Changes in the properties of Fijian Oxisols over 30 years of sugarcane cultivation
Sugarcane production plateaued in Fiji at ~4 million tonnes annually (average ~55 t/ha) during the latter part of the 1980s but, in recent years, despite the continuing release of high yielding varieties, the total production has decreased to under 3 million tonnes (average ~45 t/ha). This study was initiated to study the changes in the properties of Oxisols following sugarcane cultivation to ascertain whether yield declines were due to degradation of soil biophysical and chemical properties. The study lasted from 1978 (when the soils were first cleared for agriculture) to 2009. Soil (0-15, 30-40 and 70-80 cm) properties including pH, organic carbon, cation exchange capacity, water retention, bulk density, phosphate retention and exchangeable bases were monitored. The observed topsoil changes could generally be related to changes in organic matter and associated ion exchange properties. The major changes occurred in the first three years after clearing, but some changes continued for many years. Subsoil samples (30-40 cm) showed an increase in organic carbon after cane cultivation, probably due to soil mixing during land preparation, organic matter movement during cropping and decay of sugarcane roots. At 70-80 cm, small but significant increases in organic carbon were observed along with small decreases in pH. Overall, these changes indicate that careful management of topsoils is essential for maintaining soil fertility and hence sugarcane productivity on these highly weathered soils.