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
Organic N from SOM
NH4+
(ammonium)NO3
- (nitrate)
Plants
MineralizationFungi & Bacteria
NitrificationNitrifying bacteria
Immobilization
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
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
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
Organics as a source of nutrients
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
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
Organics as a source of nutrientsCharacteristics of organic
resource
N >2.5%
Yes No
Lignin <15%Polyphenols <4%
Lignin <15%
Yes YesNo 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 nutrientsLeaf colour
Green
Leaves fibrous (do not crush)High astringent taste
(makes your tongue dry)
No NoYes 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
Yellow
Leaves crush to powder when dry
Crop residues
Millet Sorghum Maize Rice Soybean GroundnutDryweight (g/kg)
Nitrogen 4-10 4-9 5-8 4-9 8-13 12-20
Phosphorus 1 0-1 0-1 1-2 1-2 1-3Potassium 15-27 7-15 7-17 13-27 9-18 8-12
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)
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 managementMillet Sorghum Maize Rice Soybean Groundnut
Dryweight (g/kg)Nitrogen 4-10 4-9 5-8 4-9 8-13 12-20
Phosphorus 1 0-1 0-1 1-2 1-2 1-3Potassium 15-27 7-15 7-17 13-27 9-18 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 consAdvantages•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
Mined Phosphate or Phosphate Rock (PR) from sedimentary or igneous origin
Phosphate Rock Manufactured P fertilizersProcess Needs to be ground
into fine powderPR is reacted with sulfuric or phosphoric acid
P2O5 content +/-32% - Triple superphosphate (TSP): 46%- Single superphosphate (SSP): 20%
Solubility Sparingly soluble Fully soluble
P availability - Slow release- Soil pH needs to be <5.5 to start a reaction with PR
Directly available
Suitability - Tree crops- Remineralization of degraded fields
Annual crops
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
Picture: Potash evaporation ponds, Utah. http://en.wikipedia.org/wiki/Potash
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
Multinutrient fertilizers
• 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
More expensive
Cheaper
Fertilizer: excerciseA 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 (%) Price ($/50 kg bag)
N P2O5 K2O Compound 17-17-17 17 17 17 32Urea (46% N) 46 – – 17TSP (46% P2O5) – 46 – 22
KCl (60% K2O) – – 60 30
Fertilizer: excercise worked outStep 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?)
Minimize
Maximize
Recovery fraction(RF)
• 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
Additional uptake
Internal use efficiencyIE (X): internal efficiency of nutrient X
– Units: kg crop product/kg X uptakeY_F: Yield with nutrient X
– Units: kg/haY_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
Field history: Degraded field, cultivated for many years without application of fertilizer or manure.
Yield without N application: 400 kg/haYield with 50 kg N/ha: 900 kg/ha
Farmer 2
Field history: Fertile, but N-deficient field that received moderate rates of manure in the past.
Yield without N application: 2000 kg/haYield with 50 kg N/ha: 4500 kg/ha
Agronomic efficiency: exercise worked outFarmer 1
Field history: Degraded field, cultivated for many years without application of fertilizer or manure.
Yield without N application: 400 kg/haYield with 50 kg N/ha: 900 kg/ha
Agronomic N use efficiency = (900-400) / 50 = 10 kg grain/kg N
Farmer 2
Field history: Fertile, but N-deficient field that received moderate rates of manure in the past.
Yield without N application: 2000 kg/haYield with 50 kg N/ha: 4500 kg/ha
Agronomic N use efficiency = (4500-2000) / 50 = 50 kg grain/kg N
The ‘4Rs’ for effective fertilizer use
1. Apply the right source of nutrient2. At the right rate3. At the right time4. 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 rateMatching 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 responsesFertilizer 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 non-responsive 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
Seeds
Fertilizer
Fertilizer
Seeds
A 5th right of fertilizer use in SSAMake 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 amendmentsLime•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 materials•Limestone or calcium carbonate (CaCO3)•Other materials are expressed in calcium carbonate equivalent (CCE)•Limestone: dolomitic/calcitic
Liming requirements•Depend on soil acidity level and Al3+ tolerance of crops•Strongly acidic clay soils need more lime than weakly buffered sandy soils
Gypsum•CaSO4.2H2O•Rehabilitates sodic soils •Supplies Ca•Occurs as a natural deposit in (semi-) arid regions•Sparingly soluble in water
Improved germplasmSeeds, seedlings and other planting materials that have been bred to meet particular requirements of the environment in which they are to be grown
Yield = G (genotype) x E (environment) x M (management)
Local susceptible variety Improved variety Nsansi
Yields more than doubled when both fertilizer and improved germplasm was used
Yield from unfertilized BH540 was slightly higher than fertilized local varieties
Improved germplasm
Improved germplasm
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
Seeds, seedlings and other planting materials that have been bred to meet particular requirements of the environment in which they are to be grown
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
Overriding!
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
(GL x GR) x E x M
Promiscuous Specific
Cowpea SoybeanChickpea
Groundnut Common bean
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
The greater the biomass, the larger the inputs from N2 fixation.•Tree legumes: max 600 kg N/ha/year•Grain legumes/green manures: max 300 kg N/ha/season •N inputs from grain legumes depends on residue management
Contribution to soil fertility: Amount of N2 fixed in relation to amount of N taken away with harvest.
Green manures and tree legumes•Low uptake, despite extensive research and development•>2t/ha dry matter green manure gives 1 t/ha additional grain yield in following cereal crop• High labour and land requirements!• 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
SummaryOrganic inputs-Decomposition-Crop residues-Pros and cons
Mineral fertilizers-Single nutrient-Mutiple nutrient-Solubility-Fertilizer use efficiency (AE)-The 4 rights
Soil amendments-Lime-Gypsum
BNF by legumes-Legume-rhizobia symbiosis-Promiscuous and specific legumes-Inoculation
AMF- Symbiosis between plants and mycorrhizal fungi
Improved germplasm-Benefits -Availability-Quality
