This chapter presents a preliminary assessment of the parameters that might serve as measures of the risk of fertility degradation. It shows the constraints to sustained production that are most commonly found in the lowland tropics in general, and in the Amazon in particular. Soils of the humid tropics vary a great deal in physical and chemical characteristics, and it must be recognized that these initial characteristics constrain the agricultural potential of tropical lands in different ways. The initial characteristics of soils form the first set of things that are worth examining in determining the overall risk of fertility degradation. Choosing the method of land clearing is the first, and possibly the most critical, management step in coping with the risk of fertility degradation in the lowland tropics. Acid soil infertility is due to aluminum toxicity, calcium or magnesium deficiency, manganese toxicity. The contribution of plant biomass to the total nutrient pool drastically declines when forests are brought under cultivation.