Five Fertility Principles of Humid Tropic Agriculture
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Five Fertility Principles of Humid Tropic Agriculture

(updated June 14 2002)

Principle #1: Maximize organic matter production Intercropped green manure/cover crops (gm/cc) can produce from 50 to 140 tons per hectare (22 to 62 tons/acre) (green weight) of organic matter with very little work: no transporting of material and no cutting up or layering or turning over of compost heaps. In fact, sometimes, because of the gm/cc's control of weeds, net labor costs decrease in the farming system. Soil quality often improves visibly each year. Gradually, between 1985 and 1992, we learned that villager farmers from Veracruz State in Mexico through Guatemala, El Salvador, and Honduras were intercropping velvet beans (Mucuna pruriens), cowpeas (Vigna spp.) and jack beans (Canavalia ensiformis) with their maize and sorghum. To our amazement, these systems, virtually all of them in the supposedly infertile humid tropics, allow farmers to plant maize every year for decades, with productivity increasing over time up to 4 T/Ha (1.8 tons/acre). In other words, these farmers have found a sustainable alternative to slash-and-burn agriculture. Little by little, work in a dozen countries has convinced us that the vast majority of soils can be made highly fertile. How? By using our first principle: maximize organic matter production.

Principle #2: Keep the soil covered - gm/cc mulches provide a whole series of additional benefits. They protect the soil from the heat of the tropical sun, thereby reducing burn out of organic matter. They save a tremendous amount of work; farmers can sow into the plant residue rather than tilling the soil. They keep the excess nitrogen from acidifying the upper soil horizons. And they largely prevent soil erosion, even on slopes of 40%. Migratory agriculture is most frequently motivated by decreasing fertility, increased weed problems, or both. In the Mesoamerican gm/cc systems, nitrogen fixation and biomass recycling maintain soil fertility. Mulches of crop residues and fast-growing gm/cc's drastically reduce the weed problem. Thus the third principle is to keep the soil covered.

Principle #3: Use zero tillage - Reports of zero tillage were unconvincing until visiting the work of EPAGRI in southern Brazil. Literally tens of thousands of farmers were producing harvests approaching those in the USA--with gm/cc's and zero tillage by applying massive amounts of organic matter to the soil. Brazilian farmers, after some four years of applying gm/cc's to the soil, are able to quit plowing. The advantages, in terms of better soil structure, reduced soil compaction, higher fertility, and decreased cost, are impressive. The Brazilians' discovery explains why the zero tillage gm/cc systems of northern Honduras produce so well, while many traditional zero tillage systems do not. Thus we added a third principle: use zero tillage.

Principle #4: Maintain biological diversity - EPAGRI's investigation and dissemination of over 60 species of gm/cc partly to avoid diseases and insect pests, confirmed another more widely known principle: maintain biological diversity.

Principle #5: Feed plants through the mulch - The last principle was discovered in Costa Rica. With previous attempts to solve phosphorus deficiency problem with highly acid (pH = 4.0 to 4.5) soils, virtually all the phosphorus applied became tied up almost instantly. Farmers' harvests averaged a mere 500 kg/Ha (450 lb/acre). However, when broadcasting the phosphorus on top of the mulch, the results, since confirmed in numerous additional experiments, were astounding. Bean yields rose to between 1.5 and 2.5 T/Ha (0.66 and 1.1 tons/acre). This fifth principle is undoubtedly the most unconventional: feed plants through the mulch.

Synergy of the five principles - In order for humid tropical agriculture to be both highly productive and sustainable, it must imitate the highly productive, millions-of-years-old humid tropical forest. A study from northern Honduras shows that the gm/cc/maize system there is 30% more profitable than the high-input maize system nearby. It may well be we are just beginning to fathom the full potential of low-input agriculture in the humid tropics.

[Bunch20 1985] Bunch, Roland The Overstory #20 Five Fertility Principles Discovering Principles of Agriculture for the Humid Tropics from original at