In the Sudan, the desert is reported to have grown in size by 100 km between the years 1958 and 1975. In contrast to these very depressing descriptions of what happens when rainforest is converted to pasture, other scientists have claimed that pastureland has good effects. Some light has been shed on the controversy by the construction of models showing trends of soil organic matter and nutrients when Amazon forest is converted to pasture . When the forests are cut and burned, both total soil organic matter and nitrogen immediately drop. Increase in nitrogen and organic matter after burning depends on the establishment of nitrogen-fixing species in the pasture.
Exchangeable aluminum drops after the burn .
Following an initial increase, potassium levels fall, but not to the level of the original forest . Available phosphorus clearly shows the biggest decline as the pasture is used. Phosphorus is the nutrient which is most critical for sustaining yield in these pastures. According to these models, available phosphorus drops despite maintenance of pH close to 6.0. The decline may occur because total phosphorus stocks are used up.