New Data Concerning Erosion Processes and Soil Management on Andosols from Ecuador and Martinique

Resumen

Because of their remarkable physico-chemical properties andosols are considered generally as fertile soils, well aggregated and very resistant to water erosion. Nevertheless recent measurements in cropped andosols under bananas plantations in Martinique Island and cereals in Ecuador steeplands have shown that its is necessary to moderate this opinion. In Martinique Island, intensive banana crop systems could seriously damage the environment by water erosion and chemical pollution because of the use of high inputs, aggressive tropical rainfalls and steep slopes. Therefore a field study with 10 runoff plots (200 m²) was located on 10-25-40 % slope of clay volcanic nitisol. In Ecuador, steep slopes of Andean Mountains are densely covered with little crop fields without any water management, therefore rills and gullies are abundant. 6 runoff plots (100 to 1000 m²) of 20%—40% slope were installed in the farmers fields with traditional crop rotation (barley, beans and potatoes). In each situation, under natural forest and savannah vegetation and under well mulched crops, runoff and erosion were negligible, but as soon as soils are denuded and compacted by grazing or by farmers or desiccated by the sun, it was observed in each situation moderate increase of runoff (5% to 20% of the rain amount) and severe erosion from 80 to 150 t/ha/year. Looking to soil surface features, its was observed very few sealing crust, very stable aggregates but high susceptibility to compaction. As soon as fields have more than 15%—20% slope, runoff is collected in rills and gullies. Erosion is not a selective process because runoff is able to take off aggregated topsoil as a whole. In opposite to general opinion runoff does not increase with the slope steepness but erosion increase strongly, so that from 20%—40% slope erosion processes change from sheet and rill erosion to rill and creep erosion. Between 10 and 40 % slope, when cropped plots are mulched by crops residues or mulched by strips, runoff and erosion are insignificant. In conclusion, simple anti-erosive measures can be used by farmers when they are obliged to crop very steep hillslopes. First, to increase infiltration rate, soils must be covered by intensive cropping system including crop residues mulching. Secondly, to reduce runoff velocity and energy, grass strips and earth bunds are managed each 10 to 20 meters depending of slope steepness and soil erodibility. These strips must be productive of forage, fire wood and poles for building to be acceptable for poor farmers. Keywords: Ecuador, Martinique island, volcanic soils, erosion risks, slope effect, soil and water management, soil surface features