Enhancing biological nitrogen fixation and soil biological activity. Nitrogen-fixing crop, forage, and tree/shrub species scavenge nitrogen from the soil and/or fix nitrogen from the atmosphere when soil levels are below plant requirements. Most nitrogen-fixing plant species also form symbiotic relationships with mycorrhizal fungi that improve soil aggregation, nutrient and water use efficiencies, and protect the plant roots from a variety of pathogens. This is one example of an INM practice that also contributes to IPM. Integration of nitrogen-fixing species into cropping systems diversifies inputs/outputs and reduces risk on both economic and ecological fronts. Addition of plant nutrients. The nutrient content of highly weathered soils is very low. In most cases, the export of nutrients in harvested products results in one or more plant nutrients becoming limiting. In the humid tropics, calcium and phosphorus are often limiting for crop growth and productivity. Appropriate amounts of lime and nutrients are essential to optimize plant root growth, enhance the efficiency of added nutrients, and avoid soil degradation (box 4.33). Although inorganic fertilizers such as limestone and rock phosphate are consistent with organic agriculture, inorganic fertilizers are often the most efficient means of adding soil nutrients. In many places (such as Africa) they are essential for improving productivity to levels that will then enable adoption of wider INM practices. Box 4.33 Soil nutrient recapitalization Researchers estimate that the croplands of many smallholder farmers, especially in Africa, have become depleted of the nutrients that are removed by crop harvests. Published estimates from 40 African countries indicate a net negative annual balance of 22 kilograms of nitrogen, 2.5 kilograms of phosphorus, and 15 kilograms of potassium per hectare of cultivated land. To meet increasing demand for food, soil scientists have recommended a major capital investment to replenish soil nutrients in Africa. Social scientists, however, caution against blanket nutrient recapitalization, as nutrient deficit statistics are based on limited data and fail to account for nutrient variability and transfers at the farm and watershed level. Furthermore, many socioeconomic and institutional factors influence farm management decisions. For example, smallholder farmers in western Kenya are gradually but significantly improving both crop yields and soil fertility because locally repackaged one- to five-kilogram bags of fertilizer are available, affordable, and can be transported and used on selected fields. As crop yields and input-output markets improve, farmers invest further resources for more fertilizer nutrients and improved seed. Source: Anderson et al. 2002 |
In the past, the cost of soil and crop sampling and nutrient analyses made site-specific fertilizer recommendations prohibitively expensive for most agricultural programs. Blanket fertilizer recommendations were common, but blanket application of fertilizers is often uneconomic and can lead to pollution. Recent advances in plant nutrient decision support models, improved access to high-resolution satellite images, and the improved interpretation of crop and soil spectral signatures make site-specific recommendations possible.
    
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