Soil Fertility Management Practices
Africa Soil Health
Consortium
2014
Lecture 3: Soil Fertility Management Practices in detail
Objectives
• Understanding how organic inputs function as a
source of nutrients
• Understanding how mineral fertilizers function as a
source of nutrients
– Efficiency
– The 4 rights for effective fertilizer use
• Understanding the role of improved germplasm in
ISFM
• Understanding the role of Biological Nitrogen
Fixation by legumes in ISFM
Organic inputs
Organics as a source of nutrients: a few
definitions and principles
• Decomposition: biochemical breakdown of dead organic
tissue into its inorganic constituent forms
• Mineralization: process of converting essential nutrient
elements from their organic forms into their inorganic forms
(or: nutrient release)
• Soil organic matter (SOM): organic materials in various stages
of decomposition
Soil Organic Matter
- Contains essential plant nutrients
- Improves the soil’s cation exchange capacity
- Improves the soil’s water-holding capacity (SOM can hold five times
its weight in water!)
- Improves water infiltration capacity
- Buffers soil pH
- Binds with toxic elements in the soil
- Improves soil structure by stimulating activity by soil flora and fauna
- Regulates the rates and amounts of nutrients released for plant
uptake
SOM as a source of N
Nitrification
Nitrifying bacteria
Immobilization
Organic N
from SOM
NH4+
(ammonium)
NO3(nitrate)
Mineralization
Fungi & Bacteria
Plants
Organic matter decomposition
Organic matter decomposition
depends on
- Quality of organic material
- C:N ratio
- Lignin and polyphenol contents
- Soil environmental conditions
- Micro- and macrofauna in the soil
Synchrony
Match between nutrient release and uptake
Organics as a source of nutrients
Supply of nutrients from SOM depends on
- Quantity and frequency of application of organic inputs
- Quality of organic inputs
- Soil type and environmental conditions, providing the environment for mineralization
C:N ratio
• N content >2.5% or C:N ratio<16
 nutrients are released in the
short term (e.g. Biomass of
legumes, composted crop
residues)
• N content <2.5% or C:N ratio> 16
 nutrients are immobilized for
prolonged periods (e.g. Straw)
Graph from: http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447040&topicorder=5&maxto=8
Organics as a source of nutrients
Supply of nutrients from SOM depends on
- Quantity and frequency of application of organic inputs
- Quality of organic inputs
- Soil type and environmental conditions, providing the environment for mineralization
C:N ratio
• N content >2.5% or C:N ratio<16
 nutrients are released in the
short term (e.g. Biomass of
legumes, composted crop
residues)
• N content <2.5% or C:N ratio> 16
 nutrients are immobilized for
prolonged periods (e.g. Straw)
Polyphenols & Lignin
• Some groups of polyphenols
bind nitrogen
• Ligning: main component of
wood, difficult to break down
• High polyphenol & ligning 
mineralization is very slow
Organics as a source of nutrients
Characteristics of organic
resource
N >2.5%
Yes
No
Lignin <15%
Polyphenols <4%
Lignin <15%
Yes
No
Yes
No
Incorporate
directly with
annual crops
Mix with fertilizer
or high quality
organic matter
Mix with fertilizer
or add to
compost
Surface apply for
erosion and water
control
Organics as a source of nutrients
Leaf colour
Green
Yellow
Leaves fibrous (do not crush)
High astringent taste
(makes your tongue dry)
Leaves crush to powder when
dry
No
Yes
No
Yes
Incorporate
directly with
annual crops
Mix with fertilizer
or high quality
organic matter
Mix with fertilizer
or add to
compost
Surface apply for
erosion and water
control
Crop residues
Using crop residues
- Return to the field to provide mulch and nutrients
- Animal feed (and return manure to the field)
- Composting (and return to the field)
- Fuel source
- Construction material (e.g. for wall or roof)
Nitrogen
Millet
Sorghum
4-10
Phosphorus 1
Potassium
15-27
Soybean
Groundnut
4-9
Maize
Rice
Dryweight (g/kg)
5-8
4-9
8-13
12-20
0-1
7-15
0-1
7-17
1-2
9-18
1-3
8-12
1-2
13-27
Exercise: How many kg soybean residues do you need to fertilize a 1 ha field
with a rate of 40 kg P/ha? Assume that the soybean residues contain 1.5 g P/kg
of residue.
Crop residue management
Millet
Sorghum
Nitrogen
4-10
4-9
Phosphorus
Potassium
1
15-27
0-1
7-15
Maize
Rice
Dryweight (g/kg)
5-8
4-9
0-1
7-17
1-2
13-27
Soybean
Groundnut
8-13
12-20
1-2
9-18
1-3
8-12
Exercise: How many kg soybean residues do you need to fertilize a 1 ha field
with a rate of 40 kg P/ha? Assume that the soybean residues contain 1.5 g P/kg
of residue.
Answer:
Soybean contains 1.5 g P/kg of residue. This means that soybean residues
contains 0.15 % P.
40 kg/0.15% = 40 kg/(0.15/100) = 26,667 kg soybean residues are needed to
supply 40 kg P.
26,667 kg residues is 27 full oxcarts!
Organic inputs: pros and cons
Advantages
• Builds up SOM (SOM has
many benefits)
• Provide essential nutrients
• Nutrients are released
slowly and provide a
continuous supply over the
cropping system and
nutrient losses are small
Disadvantages
• Low nutrient content
requires application in large
amounts
• Large quantities of organic
matter not always available
– Land required for production
of organic inputs
– Trade offs in using crop
residues
– Handling of organic inputs
requires labour
• Organic inputs can increase
disease pressure
Mineral fertilizers
• Mineral fertilizers can supply most of the essential macro- and
micro-nutrients
• Completely soluble: e.g. Urea, KCl, DAP
• Partly soluble: e.g. Rock phosphate, dolomite
• Basal (‘starter’) fertilizer:
– Contains nutrients (e.g. N, P, K, Mg) required for early stages of plant
growth, or nutrients which are not easily lost from the soil
– Is applied at planting
• Topdressing
– Often N based fertilizer
– Second application, later in the season
Top dressing young maize plants with N fertilizer
N fertilizers
• Atmospheric N2  NH3
– Legumes
– Industrial: Haber-Bosch process
• Nitrate (NO3-) and ammonium (NH4-)
– Directly available for plants
• Nitrification: NH3  NO3- + H+
– NO3- is easily leached
– Nitrification causes acidification through release of H+
• Types of N-fertilizer
– Urea, compound NPKs, anhydrous ammonia, calcium ammonium
nitrate (CAN), ammonium nitrate, ammonium sulfate
P fertilizers
Phosphate Rock
Process
Needs to be ground
into fine powder
P2O5 content +/-32%
Solubility
Sparingly soluble
Manufactured P fertilizers
PR is reacted with sulfuric or
phosphoric acid
- Triple superphosphate (TSP):
46%
- Single superphosphate (SSP):
20%
Fully soluble
Mined Phosphate or
Phosphate Rock (PR)
from sedimentary or
igneous origin
P availability - Slow release
Directly available
- Soil pH needs to be
<5.5 to start a
reaction with PR
Suitability
- Tree crops
Annual crops
- Remineralization of
degraded fields
Picture from: http://www.rsmm.com/miningphos.htm
K fertilizers
• Manufactured from large deposits of water-soluble K
minerals
• The salts commonly also contain Mg and S
• K-fertilizers
– Potassium chloride (KCl): 60% K2O
– Potassium sulfate (K2SO4): 50% K2O
Finite resources
- Reserves of raw materials for P and K fertilizers
are finite
- Potash reserves: sufficient for 250 years
- Phosphate reserves: sufficient for 300-400 years
Picture: Potash evaporation ponds, Utah. http://en.wikipedia.org/wiki/Potash
Multinutrient fertilizers
More expensive
• Complex multinutrient fertilizers
– Used in horticulture
• Compound fertilizers
– Mixing single nutrient fertilizers  slurry 
granulated product
• Bulk blend fertilizers
– Mixing different ‘dry’ fertilizers to achieve a
specific nutrient composition
Cheaper
Fertilizer: excercise
A farmer wants to compare the cost of applying nutrients in the
form of a compound (17-17-17) with straight fertilizers (urea, TSP
and KCl).
Calculate the difference in cost between 1 bag of compound NPK
fertilizer and applying the same amounts of nutrients from Urea, TSP
and KCL.
Nutrient content (%)
N
Compound 17-17-17
Urea (46% N)
TSP (46% P2O5)
KCl (60% K2O)
17
46
–
–
P2O5
17
–
46
–
Price ($/50 kg bag)
K2O
17
–
–
60
32
17
22
30
Fertilizer: excercise worked out
Step 1
•
50 kg bag of NPK (17-17-17) contains
– 17% of 50 kg = 8.5 kg N
– 17% of 50 kg = 8.5 kg P2O5
– 17% of 50 kg = 8.5 kg K2O
Step 2
•
For 8.5 kg N you need 8.5/0.46 = 18.5 kg urea
•
For 8.5 kg P2O5 you need 8.5/0.46 = 18.5 kg TSP
•
For 8.5 kg K2O you need 8.5/0.6 = 14.2 kg KCl
Step 3
•
18.5 kg urea costs (18.5/50) * $17 = $6.3
•
18.5 kg TSP costs (18.5/50) * $22 = $8.1
•
14.2 kg KCl costs (14.2/50) * $30 = $8.5
Step 4
•
Total costs of straight fertilizers = $6.3 + $8.1 + $8.5 = $22.9
•
One bag of compound 17-17-17 costs $32
•
Farmer saves $32 - $22.9 = $9.1 by using a combination of straight fertilizers
Mixing straight Fertilizers
Fertilizer use efficiency
• Nutrients applied to the soil are
– Taken up by crops
– Retained in the soil as nutrient stocks
– Lost from the soil through leaching or volatilization
• Agronomic efficiency (AE): The amount of additional yield
obtained per kg nutrient applied
• AE depends on
– Recovery fraction (how much of the applied nutrients is
taken up by the crop?)
– Internal use efficiency (how much additional yield per kg of
nutrient taken up by the crop?)
Recovery fraction(RF)
Additional uptake
•
RF (X): recovery fraction of applied nutrient X
– units: kg X uptake/kg X applied
•
X_uptake_F: plant X uptake at harvest when nutrient X is applied
– Units: kg X uptake/ha
•
X_uptake_C: plant X uptake at harvest without nutrient X applied
– Units: kg X uptake/ha
•
X_applied: Rate of X applied
– Units: kg X/ha
Internal use efficiency
IE (X): internal efficiency of nutrient X
– Units: kg crop product/kg X uptake
Y_F: Yield with nutrient X
–
Units: kg/ha
Y_C: Yield without nutrient X
–
Units: kg/ha
Additional yield
Additional uptake
Agronomic efficiency (AE)
Economic benefits of improving agronomic efficiency of fertilizer:
- Larger yield increases with a given quantity of fertilizer
- Less fertilizer is required to achieve a particular yield target
Agronomic efficiency: exercise
Two farmers apply 50 kg N/ha fertilizer. Their fields vary in soil fertility,
affecting crop yields and fertilizer use efficieny. Calculate the agronomic N
efficiency for each farmer.
Farmer 1
Farmer 2
Field history: Degraded field, cultivated
for many years without application of
fertilizer or manure.
Field history: Fertile, but N-deficient
field that received moderate rates of
manure in the past.
Yield without N application: 400 kg/ha
Yield with 50 kg N/ha: 900 kg/ha
Yield without N application: 2000 kg/ha
Yield with 50 kg N/ha: 4500 kg/ha
Agronomic efficiency: exercise worked out
Farmer 1
Farmer 2
Field history: Degraded field, cultivated
for many years without application of
fertilizer or manure.
Field history: Fertile, but N-deficient
field that received moderate rates of
manure in the past.
Yield without N application: 400 kg/ha
Yield with 50 kg N/ha: 900 kg/ha
Yield without N application: 2000 kg/ha
Yield with 50 kg N/ha: 4500 kg/ha
Agronomic N use efficiency = (900-400)
/ 50 = 10 kg grain/kg N
Agronomic N use efficiency = (45002000) / 50 = 50 kg grain/kg N
The ‘4Rs’ for effective fertilizer use
1.
2.
3.
4.
Apply the right source of nutrient
At the right rate
At the right time
At the right place
... To meet crop demand
R1: Right fertilizer product
Matching the fertilizer source and products to the crop’s needs
and soil’s properties
•Straight fertilizers vs. compound fertilizers
•Balanced fertilization (interactions between nutrients)
•Choice of fertilizer depends on crop, current and past use of
manure, soil properties and climate conditions
•Methods to indentify which nutrients should be applied: soil
analysis, nutrient omission trials, nutrient deficiencies on crops
•Avoid depleting nutrient stocks on the longer term
•Use good quality fertilizer
R2: Right fertilizer rate
Matching the amount of fertilizer to the
crop’s needs
Take into account:
•Nutrient requirements of the crop
•The soil’s capacity to supply nutrients
•The amount of nutrients applied in organic inputs
•The amount of nutrients applied to previous crops
•Target yield
– attainable yield under local climatic conditions
– Farmers’ goals
•Costs of fertilizers and value of crop products
•Fertilizer responses
R2: Right fertilizer rate – fertilizer responses
Fertilizer responses
• Large/poor/very poor responses, related to
initial soil fertility
Approaches to address these
• Cash constrained farmers can priotize fertilizer
use in most responsive fields only
• Additional application of organic resources or
other soil amendments or management
practice might be required on otherwise nonresponsive soils
• Applying small doses of fertilizer on fertile
fields can sustain fertility in the longer term
R3: Right time for fertilizer application
Create synchrony: Make nutrients available when crops need them
•Basal fertilizer application: N, P, K, and other nutrients required for
early crop growth are applied at or just after planting
•Top dressing: Fertilizer N is highly mobile  apply N in several split
applications at key stages during crop development
•Leaf colour charts or chlorophyll meters
to determine the crop’s N demand
•If the crop develops poorly due to e.g. low
rainfall, top dressing can be cancelled
•Slow-release N fertilizers and deep placement of fertilizer N improve
synchrony
R4: Right placement of (basal) fertilizer
Apply fertilizer there where the crop can access the nutrients
• Broadcasting
– Low labour requirements, often used for top dressing
• Banding
– Fertilizer is placed at 5-8 cm depth and covered with soil.
Seeds are planted on top. Use for basal application.
• Spot application
– Small amounts of fertilizer are placed in or close to planting hills. Preferred where
plants are widely spaced and where soil and climatic conditions increase the
chance for nutrient losses through leaching)
• Deep placement
– Slow-release N fertilizers are placed in the soil in flooded fields
R4: Right placement of (basal) fertilizer
Fertilizer
Seeds
Seeds
Fertilizer
A 5th right of fertilizer use in SSA
Make sure that scarce fertilizer resources are delivered to the
part of the cropping system that delivers the maximum
economic benefit to the farmer
• Identify the part of the cropping system where fertilizer
inputs will deliver the greatest return
• Consider the whole cropping when planning fertilizer use
– Maize/legume rotation: legumes may benefit from residual P
applied to maize.
– Maize/legume intercropping: apply N only to maize because
legumes can meet their N requirements by biological N2
fixation.
Soil amendments
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/processes
Liming requirements
• Depend on soil acidity level and Al3+
tolerance of crops
• Strongly acidic clay soils need more
lime than weakly buffered sandy soils
Liming materials
• Limestone or calcium carbonate
(CaCO3)
• Other materials are expressed in
calcium carbonate equivalent (CCE)
• Limestone: dolomitic/calcitic
Gypsum
• CaSO4.2H2O
• Rehabilitates sodic soils
• Supplies Ca
• Occurs as a natural deposit in (semi-)
arid regions
• Sparingly soluble in water
Improved germplasm
Seeds, seedlings and other planting materials that have been bred to meet particular
requirements of the environment in which they are to be grown
Local susceptible variety
Improved variety Nsansi
Yield = G (genotype) x E (environment) x M (management)
Improved germplasm
Yield from unfertilized
BH540 was slightly
higher than fertilized
local varieties
Yields more than
doubled when both
fertilizer and improved
germplasm was used
Improved germplasm
Seeds, seedlings and other planting materials that have been bred to meet particular
requirements of the environment in which they are to be grown
Genetic yield potential
- When grown in the targeted
environment
- Greater harvest index (HI)
- Additional traits (e.g. rice with high
vitamin A content or high-quality
protein maize)
- Adapted/tolerant to environmental
stresses such Al toxicity or drought
Pest and disease resistance
- Higher yields
- Healthy plants give higher returns
on used nutrient inputs
- Disease free planting materials
- GMO planting material
Nutrient use efficiency
- Higher HI  higher agronomic
efficiency
- More extensive or deeper root
system to capture more nutrients
Yield = G (genotype) x E (environment) x M (management)
Availability of improved germplasm
• Knowledge & information
– Available varieties for a particular region
– Places where they can be purchased
– Prices
• Ability to obtain
– Agrodealers or other input supply networks
– Local, community-based seed mutiplication
– Continuity of supply
• Quality of material
–
–
–
–
Purity
Free of diseases and pests
Uniform in size
High viability
Picture: Taskcape, UK
Biological nitrogen fixation by legumes
Atmosphere: 79% nitrogen (N2, gas)
Rhizobia interact with
the legume roots and
form nodules. Rhizobia
transform N2 gas into
mineral NH3+.
Picture: N2Africa
Legumes
•
•
•
•
Food
Fodder
Fuelwood and poles
Soil fertility
• Multi-purpose legumes
Dual purpose grain legumes
Components of successful BNF
(GL x GR) x E x M
•
•
•
•
GL: legume genotype
GR: rhizobium genotype (strain)
E: environment (climate and soils)
M: agronomic management
Opportunities
- Legumes benefit from residual basal fertilizer (especially P) applied
to cereals when grown in rotation or as intercrop
- Use of direct application of fertilizers on legumes when there are
good market opportunities
GL x GR
Promiscuous
Cowpea
Specific
Groundnut
Common bean
(GL x GR) x E x M
Soybean
Chickpea
GL x GR
Inoculation
Inoculation = applying rhizobia to the seed
Legumes need inoculation when
• The soil does not contain compatible
rhizobia
• The population of compatible rhizobia is small
• Indigenous rhizobia are less effective in fixing N2
compared with selected inoculant strains
Inoculants are very cost effective compared to mineral
fertilizer!
Inoculation
Be aware! Inoculants
contain living rhizobia
which die when:
- Exposed to sunlight
- Exposed to high
temperatures
- Stored in an open
package
Legume contributions to soil fertility
Contribution to soil fertility: Amount of N2 fixed in relation to
amount of N taken away with harvest.
Green manures and tree legumes
The greater the biomass, the
larger the inputs from N2 fixation. • Low uptake, despite extensive
research and development
• Tree legumes: max 600 kg
• >2t/ha dry matter green manure
N/ha/year
gives 1 t/ha additional grain yield
• Grain legumes/green manures:
in following cereal crop
max 300 kg N/ha/season
•  High labour and land
requirements!
• N inputs from grain legumes
depends on residue management •  A niche opportunity
Arbuscular Mycorrhizal Fungi (AMF)
• Many plants naturally form symbioses with AMF
• AMF can also be prepared as commercial
products and used as inoculant
• Benefits of symbiosis with AMF
– Enhance nutrient
and water uptake
– Reduce pest and
disease damage
– Improve soil structure
Summary
Organic inputs
- Decomposition
- Crop residues
- Pros and cons
Improved germplasm
- Benefits
- Availability
- Quality
Soil amendments
- Lime
- Gypsum
AMF
- Symbiosis
between plants and
mycorrhizal fungi
Mineral fertilizers
- Single nutrient
- Mutiple nutrient
- Solubility
- Fertilizer use
efficiency (AE)
- The 4 rights
BNF by legumes
- Legume-rhizobia
symbiosis
- Promiscuous and
specific legumes
- Inoculation