<P> Techniques focus on two aspects: provisioning of water, and fixation and hyper - fertilizing soil . </P> <P> Fixating the soil is often done through the use of shelter belts, woodlots and windbreaks . Windbreaks are made from trees and bushes and are used to reduce soil erosion and evapotranspiration . They were widely encouraged by development agencies from the middle of the 1980s in the Sahel area of Africa . </P> <P> Some soils (for example, clay), due to lack of water can become consolidated rather than porous (as in the case of sandy soils). Some techniques as zaï or tillage are then used to still allow the planting of crops . </P> <P> Another technique that is useful is contour trenching . This involves the digging of 150 m long, 1 m deep trenches in the soil . The trenches are made parallel to the height lines of the landscape, preventing the water from flowing within the trenches and causing erosion . Stone walls are placed around the trenches to prevent the trenches from closing up again . The method was invented by Peter Westerveld . </P>

What is the desertification of the sahara (points 3)