The effects of adding different organic amendments (maize straw, sunflower straw, and two types of manure) to Rhodic Kandiustalf from North Zimbabwe were evaluated in laboratory experiments using soil samples from a large commercial farm and from the neighboring virgin ecosystem. The study focused on a) assessing the changes in soil respiratory activity, b) comparing the accumulation patterns of stable humus substances after a 55-day incubation period, c) checking the differences in availability of plant nutrients and d) comparing the response to organic matter addition of virgin and cleared sites to evaluate the extent to which the response to organic input depends on the soil degradation status. By adding external organic matter sources the soil respiratory activity increased in the following order: sunflower straw ∼ fresh manure > maize straw > old manure. The sequestration in soil of the organic matter added was higher with lignocellulosic wastes than with old manure. Irrespective of the organic input, the mineralization coefficients evidenced the higher biodegradability of organic matter accumulated in the cultivated rather than in virgin soil. The four types of organic matter input compensated the selective accumulation of humic colloids of a low molecular weight (fulvic acids), a natural tendency of most tropical soils, and to a large extent (mainly when sunflower straw was applied) increased the humic acid/fulvic acid ratios. However, cultivation induces changes in the soil physico-chemical status and, per unit of C added, the addition of lignocellulosic wastes to cultivated soil was ca. 50% less effective in accumulating humic acid than in virgin soil. The chemical fertilizer performance of the different amendments used (i.e., the percentage of nutrients at zero time still available after the incubation experiment) only provides values above 100% (mobilizing effect) in the case of some microelements [manganese (Mn), zinc (Zn)] in soils treated with lignocellulosic wastes, but the opposite trend (microbial or physico-chemical immobilization) occurs with some macroelements [phosphorus (P), calcium (Ca), and magnesium (Mg)]. These results can be interpreted in the sense that cultivated soil displays an increasing biogeochemical activity compared with virgin soil, as corresponds to its higher mineralization coefficients of exogenous organic matter.