THE INFLUENCE OF PARENT MATERIAL ON SOIL FERTILITY DEGRADATION IN THECOASTAL-PLAIN OF TANZANIA

Differences in the vulnerability of soils to fertility degradation are compared for two major soil groups located in the coastal plain of Ta nzania and cropped with sisal (Agave sisalana). Ferralsols derived fro m intermediate gneiss of Precambrian age and Cambisols developed in Ju rassic and Neogene limestones partly covered with Quaternary deposits were selected for comparison. A clear influence of parent material was apparent when soils were continuously cropped with sisal and no ferti lizers were applied. Serious soil fertility decline occurs in Ferralso ls, but Cambisols are resilient to chemical degradation and the fertil ity decline in these soils was very limited. The differences in degrad ation rates are explained by the lower initial fertility and low nutri ent reserve of Ferralsols, while the Cambisols had higher initial fert ility levels and nutrients removed by the sisal crop are replenished b y the weathering of the underlying parent material. Sustainable soil f ertility management of Cambisols includes only NPK fertilization, but heavy dressings of lime, organic manures and/or chemical fertilizers a re required to improve the fertility status of the Ferralsols and to p roduce crops in a sustainable manner.