Improved Cropping Systems
An important way of helping smallholders manage their resources more efficiently
Widespread Nutrient Stresses from Intensive Cropping
Research into modern cropping systems is entering a critical phase. While developed countries are mainly concerned about the adverse impact of intensive cropping on the environment, developing ones are facing a constant demand for increased agricultural production, while sustaining their already fragile resource base. It is very difficult to sustain soil fertility in cropping systems which operate at a high level of production. Many farmers are in fact failing to achieve it. The effects are seen in stresses (toxicities and deficiencies) from poor availability, acquisition and utilization of nutrients. Such stresses are becoming common in a range of soils.
For example, the yield potential of acid soils, which cover large areas of Asia, is limited by deficiencies of potassium, magnesium and other elements. Conventionally, lime and fertilizers are applied to correct these deficiencies. However, total dependence on chemical inputs is neither desirable nor practical, for two reasons. Firstly, many farmers cannot afford to buy enough of them. Secondly, many soils have the capacity to fix applied nutrients into forms which are not available to plants.
Effect of Different Crops on Availability of Soil Nutrients
Many studies have been carried out on the effect of fertilizer on crops, and nutrient uptake by plants. These tend to assume that crops absorb nutrients already present in the soil. However, it seems that at least some crops actively affect the level and availability of nutrients in the soil. For example, pigeonpea is widely grown on acidic soils in India, while chickpea is common on alkaline soils. Chickpea has been found to solubilize otherwise insoluble P (phosphate) bound up in calcium. It does this by excreting a weak solution of citric acid. Pigeonpea, on the other hand, can acquire P from acidic Alfisols by excreting various organic acids. This release of P benefits, not only the crop of chickpea or pigeonpea, but subsequent grain crops grown in the same field.
Some crops may also benefit subsequent crops by stimulating soil microbes. It is well known that soil fauna play a major role in increasing nutrient availability and uptake, especially in soils poor in nutrients. For example, it is suggested that some of the fertilizing effect of soybeans on subsequent crops may be, not only because of direct N fixation by the soybean, but because the soybean stimulates microbes which mineralize soil N. (It has been found that soybean varieties which do not have N-fixing nodules, and varieties which do, both have the same effect in promoting the growth of subsequent maize crops).
Fertilizer Strategies for Resource Poor Farmers
Areas where farmers cannot afford large amounts of fertilizer benefit very much from an integrated approach using varieties which are efficient in nutrient use efficiency (NUE). This approach is easy for farmers to follow, because it does not involve any extra cost and no major change is needed in their cropping systems. NUE can be defined in several ways. It may involve increased uptake of a nutrient by the plant, or it may involve greater efficiency by the plant in converting nutrients into grain.
Different strategies must be followed in different kinds of soil. The ideal way to improve NUE in acid soils, for example, might be liming plus the use of crops tolerant of aluminum. In cropping systems where farmers cannot afford much fertilizer, it might be best to combine small amounts of high-quality organic matter with crop varieties with a high NUE. We must also remember that nutrients removed by crops with a high NUE must always be replaced. Otherwise we are mining the soil, not using it in a sustainable manner.
Nutrient Use Efficiency of the Cropping System, Not Just the Single Crop
Research into more effective use of soil nutrients must go hand-in-hand with research into how to replace them. Furthermore, NUE must be studied in terms of the entire cropping system, not just the single crop. Some cropping systems are known to release amino acids and microorganisms in the root zone which improve the mobilization of iron and copper. Traditional farming in Asia was based on sustainable cropping systems. These were replaced by high-yielding monoculture which is not sustainable in the long term, at least on many soils.
Sustainable high-yielding crop production will require high flows of nutrients to crops, which in turn will involve large nutrient reserves in the soil, and higher uptake and utilization by crops. Productivity cannot be sustained for long if we continue such practices as applying too much or too little fertilizer, or making inefficient use of crop residues and wastes.
Index of Images
Figure 1 Rice Intercropped with Melon
Figure 2 Leafy Vegetables Intercropped with Spring Onion, Using Straw Mulch
