Soil fertility concepts over the past two centuries: the importance attributed to soil organic matter in developed and developing countries

The theories about plant nutrition and soil fertility varied widely from the antiquity to the middle of the nineteenth century, with major (Thaer A. 1809. Grundsätze der rationellen Landwirtschaft (1809–1812). Berlin (Germany): Realschulbuch Ed.) or minor (Liebig J. 1840. Die organische Chemie in ihrer Anwendung auf Agrikultur und Physiologie. Braunschweig (Germany): Vieweg) importance attributed to soil humus or soil organic matter (SOM). The importance assigned to humus over the past two centuries will be developed in this historical paper. Intensification of agriculture in the twentieth century permitted an important increase in cultivated plant yield of food, fiber, wood, and biofuel production, not only in the northern countries, but also in some southern countries (e.g., India and China) with the emergence of the Green Revolution. However, the question of organic restitutions and the maintenance (or increase) of the SOM stock was, at times, not taken into consideration; consequently, there was a general decrease in SOM stock for many tropical soils. It is now widely accepted by scientists that SOM depletion is one of the major factors leading to degradation of ecosystem services and loss of ecosystem resilience: new alternatives are now necessary for the maintenance and/or increase in plant productivity and production of environmental services by agriculture. Therefore, this paper will also present some recent research in different tropical countries with a focus on agroecological management of tropical soils.