The chemical and agrobiological characteristics of 37 composts from wheat straw with different additives were evaluated through routine tests. In general, the plant yield under greenhouse conditions in soils amended with these composts was unrelated to data from the phytotoxicity germination biotest, but with the chemical parameters reflecting selective biopolymer degradation in straw. This suggests that factors such as microbial immobilization of nutrients had greater influence than phytotoxic inhibitor compounds in the plant yield of the soils amended with the composts studied. Monitoring the composition of the water-soluble fraction was also useful as regards crop yield of soils improved with compost. The factors potentially connected with the positive or depressive effect of composts in soil were assessed through factorial design experiments involving successive harvesting calcium carbonates and mineral fertilization. In general, mineral fertilization may lead to decreased yields in soils to which immature composts are applied. The effects on compost maturity of the different by-products used as additives are discussed for the system studied.