Soil erosion occurs when the vegetation cover has reduced to the point of leaving the soil material vulnerable to being washed away by rainfall or blown away by wind. It may manifest as a network of well defined deep channels, or at worst gullies, on slopes of only two or three degrees. In severe cases it leads to dust storms, rapid land degradation and desertification. The eroded soil material may bury lands downslope, or form large sedimentation areas downstream. Its occurrence has greatly increased, usually at a rate at which soils cannot be sustained by natural soil regeneration, because of the activities of modern development and population growth, particularly agricultural intensification. Agricultural soil erosion reduces the fertility of the land resulting in great loss of agricultural productivity. It is estimated that about 11 million hectares of arable land are lost to agriculture every year through soil erosion. It is also in the field of agriculture that most efforts have been made to conserve soils, with mixed success.
This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities. Agenda 21 recommends undertaking measures to prevent soil erosion and promote erosion-control activities in all sectors, in particular minimizing soil run-off and sedimentation.
The most cost-effective technology used in reducing soil erosion is considered to be contour-based cultivation. In India, contour ditches have helped to quadruple the survival chances of tree seedlings and quintuple their early growth in height. Deeply rooted, hedge-forming vetiver grass, planted in contour strips across hill slopes, slows water run-off dramatically, reduces erosion, and increases the moisture available for crop growth. Currently 90% of soil conservation efforts in India are based on such biological systems. Simple technologies involving rock bunds construction along contour lines for soil and moisture conservation have succeeded. OXFAM has promoted this technique among farmers to improve water harvesting in Burkina Faso. Bunded fields yield an average of 10% more than traditional fields in a normal year and, in drier years, almost 50% more.
In 1977, the Guatemalan government and USAID organized the Small Farmers Conservation Project in the country's highlands. Research developed and/or informed conservation activities. Contour planting, mulches, and bench terracing for steeper slopes were recommended. Positive results have been recorded. In 1993, UN Food and Agriculture Organization's (FAO) Investment Centre helped develop a 28 million dollar project to promote among others soil conservation in Tunisia.