Overarching Concepts of Soil Organic Management in the Tropics.

The term “organic matter” is misleading and should be replaced with Organic Inputs and Soil Organic Matter. SOM contents in tropical soils are similar to soils of the temperate region, and just as variable. SOM can be divided in 2 pools that provide different ecosystem services. The Active pool, with a turnover time of about 1.5 years, is responsible for providing nutrients to plants in the short term. It is now possible to classify organic inputs in terms of their quality, i.e., speed of nutrient release according to their lignin and N contents. To overcome the widespread nutrient depletion in smallholder farmers field of Africa it is necessary to begin with mineral fertilizer inputs until the soils are capable of producing sufficient organic inputs. But many such soils are not responsive to fertilizers in spite of obvious nutrient deficiency symptoms. In such cases, incorporating high quality organic inputs will provide a needed shot of C and decrease topsoil compaction to start the replenishment process. The Slow and Passive pools differ from the Active pool in that their fractions are stabilized by various forms of physical or chemical protection, due to interactions with mineral particles that form micro-aggregates inaccessible to many microorganisms. These pools are where carbon sequestration takes place. The main determinants of formation and stabilization of aggregates are texture and clay mineralogy. Sandy topsoils are unlikely to be relevant for carbon sequestration. The tropics have a huge carbon sequestration potential particularly with tree-based systems. This is because many soils in the tropics have lost much of its topsoil organic carbon; trees grow fast in the tropics, and there is lots of deep soil carbon that is probably unsaturated.