Soil Fertility: Module 4

Module 4. ISFM Strategies to Maximize Profits and Agronomic Use Efficiency

Module Overview

You have now come a long way toward your goal of developing a sound ISFM program strategy. You’ve selected a site from which to do your work, identified potential problems and come up with a plan to verify your diagnosis. In this module you will have the opportunity to think about what can be done to address the problems you have identified and in the following lessons you will find a suggested analytical framework and range of interventions for you to consider and adapt for your situation. Although we focus on nutrient management in this framework with special emphasis on the macro-nutrients, you should realize by now that soil fertility is more than nutrients alone. Certain interventions may improve the physical and biological properties of the soil, yielding important benefits, such as better water retention and improved recovery of mineral fertilizer by the plant.

Your goal after going through the various lessons is to be able to formulate a list of key soil fertility management recommendations for farmers in your target area. What actions can be taken to insure that soil fertility is maintained at a level where nutrient supply won’t be a limiting factor at any stage of plant growth, from germination to harvest?

The framework we will present asks you to consider 4 main options:

  • adding nutrients to replenish stocks and flows in the soil
  • blocking nutrient flows leaving the farm (‘leaks in the system’)
  • doing a better job in recycling nutrients that are not optimally used within the farm
  • increasing the efficiency with which nutrients are used by the various production systems.

Lesson 4.1: Adding Organic or Inorganic Fertilizer

It is a simple fact that plants use soil nutrients to grow and reproduce. If whole plants or major portions of them are continuously removed from the field and no nutrients are added, the soil’s reserve of some or all of the elements will not be sufficient for economic agricultural production. This continuous removal of nutrients is known asnutrient mining. Some information on the nutrient removal by rice can be found in the table below. Information on nutrient removal by various other crops can be found in the Supplementary Readings below.

Average nutrient removal of modern irrigated rice varieties and mineral concentrations in grain and straw.





Total nutrient removal with grain + straw (kg / t grain yield)





Nutrient removal with grain (kg nutrient in grain / t grain yield)





Nutrient removal with straw (kg nutrient in straw / t grain yield)





Mineral content in grain (%)





Mineral content in straw (%)





Much of the last module dealt with how to identify specific nutrient deficiencies. Depending on availablily of nutrient sources, these single element deficiencies are perhaps the easiest to address. For example, if a farmer observes widespread symptoms of Zinc deficiency in his or her field, it is a relatively easy matter to apply additional Zinc to the field.

While important, adding nutrients should not be considered only in terms of just single elements. As we discussed in the first module, a soil with adequate fertility levels is not necessarily a productive one. Care must be taken to enhance and maintain soil structure and texture and ensuring adequate levels of organic matter and balanced plant nutrition, i.e. crops need a variety of nutrients in a balanced proportion to ensure optimal growth. Also, managing such factors as soil pH and CEC must also be considered if nutrients are to be made available to plants.

Adding nutrients is done to achive two main objectives – short- and long-term. To achieve short term gains in soil fertility farmers may grow a green manure crop, add mineral fertilizers or apply farmyard manure. Long-term strategies include fallowing, application of organic matter with a high C/N content, application of one-time high doses of inorganic P or phosphate rock.

Lesson 4.1.1: Adding Nitrogen

Although N is one of the most abundant elements on earth, its deficiency is probably the most common nutritional problem affecting plants worldwide. Most plants take N from the soil continuously throughout their lives, and demand usually increases as plant size increases.

Building soil nitrogen is made difficult because of the dynamic nature of N cycling in the soil. There are three main forms of N capital: mineral N (ammonium NH4 and nitrate NO3), N in relatively labile soil organic matter and N in a more stable form of soil organic matter. NH4-N can be held as exchangeable cation or trapped as an interlayer cation in some 2:1 clay minerals, such as vermiculite and illite in vertisols. Under anaerobic conditions nitrifying bacteria quickly transform NH4-N into NO3-N (nitrification).

N is sometimes refered to as the “nutrient that doesn’t want to stay put”. Nitrate is highly mobile and easily lost by leaching or by denitrification (gaseous losses as NO, N2O and N2). Substantial losses of NH4-N can also occur through volatilization (gaseous losses as NH3).

Legumes may be used to build soil N capital, however often a majority of N is fixed in the grain. The growing of soybeans, for instance, may even result in a net removal of N from the field. When soils are cultivated continuously it may be impossible to build up soil N capital. Even when the capital store of N is replenished, continued use of crop sequences and intercrops with grain legumes and green manures, better integration of crops and livestock and optimal use of mineral fertilizers are essential to ensure improved yields are maintained. In sandy soils, with weak microbial activity and poor structural protective capacity of the soil opportunities for the build-up of nitrogen capital associated with organic matter may be limited. This is especially the case in lowland areas where temperatures are high.

Lesson 4.1.2: Adding Phosporus

The two main ways P is lost is though crop-harvest removals and soil erosion. The P content of plant residues and manure is normally insufficient to meet crop requirements. P fertilizers are, therefore, often necessary to overcome P depletion. Soil P can be replenished with soluble P fertilizers, direct application of sufficiently reactive phosphate rock (PR), or the combination of soluble P fertilizer and PR. In high P-sorbing soils, high levels of P addition may be required before a response is achieved, although the residual benefits of such applications are potentially high. By contrast, in sandier soils, lower applications may be given. However, the residual benefits will probably be limited, due to the combined effects of leaching of inorganic P and the limited existing fraction of organic P.

The suitability of PR for direct application to soil depends upon the mineralogy and reactivity of the PR, soil properties, climate, crop, and economics of use associated with the PR. Dissolution of PR requires low pH, low soil exchangeable Ca and low soil solution P concentration. Plants can enhance the dissolution of PR through acidification of the rhizosphere. A high P sorption capacity can promote more rapid dissolution of PR, but the low soil solution P concentration resulting from high P sorption may limit plant growth. Combined use of P fertilizers and legumes can result in synergism where nitrogen fixation can be enhanced because of reduction of P deficiency.

Lesson 4.1.3: Adding Other Elements and Lime

While N and P are arguably the most commonly deficient elements, a deficiency of any of the other critical elements will adversely affect crop growth and the profitabilitiy and sustainability of a farm. For details on the addition of these other elements participants should refer to the supplementary readings below.

If soils in your area are acidic, it might be necessary to add lime. Acid soils are widespread around the world. Their occurrences are caused by natural processes (weathering) and/or man-made processes (adding NH4 producing fertilizers to soils, releasing acid forming gases to the atmosphere). Acid soils are infertile because of (i) Al and/or Mn toxicities and (ii) Ca and/or P deficiencies. Acid soils can be managed by liming based on appropriate lime requirement curves.

Lesson 4.1.4: Adding Organic Matter

Adding of organic matter can improve virtually almost all soil properties. It will result in looser and more porous soil, lower bulk density, higher water-holding capacity, greater aggregation, increased aggregate stability, lower erodibility, greater soil fertility and increased CEC just to mentions some of the most important.

However, contrary to what some may say, adding organic matter alone is not the complete answer to soil fertility problems. Soil organic matter contributes to soil productivity in several ways, but there is no direct quantitative relationship between soil productivity and total soil organic matter. In fact, it has been the decline in organic matter that has contributed to the productivity of the crop-fallow system.

Soil organic matter cannot be increased quickly even when management practices that conserve soil organic matter are adopted. The increased addition of organic matter associated with continuous cropping, and the production of higher crop yields, are accompanied by an increase in the rate of decomposition. Moreover, only a small fraction of crop residues added to soil remains as soil organic matter. If the rate of addition is less than the rate of decomposition, soil organic matter will decline and, conversely if the rate of addition is greater than the rate of decomposition, soil organic matter will increase.

Lesson 4.1.5: Adding a Combination of Organic and Inorganic Fertilizers

While adding organic matter can contribute to maintaining soil fertility, organic sources of nutrients have low nutrient contents and are usually not abundantly available. Sustaining soil fertility and increasing productivity using organic resources alone is, therefore, often a loosing battle. An enormous amount of organic fertilizer would be required to maintain soil fertility levels in each and every field. However, the opposite strategy, the unique use of inorganic fertilizers may lead to yield gains in the short term, but to serious damage to soil fertility (e.g. acidification) and yield decline in the long term. The best remedy for soil fertility is, therefore, a combination of both inorganic and organic fertilizers, where the inorganic fertilizer provides the nutrients and the organic fertilizer increases soil organic matter status, soil structure and buffering capacity of the soil in general. Use of both inorganic and organic fertilizers often results in synergism, improving efficiency of nutrient and water use.

Lesson 4.2: Reducing Nutrient Losses – Blocking Nutrient Flows Leaving the Farm

Nutrient losses can be reduced by controlling erosion, run-off and leaching. Erosion losses can be reduced through the construction of bunds, terraces, or stone lines. Trees can be instrumental in re-capturing nutrients leached from the subsoil. Nitrogen losses through NH3 volatilization during storage and handling of manure limit its effectiveness as a nutrient source. Anaerobic storage in pits with or without addition of crop residues can significantly reduce N losses.

Supplementary Reading

Lesson 4.3: Better Management of Available Resources – Managing Internal Flows of Nutrients

Better integration of crop and livestock management, use of household waste, composting and incorporating crop residues into the soil are promising ways to improve nutrient cycling within the farm. Bedding in stables absorbs urine and conserves nutrients.

Organic matter turnover is very rapid in tropical soils. Organic matter sources with a low C/N ratio mineralize very quickly and will supply nutrients for plant growth, but this will not lead to a rapid increase in soil organic matter in the soil. The effectiveness of an organic resource as fertilizer decreases with increasing C/N ratio. Chicken manure and vegetable residues have C/N ratios of about 10 and are most effective as an alternative to mineral fertilizer. Cattle and pig manure are intermediate (C/N ratios of about 20), and straw is least effective as fertilizer. The quality as soil amendment increases with increasing C/N ratio, but decreases at extreme values. Soil N availability may even decrease if soil amendments are used with very high C/N ratios as the microorganisms that decompose the material temporarily block N otherwise available to the crop.

Composting is a process where material with a high C/N ratio (e.g. rice straw) is converted into material with a low C/N ratio. Farmers may improve the nutritional quality of compost by adding ashes, eggshells and droppings of small ruminants.. During composting, about 50% of the carbon in the initial material is typically lost but mineral nutrients are mostly conserved. Finished compost is therefore generally more concentrated in nutrients than the initial combination of raw materials used and can serve as an effective means of building soil fertility.

Lesson 4.4: Improving the Efficiency of Nutrient Uptake

Improving input use use efficiency is a key intervention as it results in reduced production costs and environmental risks. The more nutrients a crop converts to grain or fiber, the less opportunity for nutrients to reach streams, lakes or groundwater. Nutrient recovery may be enhanced in several ways. Perhaps the most effective of these is through improved crop management. It is important that nutrient addition be synchronized with plant demand for nutrients and fertilizer application may greatly enhance recovery. Better management of yield reducing factors like weeds, pests and diseases may greatly increase nutrient recovery from fertilizers.

Paying attention to placement of fertilizers is another important tactic. Plants take up nutrients more efficiently if fertilizers are applied close to the roots. It has been shown that micro-doses of growth-limiting nutrients placed near the roots may greatly enhance crop performance. Mulching (e.g. with rice straw or other plant residues) may conserve moisture and smother out weeds, enabling better crop establishment and nutrient uptake. It is not uncommon to see that farmers tend to wait with weed management until weeds are clearly visible, a period when most of the damage will already be done.

It is also important to consider the value of balanced fertilization. When nutrient supply is unbalanced, yields and profits decline and, quite often, the quality of the crop is impaired.

Supplementary Reading

Lesson 4.5: Economic Considerations

Although we try to highlight some of the key economic considerations in ISFM below we know that what is presented here is just enough to give participants a feel for these concepts. We hope to make available a more comprehensive treatment of economic considerations in ISFM in future versions of this course. It should be obvious however, that economics are the basis for farmer decision making and judging ISFM recommendations.

In the majority of cases, farmers are not motivated to use ISFM technologies because they are not profitable. For example, in sub-Saharan Africa, it is estimated that the cultivatable land area on which productivity-enhancing technologies (improved seed, inorganic and organic fertility, good agronomic practices) have been successfully used does not exceed 15 to 20%. Socio-economic factors also play an important role. Soil fertility management can be strongly related to the degree of access to resources (e.g. land, carts, cattle, labor, and cash). Land tenure is a very important issue. Farmers that do not own the land they cultivate may well be hesitant to invest in soil fertility, as the pay-off is not always directly visible. Access to resources often differs among household members, e.g. women may have only limited access to certain resources. In measuring the profitability of fertilizer use, several simple criteria are used. These are presented in the table below.

Method What it measures Profitability Decision Criteria
Input Output price Ratio (IOR) (Usually fertilizer/crop ratio) Measures how much produce in kilograms is required to acquire a kilogram of the input (e.g. fertilizer). It is a price incentive. The smaller the ratio, the higher the incentive. Typical values for West Africa are between 2 and 4 for maize, crop and millet, 2 for irrigated rice, 3 for groundnuts and 1.9 for cotton.
Benefit Cost Ratio (BCR) Measures the value of production less the cost (benefit), and expressed as a ratio of the direct and indirect costs incurred in the production. It is a profit incentive. A ratio exceeding 1 shows profitability.
Net Present Values (NPVs) and Internal Rates of Return (IRR) Measures returns to investments made in the fertility-enhancing technologies over time. Are profit incentives. Because it takes into account the discount rate, IRRs greater than the going interest rates on savings accounts are usually considered good investments.
Break-even analysis Measures several aspects of profitability. For example:

  • estimates the minimum price at which the product must be sold for the fertilizer to just pay for itself
  • estimates how much fertilizer must cost for the product to just pay for it
  • to achieve a specified level of profit from the use of fertilizer and
  • to compare the profitability of fertilizer use across various enterprises
Varies according to the aspect required
Value Cost Ratio (VCR) Measures the value of additional production due to fertilizer application. Estimated by dividing the value of the yield increase by the cost of fertilizer used in procuring the increased yield. It is a profit incentive. Values must be equal to at least 2 in assured and less risky environments and at least 3 or 4 in more risky environments.

If available at all, the adoption of ‘external’ inputs, and in particular of mineral fertilisers, gives rise to considerable financial risks. These financial risks are determined by:

  • the prices farmers have to pay for the ‘external’ inputs;
  • the availability and cost of credit to buy ‘external’ inputs;
  • the agricultural technology – and knowledge about technological options;
  • the prices farmers receive for their agricultural produce.

While we have highlighted the importance of economic factors in this lesson, in reality, these cannot be considered independently of the agronomic considerations we presented earlier. Unfortunately the “agronomic environment” of much of the farm land in developing countries is so poor that crop response to inputs are generally low and will continue to be low until investments are made in the improvement of the fertility status of the soil through amendments and other agronomic practices. Farmers are making efforts to improve the fertility of the soil through labour-intensive water harvesting and conservation technologies, and for the production, collection and use of manure to improve the agronomic environment. The cost is high and farmers need to be supported to invest in animal traction and other equipment.

Lesson 4.6: Computer-based Decision Support Tools

Just as computer-based decision support tools can be useful in diagnosing soil fertility problems, so too can they be useful for formulating and evaluating potential solutions. These tools can provide quick and cost effective answers to such questions as – When to apply fertilizer and how much’, ‘when to weed’, ‘when to irrigate’ etc. They can help to guide medium term decisions on choice of production system (e.g. ‘growing a leguminous crop in association with maize or two times a maize crop’), tillage system (e.g. ‘plowing or conservation tillage’), cultivar choice (e.g. short or medium duration) and sowing date (early or late). They may help evaluate such long-term decisions such as whether or not to apply rock phosphate to improve the phosphorus supplying capacity of the soil over a number of years, inclusion of agro-forestry options into the production system, or investment in improved irrigation and drainage facilities.

For mineral fertilizers, the QUEFTS model can be used to determine best-bet fertilizer strategies for a range of target yields, given soil nutrient supplying capacity, potential yields and nutrient recovery rates. Dynamic crop simulation models that simulate phenology, such as APSIM or DSSAT can be employed to compare timing of management interventions by farmers to what would be optimal in agronomical sense. The organic resource database developed by TSBF-CIAT helps with organic matter management decision-making based on N, lignin and polyphenol content. We introduced these and several other DSTs earlier and interested participants might want to revisit their descriptions.

Another good use of these tools is that discrepancies between modeled outcome and farmer reality can be used as input into discussions with farmers. Farmers may be aware of the best timing for a given crop management intervention, and other constraints (e.g. lack of credit, lack of labor) may be the real cause of the delay.

Of course, results from simulation – and other – models, representing a complex, and too a large extend still poorly understood, reality of crop, soil, capital and labor interactions, should be interpreted with much care. For more information and supplementary materials on these models click on the links below. The online Bontkes and Wopereis book may also be of interest and participants can find a link to this publication in the Supplementary Reading section.

To get a feeling for how these tools can be used you might want to take some time to play around with 3 online crop response models that have been published on the Web by QPAIS Pages. These dynamic models simulate crop response to the application of Nitrogen, Phosphorus and Potassium fertilisers. Users input their initial values for sowing and harvesting dates, climatic and soil conditions, and fertiliser application in a simple online form using their web browser. The program output consists of estimates of crop response in numerical or graphical form. Each model also has an associated model description and diagram showing the compartments and flows. For additional information on other models please click on the links available in the Supplementary Reading section below.

Supplementary Reading