Laws

The laws of diminishing yields in the tropics

R. Derpsch und M. FlorentÍn
Published in: Derpsch, R., Florentín, M. & Moriya, K., 2006: The laws of diminishing yields in the tropics. Proceedings on CD, 17th ISTRO Conference, Kiel, Germany, August 28 - September 3, 2006, p 1218 - 1223

In nature there are laws that rule the diminishing productivity of soils, which have to be taken into account in agricultural and livestock production. Those who disrespect these laws are promoting the degradation of soils and the loss of soil productivity. To respect these laws is indispensable if we aim to obtain a sustainable agricultural production.

Any agricultural or livestock production system that contributes to constantly reduce organic matter content of the soil is not sustainable and results in poor soils and farmers.
Under tropical and subtropical conditions intensive and repeated tillage will generally mineralize (reduce) organic matter at rates greater than possibilities of reposition. This results in a decreasing organic matter content of the soil and diminishing crop yields over time.
High rainfall and wind intensities prevailing in the tropics and subtropics are generally associated under intensive and repeated tillage, with soil loss rates (due to wind or water erosion) greater than natural soil regeneration. This results in loss of nutrients and organic matter and in diminishing yields over time.
Under tropical and subtropical conditions intensive and repeated tillage will generally damage the soil structure and lead to excessively high soil temperatures. This will have negative effects on root growth, soil flora and fauna (soil biological processes) and on soil moisture resulting in diminishing yields over time.
Any agricultural or livestock production system in which important losses of nutrients occur through extraction without reposition (i.e. soil exploitation) through volatilization (i.e. regular burning), and/or through leaching (i.e. fallow periods without crops), is not sustainable and results in poor soils and farmers.

Additionally soil carbon is lost very fast to the atmosphere (as carbon dioxide) after the soil is intensively tilled. This results in unacceptable CO2 emissions into the atmosphere, and instead of carbon being deposited in the soil improving its fertility, tillage contributes to the greenhouse effect and to the global warming of the planet.

In summary:

The unavoidable negative effects of intensive and repeated soil tillage in the tropics and subtropics on organic matter content, soil erosion, soil structure, soil temperature, soil moisture, water infiltration, soil flora and fauna (soil biological processes) and loss of nutrients, result in chemical, physical and biological soil degradation. This results in diminishing yields over time and in productivity losses of the soil and leads to poor soils and farmers.

As a consequence of the laws of diminishing productivity of tropical soils, sustainability of agricultural or livestock production can not be achieved as long as repeated and intensive soil tillage is performed in the tropics and subtropics. Sustainability also can not be achieved as long as the soil is exploited without reposition of nutrient losses through leaching and/or extractions that occur with harvests, and as long as frequent burning of fields is performed.

In order to maintain and improve soil fertility and achieve a sustainable agriculture in the tropics and subtropics, it is necessary to stop mechanical soil preparation and keep a permanent cover of the soil. At the same time adequate quantities of plant residues should be added to the system (more than 6 t/ha/year of dry matter in semi-arid climates and more than 10 t/ha/year of dry matter in humid climate). It is impossible to achieve a sustainable agriculture and at the same time perform intensive mechanical soil preparation.

No- tillage with green manure cover crops and crop rotations is the only truly sustainable production system in most forms of agriculture in the tropics and subtropics.