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Soil & soil fertility
Africa Soil Health Consortium
2014
Lecture 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
Soil
Soil s
olids
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
Pore space
Porosity: 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.
Soil particle
Water film
Air space
mm0 0.5 1.0 1.5 2.0 2.5
Mineral fraction
Sand: 0.05 - 2.0 mmSilt: 0.002 - 0.05 mmClay: < 0.002 mm
Illustration adapted from: www.iconn.org
SiltClay
mm0 1 2 3 4 5
Sand
Clay % Silt %
Sand %
Mineral fraction
Mineral fractionThe finger test
Mineral fraction & PorositySoil texture affects-Porosity-Water holding capacity-Nutrient retention and supply-Drainage-Nutrient leaching
Illustrations adapted from: http://wegc203116.uni-graz.at/meted/hydro/basic/Runoff/print_version/04-soilproperties.htm
Pore Space in Sandy Soil vs. Clay Soil
Sandy soil Clay soil
Larger pores Smaller
poresLess total pore
volume =
Less porosity
Greater total pore volume
=Greater porosity
Infiltration Variations by Soil Texture
Sand Silt Clay
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
H+
Ca2+
Mg2+
NH4+
Na+
K+
H+K+H+H+
Sand
Clay
Clay – Many positions to hold cations
Sand– Few positions to hold cations
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’ 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: SOM
- Contains 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: http://www.tekura.school.nz/departments/horticulture/ht106_p4.html
Top soil
Sub soil
% Organic matterLitter layer 4321 5
Organic matter
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!
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-dividing7.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.
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 mobility
Very mobile
Very mobile
Very mobile
Very mobile
Quite poor mobility
Very
mob
ile
Very mobile
Poor
mob
ility
Qui
te m
obile
Quite
poo
r mob
ility
Med
ium
mob
ility
NutrientsMicronutrients: 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-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 deficiency
Healthy
N-deficient
P-deficient
K-deficient
Diseased
Nutrient deficiencies
Nutrient deficiency: exercise
Nutrient deficiency: exercise
P-deficient-Stunted growth-Purplish colouring
K-deficient-Browning of leaf edges
Nutrient uptake Nutrient Plants take upN NO3
-, NH4+
P H2PO4- , HPO4
2-
K K+
S SO42-
Mg Mg2+
Ca Ca2+
Fe Fe2+ and Fe3+
Mn Mn2+ and Mn3+
B (BO3)3-
Zn Zn2+
Cu Cu2+
Mo Mo42+
Cl Cl-
Co Co2+
Ni Ni2+
Na Na+
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 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
Inherent Dynamic
Soil texture Soil organic matter (SOM)
Depth Nutrient- and water-holding capacity
Parent material Soil structure
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
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 acidity correction
Breaking hardpans
Water harvesting
Erosion control
Land preparation
Planting date
Spacing
Planting practices
Weeding
Pest and disease management
Intercropping
Lime•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 practices
• Compaction 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
Intercropping
Illustration adapted from: http://locallygerminated.wordpress.com/
Porous soil allows good root development
Sub-surface barrier to roots
Surface crust
Soil fertility management practices
• Capture more rainfall in areas that are prone to drought– Harvesting additional water (e.g. Zaï)– Promoting infiltration by coversing the soil surface with mulch
• Labour intensive
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
Zaï pits in Niger Mulching of bananas, western UgandaPictures: fao.org
Soil fertility management practices
• Prone to erosion: fields on steep slopes, or on gentle slopes with course-textured top soil
• Measures: live barriers (e.g. grass strips), teracces, surface mulch
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
Bunds on sloping land in Burundi
Soil fertility management practices
• Good seedbed preparation improves germination and reduces the chance for diseases
• A delay in planting date often affects yield negatively• Planting time is important especially when the growing
season is short
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
Soil fertility management practices
• Crops compete for nutrients, water and light• Use 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 deficiencies
Soil acidity correction
Breaking hardpans
Water harvesting
Erosion control
Land preparation
Planting date
Spacing
Planting practices
Weeding
Pest and disease management
Intercropping
Crop Optimal rainfall Poor rainfallDensity Between
rowsWithin rows
Density Between rows
Within rows
‘000 Plants/ha
cm cm ‘000 Plants/ha
cm Cm
Beans (common) 200 50 10 133 50 15
Maize 44 75 30 37 90 30
Soybean 444 45 5 333 60 5
Soil fertility management practices
• Use viable seed (at least 80% germination)• Plant seeds at the correct depth and insert cuttings at correct angle• Plant more seeds than required for optimal plant density.
• Weeds compete with crops for nutrients, water and light.• Timely removal of weeds is essential• Weed before top dressing crop with fertilizer
• Control pests and diseases at specific growth stages
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
Delayed weeding reduces the crop response to fertilizer
Soil fertility management practices
• Intercropping 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 deficiencies
Soil acidity correction
Breaking hardpans
Water harvesting
Erosion control
Land preparation
Planting date
Spacing
Planting practices
Weeding
Pest and disease management
Intercropping
Maize-pigeonpea Maize-cassava
Cassava-soybean
Conservation agriculture (CA)Basic principles1.Soil disturbance is minimized by reduced or zero-tillage2.Use of at least 30% soil cover (mulch or cover crops)3.Use of crop rotations/associations
Advantages-Rapid planting of large areas-Reduction of soil erosion
Pitfalls-Competing uses of crop residues needed for mulch-Yields may decrease on the short-term (the increase often comes on the longer-term)-Increased weed pressure caused by reduced tillage-Full CA requires a fundamental change in the farming system. This may not be practical or enomic for the farmer-Possible decrease in agronomic efficiency of fertilizer use
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 SSA•Soil nutrients stocks in large parts of SSA have already become depleted and require replenishment•Organic resources are not available in large enough quantities to replenish and sustain nutrient stocks in the soil•Large and economic responses to mineral fertilizer are obtained in many parts of SSA•Organic 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 nutrients
Losses of nutrients into the environment•Depletion of nutrients in farming systems• Eutrophication in case of excessive mineral fertilizer use (not common in
SSA)
Losses through•Harvesting crops recycling•Water and wind erosion•Leaching•Volatilization
Nitrogen is the most susceptible to losses•Very mobile, can be lost through different ways•NO3- is susceptible to leaching.
Losses: Water and wind erosion
10 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. Zaï), improving SOM
Tied rigdes
Bunds on sloping land in Burundi
Losses: Leaching
• Problematic 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
Summary
Porosity
CEC
Texture
Soil organic matter
Nutrients-Functions-Availability-Mobility-Deficiencies
Soil fertility management options
Conservation agriculture
Organic agriculture
Mimimizing losses of added nutrients-Erosion-Leaching-volatilization