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Wageningen Universiteit Pays Bas (1999)

Tillage for soil and water conservation in the semi-arid Tropics

Hoogmoed, W.

Titre : Tillage for soil and water conservation in the semi-arid Tropics

Auteur : Hoogmoed, W.

Université de soutenance : Wageningen Universiteit

Grade : PhD thesis 1999

Résumé partiel
Soil tillage is the manipulation of soil which is generally considered as necessary to obtain optimum growth conditions for a crop. In the same time the resulting modification of soil structure has serious implications for the behaviour of the soil to erosive forces by water and wind. In Chapter 1 an introduction is given to the most important aspects : the objectives of tillage, the conflicting requirements set to tillage, the characteristics of soil and water conservation in the semi-arid tropics, and the nature of tillage research including modelling.
Chapter 2 treats in detail the characteristics of the soils often found in the semi-arid tropics : the SCH soils (sealing, crusting and hardsetting). Sealing and crusting causes problems with emergence of seedlings and with infiltration. The hardsetting soils are difficult to manage, particularly when tillage has to be performed with limited energy inputs. Physical characteristics and low organic matter contents are primarily responsible for SCH behaviour. In case 1, research undertaken in Mali is reported. Sandy soils of the Sahel area, mainly cropped to millet (Pennisetum glaucum (L.) R.Br.) are very sensitive to crust formation. These crusts were found to strongly reduce infiltration capacity. On the typically gently (1-3%) fields runoff is a widespread phenomenon ; on the average 25% of the rain (mainly in the form of a few large storms during the rainy season) is lost by runoff. Crust formation and its effect on the infiltration rate was studied in experiments using a rainfall simulator are discussed. On untilled soils the presence of a crust is a permanent feature, and the effect of superficial tillage on crust disturbance disappeared quickly under subsequent rainfall. It was established that rainfall characteristics (aggressiveness, intensity) play a key role in crust formation. Research reported in case 2 was carried out in Niger. Here, important processes of soil structural changes under rainfall were assessed, to obtain a basis for a proper development of improved soil management methods. Soil and rainfall characteristics of a millet growing area close to Niamey, were determined. Laboratory tests showed a confirmation of what was observed in the field, namely that the coarse sandy soil of the area shows a mechanical behaviour which is extremely dependent on the moisture content at the time of soil handling. Therefore, the workability range is very narrow. Special tillage under wet conditions, resulting in smearing of the surface layer caused a condition which was more resistant to wind erosion. In an extension of the analyses reported in case 1, it was found the rainfall in this region is aggressive ; even small storms may fall with high intensities. The major rainfall characteristics of the Sahel differ significantly from those of other semi-arid areas (such as India). The erratic rainfall pattern in combination with the sandy nature of the soil in the region studied, leads to an extremely small number of days available for planting millet, on average around 11 for the season. Therefore, time-efficiency of soil preparation and planting methods is even more important than a positive effect on SWC and crop emergence aspects.

Mots clés : tillage / sealing / loosening / crusts / soil conservation / conservation tillage / infiltration / west africa / brazil

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Page publiée le 10 juin 2008, mise à jour le 31 décembre 2017