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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
– 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
Organic fraction:
- Soil organic matter (SOM)
- Key issue in soil fertility management
Mineral fraction:
- Provides support to plant roots
- Slowly releases nutrients into the
soil solution
Soil
Pore space:
-space for roots and
micro-organisms
-air for micro-organisms
-water storage
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
0 0.5 1.0 1.5 2.0 2.5
Water film
mm
Soil particle
Air space
Illustration adapted from Brady 1984, The nature and properties of soils, 9th edition.
Mineral fraction
Sand
Silt
0
1
2
3
mm
Clay
4
5
Sand: 0.05 - 2.0 mm
Silt: 0.002 - 0.05 mm
Clay: < 0.002 mm
Illustration adapted from: www.iconn.org
Mineral fraction
Silt %
Clay %
Sand %
The
finger
test
Mineral fraction
Mineral fraction & Porosity
Pore Space in Sandy Soil vs. Clay Soil
Sandy soil
Larger
pores
Less total pore
volume
=
Less porosity
Clay soil
Smaller
pores
Soil texture affects
- Porosity
- Water holding capacity
- Nutrient retention and supply
- Drainage
- Nutrient leaching
Infiltration Variations by Soil Texture
Greater total pore
volume
=
Greater porosity
Sand
Silt
Clay
Illustrations adapted from: http://wegc203116.uni-graz.at/meted/hydro/basic/Runoff/print_version/04-soilproperties.htm
Mineral fraction & CEC
Cations: 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
Clay – Many positions
to hold cations
H+
Sand– Few positions
to hold cations
Ca2+
Mg2+
Sand
NH4+
Clay
Na+
K+
H+
H+
K+
H+
Illistration adapted from: http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm
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’ www.soilsurveys.org
Organic fraction: SOM
SOM: 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
% Organic matter
Litter layer
1
2
3
4
Top soil
Sub soil
Organic matter
 SOM is a key issue in soil fertility
management!
Illustration adapted from: http://www.tekura.school.nz/departments/horticulture/ht106_p4.html
5
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.
2.
3.
4.
5.
6.
7.
8.
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 2040 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.
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 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
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
- 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
N
P
K
S
Plants take up
NO3-, NH4+
H2PO4- , HPO42K+
SO42-
Mg
Ca
Fe
Mn
B
Zn
Cu
Mo
Cl
Co
Ni
Na
Si
Mg2+
Ca2+
Fe2+ and Fe3+
Mn2+ and Mn3+
(BO3)3Zn2+
Cu2+
Mo42+
ClCo2+
Ni2+
Na+
(SiO4)4-
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
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 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
Correcting nutrient
deficiencies
Soil acidity correction
•
•
Use mineral (fertilizer) or organic (manure, crop residues) to
supply nutrients
Use special fertilizer blends containing micronutrients or manure
in case of micronutrient deficiencies
Healthy
Ndeficient
Breaking hardpans
Water harvesting
Erosion control
Land preparation
Planting date
Spacing
Planting practices
Weeding
Pdeficient
Kdeficient
Pest and disease
management
Intercropping
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
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
Planting date
Spacing
Planting practices
Weeding
Pest and disease
management
Intercropping
•
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
Porous soil allows
good root
development
Erosion control
Surface crust
Land preparation
Planting date
Spacing
Planting practices
Sub-surface Weeding
barrier to
Pest and disease
roots
management
Intercropping
Illustration adapted from: http://locallygerminated.wordpress.com/
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
Pictures: fao.org
Mulching of bananas, western Uganda
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
Correcting nutrient
deficiencies
Soil acidity correction
Breaking hardpans
Water harvesting
• A delay in planting date often affects yield negatively
• Planting time is important especially when the growing
season is short
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.
Crop
Optimal rainfall
Correcting nutrient
deficiencies
Soil acidity correction
Breaking hardpans
Water harvesting
Erosion control
Land preparation
Poor rainfall
Planting date
Density
Between
rows
Within
rows
Density
Between
rows
Within
rows
‘000
Plants/ha
cm
cm
‘000
Plants/ha
cm
Cm
Beans (common)
200
50
10
133
50
15
Pest and disease
management
Maize
44
75
30
37
90
30
Intercropping
Soybean
444
45
5
333
60
5
Spacing
Planting practices
Weeding
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.
Correcting nutrient
deficiencies
Soil acidity correction
Breaking hardpans
Water harvesting
•
•
•
•
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
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
Maize-cassava
Maize-pigeonpea
Correcting nutrient
deficiencies
Soil acidity correction
Breaking hardpans
Water harvesting
Erosion control
Land preparation
Planting date
Spacing
Planting practices
Weeding
Cassava-soybean
Pest and disease
management
Intercropping
Conservation agriculture (CA)
Basic principles
1. Soil disturbance is minimized by reduced or zero-tillage
2. 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 longerterm)
- 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 agriculture
Reliance 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: Volatilization
Denitrification 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 manure
Measures: use anaerobic storage pits
Summary
Soil organic matter
Soil fertility management
options
CEC
Porosity
Texture
Conservation
agriculture
Organic
agriculture
Mimimizing
losses of added
nutrients
- Erosion
- Leaching
- volatilization
Nutrients
- Functions
- Availability
- Mobility
- Deficiencies
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