INTEGRATED SOIL FERTILITY MANAGEMENT
Concept and Definition:
A set of soil fertility management practices that necessarily include the use of fertilizer, organic inputs and improved germplasm combined with the knowledge on how to adapt these practices to local conditions, aiming at optimizing agronomic use efficiency of the applied nutrients and improving crop productivity. All inputs need to be managed following sound agronomic and economic principles. Integrated Soil fertility Management focuses on the agronomy of crops and inorganic fertilizers. ISFM also deals with interventions on germplasm which involve the selection of varieties, spacing and planting date. Furthermore, interventions on fertilizer use respectively targeting its formulation, placement, rate and timing of inorganic nutrient inputs. In addition, ISFM targets interventions on organic resource management, including the return of crop residues, manure, compost and other types of organic wastes, next to rotation or intercropping with legumes and use of plant growth promoting micro-organisms. The last focal point of ISFM deals with any other amendments that may be needed to lift limitations to productivity such as soil acidity, micronutrient deficiency, erosion, soil compaction or pests and diseases.
By definition, ISFM prescribes that interventions have to be aligned with prevalent biophysical and socio-economic conditions at farm and plot level.
COMPONENTS OF ISFM
In mathematical terms, ISFM is simplied into an equation as follows:
Yield = G (genotype) x E (environment) x M (management).
The above equation is explained as yield is a function of crop genotype (improved varieties) with the requisite environment and good agronomic practices.
It is important that the farmer uses the crop planting materials (usually seed but sometimes seedlings) best adapted to the particular farm in terms of:
- Responsiveness to nutrients (varieties differ in their responsiveness to added nutrients);
- Adaptation to the local environment (soils, climate); and
- Resistance to pests and diseases (unhealthy plants do not take up nutrients effi ciently).
Integrated nutrient sources: This refers to the strategy of adoption of more than one source of an essential nutrient source for effective crop growth and development. This has become necessary owing to the numerous challenges when sole source of nutrient of significance is fraught with implementation challenges which cut across many spheres. The challenges associated with sole source of nutrient supply for crop growth and yield is explained below:
Inorganic sole source:
The following buttresses why farmers use inorganic nutrient sources:
- Supplement recycled or added nutrients from organic sources
- Contain essential nutrients in a form readily available for plant uptake
The reliance on inorganic sources has been difficult due to the following:
- Fertilizer is ‘too costly’ for most farmers
- Fertilizer use is uneconomic in poorly responsive environments
- Fertilizer recommendations were not tailored to farmer’s specific circumstances, owing to;
- Heterogeneous soil fertility
- The farmer’s social and economic situation and goals which vary
Organic sources of nutrients: The adoption of organic sources of nutrients was justified based on the following:
- Conserving nutrients: through compost and manure
- Recycling nutrients : through deep rooting trees
- Adding nitrogen: through biological N2 fixation (BNF) by leguminous cover crops, trees, shrubs and grain legumes.
- Source of nutrients, including nutrients not contained in mineral fertilizers
- Replenish soil organic matter
- Increase the crop response to mineral fertilizer
- Improve availability of phosphorus for plant uptake
- Regulate soil chemical and physical properties
- Create a better rooting environment due to the improvement of the soil structure
- Improve the soil’s capacity to store moisture
- Maintain the biodiversity in the soil
However, the reliance on organic source as sole source of plant nutrient is faced with challenges due to the following:
- Quality of organic resources is often poor in the tropical environment
- Quantity of manure or organic resources is not sufficient
- Competing uses for plant residues
- Organic materials are bulky and costly to store, transport and apply
- Adoption and suitability of leguminous cover crops is limited by;
- high labour requirements
- only N can be supplied
- availability of other nutrients (e.g. P) need to be sufficient for effective BNF since the currency for biological nitrogen fiation (BNF) is P driven in the form of ATP.
- drought and low soil pH limit BNF
- lack of useable yield (except grains in most cases)
The positive interaction between inorganic and organic fertilizer is shown in the figure below:
Figure 2: Evidence of the positive synergy between inorganic and organic fertilizer in yield increases
The above figure indicates the positive interaction called synergy between inorganic fertilizer and organic crop residue applied to millet over a 7 year period.
Note: Fert (fertilizer from inorganic source), CR (Crop residue which represent organic source of nutrient), Control (meaning yields from fields with fertilizer or crop residue addition) and CR+Fert( representing addition of both fertilizer and crop residue).
OTHER ADD-ONS TO ISFM
- LOCAL ADAPTATION
In the definition of ISFM emphasis was also placed on the need for ‘local adaptation’ because one needs to take into account variability. This variability stems from two major sources such as:
- Between farms, where in terms of farming goals, and objectives, size, labour availability, ownership of livestock, importance of off-farm income differ; and
- In the amount of production resources (i.e. land, money, labour, crop residues and animal manures) available to different farming families are their ability to invest in the fields in their farm.
The ISFM definition places emphasis on the importance of using often scarce resources like fertilizer and organic inputs efficiently while reaching economic goals that are achievable for each farm household. We should bear in mind that on farmers’ field lie three different types of soils regarding their ability to respond to external inputs. These are:
- Poorly responsive fertile ‘in-fields’ are often found close to the farmer’s house and have benefitted over the years from inputs such as household waste, crop residues, animal manures and sometimes human waste.
- Responsive ‘out-fields’ are often found some distance from the farmer’s house where crop residues and animal manures have not been applied.
- Poorly responsive ‘bush-fields’ are also found at a greater distance from the farmer’s fi eld and have become degraded, perhaps because they are under communal use and farmers are reluctant to invest in soil fertility improvement because they are unsure of whether they will be able to grow crops on the land in the future. This farmer field gradient is illustrated below:
Figure 3: Farmer field fertility gradient near a home.
Under such high gradient field, addition of fertilizers should be graduated. For instance, the poorly responsive fertile fields should receive minimal amounts of fertilizer, if need be. On the other hand, for responsive soils, fertilizer recommendations should be targeted to each field based on anticipated or proven responses, whilst for non-responsive soils, due to their often complex and less understood sets of constraints to crop production, rehabilitation should only be carried out where solutions have been developed and tested and have been found to be practical and economical.
The variable response to fertilizer addition to such highly variable farmer field is further illustrated below:
Figure 4: Yield responses of fertilizers on three different responsive soils.
- OPTIMIZING AGRONOMIC EFFICIENCY
Agronomic efficiency (AE) is a measure of the amount of additional yield obtained per kilogram of nutrient applied. In other words, the AE of applied nutrients is equal to the additional crop yield obtained with the application of nutrients (i.e. the yield in the treatment with fertilizer minus yield in the treatment without fertilizer) divided by the amount of nutrients applied (in kilograms per hectare). This is represented in the equation below:
- YF and YC refer to yields (in kg/ha) following treatment where nutrients have been applied (YF) and in the control plot (YC ).
- Xappl is the amount of nutrient X applied (kg nutrient/ha) from fertilizers and organic inputs.
One should note that to increase AE (and yield) at a particular fertilizer application rate, the following should be considered:
- Plant the crop at the right planting density
- Apply fertilizer at the right time
- Apply fertilizer in the right place
- Apply fertilizer in several split applications
In ISFM, the practices put together must be sound economically in order to ensure maximum return to investments and increase AE. There are number of sound agronomic principles that must be adhered to. This includes:
- Appropriate varieties
- Appropriate land preparation
- Planting dates and practices
- Pest and disease management practices
- Appropriate intercropping arrangements
In addition, ISFM makes economic decisions as well. This is referred to sound economic principle. In other words, sound economic principles compare the value of additional yield with the costs of the inputs required. It is illustrated below:
Figure 5: Ilustrations on sound economic yields using the maximum economic yield response scenarios
One other easy way to establish economic sense in fertilizer application is the use of the value cost ratio (VCR). It makes an assessment of the economics of fertilizer application by comparing the value of additional yield with the cost of the inputs required to achieve the yield increase. It is represented in the following equation as follows:
Using the figure below, answer the following question
- Calculate the agronomic efficiency:
- applying 100 kg fertilizer per ha
- applying 200 kg fertilizer per ha
- Calculate the value:cost ratios for:
- Increasing yields from point B to point C
- Increasing yields from point C to point D
- Is it economically sound to increase yields up to point E with the use of fertilizer?
- What happens to the maximum economic yield when the price of fertilizer increases?
The coordinates for the respective points are as follows:
Point A: (0, 500)
Point B: (50, 2300)
Point C: (100, 3000)
Point D: (150, 3300)
Point E: (200, 3380
Note: The price of N fertilizer is 1 US$/kg and Yield can be sold for 0.5 US$/kg
BENEFITS OF INTEGRATED SOIL FERTILITY MANAGEMENT
Numerous ISFM-based practices have been studied and demonstrated significant benefits on productivity, profitability, resilience, and/or greenhouse gas (GHG) emissions. Some benefits of adoption of ISFM are discussed below:
- ISFM increase productivity at the farm level, livelihoods and food security
ISFM focusses on the management of crops that respectively involves the timing and spacing of planting up to dissemination of elite varieties and healthy seed systems. Such interventions on germplasm (improved varieties) are very important for pushing up yield potentials as well as combating pests and diseases. Furthermore, ISFM embeds different fertilizer practices that have been proven to enhance nutrient uptake and productivity of crops such as micro-dosing, deep placement, banding, and harmonizing of inputs with rainfall and nutrient demands. Regarding farm livelihood and food security, greater profitability of the ISFM system is attributed to lower production costs and better retail prices for some food crops. It has been shown that the gains in food production and income from practicing ISFM significantly benefited the intake of calories and proteins by farmers in a research.
Figure 6: shows the interaction between organic and inorganic fertilizers effects on grain yield, yield variability and soil organic carbon stocks.
Figure 7. Scenarios of ISFM interventions in improving livelihood of farmers against poverty
- ISFM mitigate greenhouse gas emissions
Fertilizer micro-dosing, disseminated under the first ISFM entry point, has been shown to significantly increase the recovery of N by crops. Greater recoveries of N fertilizers by crops, and retention of nitrate in soils, are two of the most important indicators for reduced emissions of nitrogen oxides in tropical farming systems. Combining fertilizers and organic inputs also enhances fertilizer uptake and retention by balancing immobilization and release processes. Combining fertilizers and organic inputs benefits the conservation and build-up of soil C stocks, hence mitigating CO2 emissions from soils.
- ISFM help adapt to and increase resilience to climate change impacts
ISFM contributes to strengthening the resilience of crop production to climate impacts. Practices on germplasm and crops respectively involve tactical decisions such as use of early maturing and drought tolerant varieties, or harmonizing of planting time with rainfall predictions. At the same time, one of ISFM strategies is disseminating strategic fertilizer practices that minimize the risk of input loss to adverse weather. For instance, interspersing or split application of N fertilizer inputs across periods when soils have optimal water content has shown significant benefits for N uptake by crops under varying climate. The ISFM principle of combining organic inputs and fertilizers makes important contributions to reducing the sensitivity of crop production to climate impacts
Challenges to adoption of ISFM
Despite the significant benefits of ISFM for food security, household income and environmental protection, the adoption of practices by farmers is usually low and incomplete, especially in African smallholder systems. The most important factors impeding the adoption of ISFM are as follows:
- High transaction costs of input and produce trading
- Low awareness and common disbeliefs about the benefits of soil fertility management
- Inadequate credit facilities for making initial investments in ISFM
- Aversion to risks surrounding the profitability of inputs
- Cost and availability of labour
- Land size and property rights
- Weak social networks and pervasive distrust
- Lack of information about soil fertility and rainfall forecasts
- Scarcity of organic residues and competition for residues with livestock