Conservation tillage: an important
The problem of soil water losses through surface runoff and evaporation is one of the major limiting factors in agricultural production today. Especially in arid and semi-arid lands, short intense storms coupled with prolonged dry spells make crop production difficult, if not impossible. A rainstorm brings about soil water conservation considerations, within the context of the surface storage, infiltration and water holding capacity of the soil and the capacity to minimise evaporation losses especially through the dry periods.
Tillage is defined as the mechanical manipulation of soil for any purpose. Manipulation involves soil disturbance and this can have great deteriorative consequences if not carefully or adequately incorporated. Tillage modifies the soil surface where the complex and crucial partitioning of rainfall into runoff, infiltration and subsequent evaporation. Tillage modifies soil surface structure, total porosity, macro-porosity, pore continuity and pore size distribution and therefore has great influence on the hydrology of an agricultural catchment (Mwendera, 1992).
Tillage influences the upward movement of moisture to the soil surface, vapour transfer from the surface to the atmosphere and heat transfer to the soil. Tillage therefore affects soil water evaporation and will do so differently in arid and humid environments. The properties of the plough layer and particularly the surface characteristics are time variant. Models of soil water transport can and have been used to help understand the effects of tillage (Klute, 1982).
Conservation tillage (Contil) is but one aspect of global, regional and national interest and importance in environmental conservation. For East and Southern Africa (ESA) the subject has special importance, considering that it touches directly on agricultural production and more so, in the majority semi-arid and arid tropics, which carry over 50% of the population. Additionally, ESA has about 80% of the population involved in smallholder agricultural production, utilizing traditional means of land preparation. The region also has some of the world's poorest population and it is unlikely that such people can have time for environmental preservation among other pressing needs, in a life of uncertain food security.
This situation makes Contil work and development in ESA to be incomplete, unless it addresses somewhat unusual or extra-thematic issues which are non-technical, economic and socio-political. Compared to the mechanized high input agriculture of the western world, conservation tillage in the tropics of sub-saharan Africa must be considered much more broadly, even if just to accommodate the highly variable eco-system.
Between the semiarid and arid planes of Namibia and the highly humid highlands of Cameroon and every climate and soil condition in-between, ESA is indeed a region of contrast. The region presents a highly defined ethnic and loosely structured and variable socio-political and other development scenario, which is highly influenced by practices or issues such as land tenure, pastoralism, shift-cultivation and others. Like the rest of Africa, ESA presents a complex system in which to address the Contil challenge. Indeed, in ESA the environment has become everybody's concern as well as frustration. Urban migration, movement to lower potential land in lower altitude locations, among other tendencies has brought in many factors of environmental sustainability, which have placed the region under great threat of total destruction.
In the region, environmental degradation is most likely to be associated with urban areas, while the erosion of large straits of idle semi-arid and arid lands goes un-addressed. High potential land is not spared as can be observed in streams and rivers which remain dark brown, throughout the rain season. Pollution by factories and motor vehicles which have recently been associated with subsequent global warming are small subjects in this region where there always seems to be more urgent problems in economically suppressed political economies.
In the agricultural sector soil and water conservationists have mostly addressed soil erosion and how mechanical approaches such as terracing, can be the answer. More recently, agroforestry efforts and promotion have brought in the tree as a structural aid and a source of biological wealth to the otherwise degraded land. The agroforestry approach has progressed a step into multi-disciplinary and multi-sector approach to agricultural land management. It also built on the indigenous techniques of soil conservation practiced for centuries by smalholder farmers in humid areas.
This paper is published in: Kaumbutho P G and Simalenga T E (eds), 1999. Conservation tillage with animal traction. A resource book of the
Animal Traction Network for Eastern and Southern Africa (ATNESA). Harare. Zimbabwe. 173p. A publication supported by French Cooperation, Namibia. For details of ATNESA and its resource publications, see http://www.atnesa.org