Input and output of major nutrients under monocropping sisal in Tanzania

Abstract

A semiquantitative nutrient balance is presented for a field monocropped with sisal on Ferralsols in Tanzania. Input of nutrients included wet deposition, non-symbiotic nitrogen fixation and nutrients added with planting material. Nutrient output consisted of the harvested product. The average annual shortfall between 1966 to 1990 was 12 kg N ha−1, 2·8 kg P ha−1, 38 kg K ha−1, 44 kg Ca ha−1 and 19 kg Mg ha−1. The nutrient balance was compared to changes in topsoil (0–20 cm) nutrient contents of the sisal field during the same period. Average annual decrease in soil nutrient contents was: 104 kg N ha−1, 1·8 kg P ha−1, 11 kg K ha−1, 29 kg Ca ha−1 and 10 kg Mg ha−1. Much more nitrogen was lost from the topsoil than can be explained by the nutrient balance, indicating significant losses. Changes in soil phosphorus content are almost explained by the nutrient balance. More exchangeable cations were removed with the yield than were lost from the topsoil, which may imply that cations are extracted from the subsoil. Both the nutrient balance and the changes in soil nutrient contents showed that monocropping sisal is mining nutrients. © 1997 John Wiley & Sons, Ltd.