SOIL FERTILITY DECLINE AND FALLOW EFFECTS IN FERRALSOLS AND ACRISOLS OF SISAL PLANTATIONS IN TANZANIA

Soil fertility decline and fallow effects were studied in Ferralsol-Ac risol catenas of plantations of sisal (Agave sisalana) in north-east T anzania. The fertility of Ferralsols that had been subject to continuo us sisal cultivation in the absence of fertilizers was extremely low b ur that of Ferralsols that had been under 18 years of bush fallow or u nder secondary forest was slightly better. Acrisols that had been unde r continuous sisal cultivation were less depleted than the Ferralsols because of greater intrinsic fertility. A comparison of soil analytica l data from the 1950s and 1960s with recent data from the same sisal f ields showed that the topsoil pH of the Ferralsols had decreased by 1. 5 (r(2) = 0.807) and that of the Acrisols by 1.2 (r(2) = 0.494) under continuous sisal cultivation. Thus there had been a serious decline in soil fertility under sisal cultivation, and this decline was not adeq uately reversed by fallowing.