Changes in Soil Fertility Induced by Trash Reten- Tion and Fertiliser Applications on the Long-term Trash Management Trial at Mount Edgecombe

The long-term effects of green cane harvesting with trash retention, compared with burning, on soil fertility were evaluated using data from the long-term trash management trial at Mount Edgecombe (BT 1). Total and potentially mineralisable soil N were greater under green cane harvesting than burning in the surface 0-10cm layer of soil. As expected, fertiliser applications induced increases in extractable soil P and also some accumulation in soil organic P (particularly under trash retention). Both exchangeable and non-exchangeable soil K concentrations were higher in fertilised than non-fertilised treatments. Exchangeable K concentrations were also increased markedly by trash retention reflecting the large amounts of K that are returned in trash and demonstrating their positive effect on K fertility. Soil pH was lowered considerably by fertiliser applications and this effect was evident to 30cm. Mean pH to a depth of 20cm was 6.0 for unfertilised plots and 5.4 for those fertilised. Fertiliser induced acidification is attributed to annual applications of nitrogenous fertilisers at a rate of 140 kg N/ha. Trash retention also tended to result in a decline in pH due to the greater amount of N cycling in the system. It was concluded that recommended fertiliser rates are likely to be reduced under green cane harvesting and that regular lime applications are important when acidifying nitrogenous fertilisers are being used routinely.