The planting of "Eucalyptus" sp. has been criticised because of concerns that soil would be degraded by nutrient depletion and other biological changes. During the last ten years, in the northwestern regions of Uruguay the "Eucalyptus" sp. plantations have reached annual forestation rates of 300%. That zone has a potential forested area of 1 million hectares, approximately 29% of the national territory dedicated to forestry, of which approximately 800,000 hectares are currently forested by monoculture of "Eucalyptus" spp. It is expected that the radical and durable substitution of vegetation cover leads to changes in the quantity and quality of soil organic matter. Such changes may also influence soil fertility and soil physical and chemical properties. The soil quality effects associated with "Eucalyptus" sp. plantations could have adverse effects on soil chemistry; for example: soil acidification, iron leaching, allelopathic activities and a high C:N ratio of litter. Additionally, as most scientific understanding of land cover change effects is related to ecosystems where forests were replaced by grasslands or crops, or grassland was replaced by crops, the environmental effects of the current Uruguayan land cover changes are not well understood. The first scientific publication on soil studies in western zone tree plantations (focused on pulp production) appeared in 2004 and described soil acidification and soil carbon changes, similar to a podzolisation process, and destruction of clay (illite-like minerals), which is the main reservoir of potassium in the soil. Although these studies were carried out in an important zone for forest cultivation, they cannot define the current situation in the rest of the land area under eucalyptus cultivation. Moreover, recently Jackson and Jobbagy have proposed another adverse environmental impact that may result from "Eucalyptus" culture on prairie soils—stream acidification.