Overstory #119 - Five Fertility Principles
Understanding the relationship between nutrients in the soil and crop productivity is crucial in achieving and maintaining high levels of agricultural productivity. This understanding is especially important for productivity at the lowest possible costs, both economic and ecological. Soil fertility is not an easy concept to define. For the purposes of this paper, we use the definition of soil fertility presented by Anthony Young (Young 1989), "soil fertility...is the capacity of soil to support the growth of plants, on a sustained basis, under given conditions of climate and other relevant properties of land." Notice that this definition goes beyond the simplistic concept of soil content of available nutrients, and allows for other critical aspects of soil fertility such as physical and biological properties.
The five principles of agriculture for the humid tropics
An increasing number of institutions involved around the world in small farmer agriculture have begun to use some or all of the following principles of soil management:
1) Maximize organic matter production.
Frequently, small-scale farmers can dramatically increase the amount of organic matter their fields produce while maintaining yields and only increasing costs slightly, if at all. In fact, many green manure/cover crop (gm/cc) and agroforestry systems reduce the amount of labor needed for controlling weeds, thereby increasing overall organic matter production while decreasing costs.
Increases in organic matter production can be achieved by using the intercropping of either various crops or gm/cc's with annuals or tree crops (as in dispersed tree systems), by establishing two- to four-story fields and gardens, and by growing trees or gm/cc's on wasteland or during the dry season. In drought-prone areas, an increased provision of water in whatever form can also result in greater levels of biomass production.
Obviously, the more biomass we produce on site, the more we will have for applying to the soil, thereby making the provision of nutrients to the soil greater and more constant. If animals are present in the farming systems, they will also be more numerous and/or produce more manure per animal if they have more biomass to consume.
2) Keep the soil covered.
Soil exposed to the tropical sun produces more weeds (which are another form of biomass, but may compete with crops and/or occasion a good deal of work). Unprotected soil also becomes very hot, causing a series of problems, including the more rapid rate of soil organic matter burn-out, the reduction in crop growth rates and the death of beneficial macro and microorganisms.
Shifting agriculture has been motivated in most places in the world by either declining soil fertility or increasing noxious weed growth. Once we maximize biomass production and keep the soil shaded, both of these problems are largely eliminated. The need to let the land lie fallow for years is thus also eliminated, as has been shown in country after country with the use of gm/cc's.
It is interesting to note that virtually all the systems of improved fallows or gm/cc's that farmers have developed on their own, have increased both soil cover and the overall production of biomass, as compared to the previous shifting agriculture systems.
Keeping the soil covered reduces the decomposition rate of soil organic matter, thereby making sure the provision of nutrients to the soil lasts longer and is more constant, even if mulches tend to lose a certain amount of nitrogen to volatilization.
3) Use zero tillage.
Tillage both damages soil structure and increases the rate of soil organic matter burn-out. Furthermore, tillage exposes the soil (i.e. violates the principle of keeping the soil covered) and removes or incorporates the mulch, which violates the fifth principle below.
Many traditional agriculture systems use zero tillage. However, these systems are often not very productive over time, because, in the absence of large amounts of soil organic matter, nutrients are no longer constantly supplied to the soil and soil structure deteriorates quite rapidly. If, however, zero tillage is used in the presence of a maximum production of biomass, then both the supply of nutrients and good soil structure can be maintained. Thus, in contrast to many traditional zero till systems, those systems with plentiful biomass production can remain highly productive over decades, as a whole series of gm/cc and agroforestry systems have proven.
Often zero tillage cannot be practiced the first or second year of the transition, but as soil organic matter levels increase and the soil becomes covered, the populations of organisms that naturally till the soil increase rapidly, making further tillage by the farmer unnecessary. Scientists have shown, for instance, that earthworms alone can move more soil/ha/year than is moved with one ploughing using a tractor-pulled moldboard plough (Minnich 1977).
In conventional textbooks, zero tillage is linked with a major increase in the use of herbicides. However, if the soil is kept covered through an adequate use of gm/cc's and agroforestry, most small-scale farmers will find they never, or only very rarely, need to use herbicides.
4) Maximize biodiversity.
Some gm/cc users report achieving slightly better yields with a mixed selection of gm/cc's. Nevertheless, this principle will find its primary importance not in the short run, but rather in maintaining the systems' long-term sustainability. It can also be very important in maintaining the balance of soil nutrients (Primavesi 1982).
5) Feed the crops largely through the mulch.
Many humid tropical soils, with low pH (below 5.0), aluminum toxicity, and compaction layers, are not very hospitable environments for crop roots. Thus, crops will often grow much better if they can also access nutrients from a thick litter layer or mulch. Most, if not all, crops that grow in the humid tropics spread the vast majority of their feeder roots immediately under or even up into a mulch layer as long as it remains fairly moist. That is, they will feed much more readily from inside and immediately below the litter layer than from the soil itself.
Even the impact of chemical fertilizers can sometimes be greatly increased by being applied to the mulch rather than the soil. In Costa Rica, edible bean yields in the "frijol tapado" system, a traditional slash-mulch system, were not increased much at all above the traditional 500 kg/ha when chemical phosphorus was applied to the soil. However, yields rose two to three times traditional yields (to above 2 t/ha) when the inorganic P was applied directly to the mulch. Researchers in Africa have also noted that fertilizers applied to mulching materials are more efficient than when incorporated into the soil (Thurston 1997).
Feeding plants through the mulch helps compensate for less than ideal conditions of soil structure or root growth, providing a supplemental source of readily available nutrients in small but constant quantities right at the soil surface, thereby making it less necessary for crops to develop huge root systems that extend deep into the soil profile. Obviously, plants' access to nutrients will be better if the nutrients are on the soil surface than if they are three feet below it, especially in impoverished, acidic soils with problems of aluminum toxicity.
These same principles may well apply not only to the humid tropics, but to the semi-arid tropics, as well. Reports from some semi-arid areas indicate that this is the case. Nevertheless, there still exist some doubts as to what extent crops can survive during, and recover after, the mulch has dried out completely due to the frequent droughts in such areas. Much more experimental evidence is needed in this case.
Small farmers and NGO's have developed a number of other simple ways that plants access to nutrients can be inexpensively enhanced during the transition period. Edwin Asante, of World Vision/Rwanda, for instance, has developed a sort of small farmer version of precision planting for potatoes. In this case, an 8-cm ball of organic matter, lime, and about one-fourth the normally recommended amount of chemical fertilizer are placed directly below the seed. Yields in impoverished soils with a pH of 3.5 have averaged 20 t/ha, as opposed to 9 t/ha without precision planting (personal communication). In Honduras, Elías Sánchez developed a type of strip tillage or in-row tillage (locally called "minimum tillage" or "labranza mínima") which concentrates the organic matter in the crop row, where it is more accessible. And Dr. Erich Raddatz is developing strains of mycorrhizal fungi that can double fruit production by increasing plants' access to nutrients (personal communication).
These five principles, apart from having proven themselves time and time again among small farmers around the world, are the self-same principles a humid tropical forest employs to maintain its high "productivity" during millennia, even on soils with very low cation exchange capacities (CEC's). A tropical rainforest maximizes biomass production and biodiversity, keeps the soil shaded at all times, and feeds its plants largely through the litter layer. And, of course, no human being has to plough a forest to keep it growing lush and green, century after century.
Thus, the sustainability of forest ecology over the millennia provides important evidence that tropical agriculture following these Five Principles should also be sustainable over long periods of time. The small amount of scientific research done on this issue so far tends to support this conclusion.
- Buckles, Daniel, et al. (1998) Cover Crops in Hillside Agriculture, Farmer Innovation with Mucuna, Ottawa, Canada, International Development Research Centre (IDRC) and International Maize and Wheat Improvement Center (CIMMYT).
- Bunch, Roland (1995) "An Odyssey of Discovery, Principles of Agriculture for the Humid Tropics," ILEIA Newsletter, Vol. 11, No. 3, October.
- Bunch, Roland (2001) "A Proven Technology for Intensifying Shifting Agriculture, Green Manure/Cover Crop Experience Around the World," and "Achieving the Adoption of Green Manure/Cover Crops," both presented at the International Institute for Rural Reconstructon (IIRR's) Conference on "Best Practices in Shifting Agriculture and the Conservation of Natural Resources in Asia," held August 14-26 at Silang, Cavite, the Philippines. Both are soon to be published by IIRR.
- Cresser, Malcolm, et al. (1993) Soil Chemistry and its Applications, Cambridge, UK, Cambridge University Press.
- Minnich, Jerry (1977) The Earthworm Book, How to Raise and Use Earthworms for Your Farm and Garden, Emmaus, Pennsylvania, Rodale Press.
- Primavesi, Ana (1982) Manejo Ecológico del Suelo, La Agricultura en Regiones Tropicales, Quinta Edición, Buenos Aires, Librería "El Ateneo" Editorial.
- Thurston, H. David (1997) Slash/Mulch Systems: Sustainable Methods for Tropical Agriculture, Boulder, Colorado, Westview Press.
- Young, Anthony (1989) Agroforestry for Soil Conservation, Oxon, UK, C.A.B International.
This article was adapted with the kind permission of the author from:
Roland Bunch. 2001. "Nutrient Quantity or Nutrient Access?: A New Understanding of How to Maintain Soil Fertility in the Tropics." COSECHA, Tegucigalpa, Honduras.
This edition is a substantial update to The Overstory #20--Five Fertility Principles, which was published in November 1998.
About the author
Roland Bunch pioneered farmer-to-farmer extension and participatory technology development in the late 1960's, and wrote about these and other facets of farmer-empowering agricultural extension in his book Two Ears of Corn: A Guide to People-Centered Agricultural Improvement. The book has now been published in ten languages. Since then he has worked in some 35 nations as a consultant to the Ford Foundation, OXFAM, ILEIA, HELVETAS, GTZ, several national governments and many other development organizations, while coordinating the work of COSECHA, a Honduran nongovernmental organization. Roland was nominated in 1999 for the World Food Prize.
The Association of Consultants for a Sustainable, Ecological a People-Centered Agriculture (know by is Spanish acronym COSECHA), is a not-for-profit, non-secretarian, non-governmental development organization based in Honduras, Central America. COSECHA'S primary purpose is to spread the knowledge and effective use of the "people-centered development" process, as described in Two Ears of Corn, A Guide to People-Centered Agricultural Improvement by Roland Bunch, Agricultura Ecologicamente apropiada by Bernard Neugebauer, et al, and Desarrollo Agropecuario, De la Dependencia al Protagonismo del Agricultor by the Food and Agriculture Organization of the United Nations.
COSECHA's mailing address is: Apartado 3586, Tegucigalpa, Honduras Tel and Fax: (504) 766-2354
Related editions to The Overstory
- The Overstory #111--Land Husbandry
- The Overstory #96--Sheet Mulch
- The Overstory #81--Soil Foodweb
- The Overstory #70--Rhizosphere
- The Overstory #66--Carbon Sequestration: Storing Carbon in Soils and Vegetation
- The Overstory #61--Effects of Trees on Soils
- The Overstory #29--Tropical Green Manures/Cover Crops
- The Overstory #22--Pioneering Difficult Sites