the area to be sampled for notable features (e.g. slope, soil types, vegetation, drainage). 2.Draw a sketch map, and identify and mark the location of sampling sites. 3.Take soil samples with a soil auger at the sampling depth (0-20 cm or 20- 40 cm). 4.Take 10-35 sub-samples per site, the number depending on the size and heterogeneity of the field. 5.Combine the sub-samples to one composite per site and mix thoroughly. 6.If necessary, reduce sample weight by sub-dividing 7.Label the sample of soil properly. 8.Air-dry the sample and when dry, store it, properly labelled, in a plastic bag or a glass bottle for further analyses.
  • Slide 15
  • Nutrients Macronutrients: 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 N 2 -fixation by legumes Potassium (K): -Plant turgor pressure maintenance -Accumulation and transport of the products of plant metabolism -Disease resistance -Required for N 2 -fixation by legumes Sulphur (S): -Part of amino acids (protein formation) -Synthesis of chlorophyll and some vitamins -Required for N 2 -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 mobility Very mobile Quite poor mobility Very mobile Poor mobility Quite mobile Quite poor mobility Medium mobility
  • Slide 16
  • 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 activation Copper (Cu): -Chlorophyll formation -Seed formation -Protein synthesis Molybdenum (Mo): -Protein synthesis and N uptake -N 2 -fixation by legumes Chlorine (Cl): -Movement of water and solutes -Nutrient uptake -Photosynthesis -Early crop maturity -Disease control Cobalt (Co): -N 2 -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
  • Slide 17
  • Nutrient deficiency Healthy N-deficient P-deficient K-deficient Diseased
  • Slide 18
  • Nutrient deficiencies
  • Slide 19
  • Nutrient deficiency: exercise
  • Slide 20
  • P-deficient -Stunted growth -Purplish colouring K-deficient -Browning of leaf edges
  • Slide 21
  • Nutrient uptake NutrientPlants take up NNO 3 -, NH 4 + PH 2 PO 4 -, HPO 4 2- KK+K+ SSO 4 2- MgMg 2+ CaCa 2+ FeFe 2+ and Fe 3+ MnMn 2+ and Mn 3+ B(BO 3 ) 3- ZnZn 2+ CuCu 2+ MoMo 4 2+ ClCl - CoCo 2+ NiNi 2+ NaNa + Si(SiO 4 ) 4-
  • Slide 22
  • Nutrient availability Readily 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
  • Slide 23
  • Soil fertility 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. 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 InherentDynamic Soil textureSoil organic matter (SOM) DepthNutrient- and water-holding capacity Parent materialSoil structure
  • Slide 24
  • Soil fertility management practices Nutrient 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 Intercropping Healthy N- deficient P- deficient K- deficient
  • Slide 25
  • Soil fertility management practices Acidity 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 acid


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