Misconception Number 12: In tropical developing countries, the surplus of organic matter that can be returned to the soil is too small and mineralization of organic matter is too quick to provide sufficient nutrient inputs to the plants. Therefore, the only way to avoid depletion of agricultural soils is to provide them with regular synthetic fertilizer inputs.
Summary of Counter-Arguments:
Details of Counter-Arguments:
The agronomic importance of organic matter cannot be overemphasized and accelerated degradation of soil organic matter in tropical areas is indeed a challenge in the aim of obtaining high productivity, but synthetic fertilizers are no substitute for organic matter. Organic matter contributes substantially to nutrient supply, cation exchange capacity (CEC), and favorable soil structure. A loss of soil organic matter due to unsustainable agricultural practices mines the soil fertility in a way for which mineral fertilizers can not compensate. In soils that are depleted of organic matter, application of single compound mineral fertilizers remains ineffective because the low CEC of the soils and limited availability of other nutrients (e.g., micronutrients) often become the limiting factors.
In addition, synthetic fertilizer use has not been proven as an economically sustainable way of improving soil fertility and yields in Africa and parts of Asia, partly because it is too expensive and risky to use for poor farmers faced with poor infrastructure and risky rainfall patterns, and partly because relying on synthetic fertilizers might work in the short term, but it depletes the soils in the long term. Therefore, it is crucial to improve the use of locally available resources and to build soil organic matter content as much as possible.
The production of organic matter is generally not a constraint in the semi-humid or humid tropics. In fact, the humid tropics have more potential for organic matter production than temperate regions due to year-around plant growth. However, the availability of organic matter resources for agriculture might be reduced by other competing uses such as fuel and house construction. Appropriate technologies, such as solar cookers, are yet to be developed and disseminated to reduce this competition.
The production of organic matter is mainly a constraint in arid and semi-arid tropics, where the low availability of water limits plant growth. Despite these constraints, farmers in these areas sometime waste organic matter by burning crop residues. They may also not use all available organic matter that could be added to the soils through mulching or other techniques. In many farming systems with livestock, the manure deposited in the night kraals (or in stables in the case of zero grazing) is often not returned to the cultivated fields, but left to decompose in a small area. With improved transportation and targeted application techniques (such as planting pits often promoted in African organic projects), this manure resource could contribute to building soil organic matter. Organic agriculture, through the emphasis on building soil fertility with organic matter inputs, can help farmers make better use of existing organic matter sources.
In the semi-arid Great Plains of Colorado, organic management has enabled increases in the organic matter content of soil and has maintained or increased the stocks of macro and some micro nutrients available to the plants.  Although there are many examples of such promising results, more research is needed on organic techniques that can help increase production of organic matter on the farm, especially in dry areas. Some studies have shown that there are promising leguminous plants that can be cultivated in very dry conditions, for instance during the dry season in semi-arid regions, and produce a substantial amount of organic matter and serve to fix nitrogen. One example of this is the Jack beans (Canavalia ensiformis) that will grow where either the climate is so dry or the soils so poor that virtually nothing else will grow.  This plant and other leguminous crops (e.g., Crotalarias and Mucuna) provide especially large amounts of biomass that can be used as reliable supplies of organic matter.
The degradation of organic matter is determined by the quality of the added material, the soil water content, and the soil temperature. In semi-arid areas degradation is often delayed due to low soil water content. Many organic materials are rather recalcitrant to degradation, including phenol- and tannin-rich, leguminous tree leaves. Moreover, the use of certain organic techniques can help address the problem of quick mineralization. For instance, the use of hedgerows can slow down mineralization, due to the lower degradability of leaves and twigs.
Finally, the use of various non-chemically processed mineral fertilizers (such as rocks) is allowed in Organic Agriculture, if they are used with the aim of addressing long-term fertility needs.
|IFOAM - International Federation of Organic Agriculture | email@example.com|