Volcanic fertilization of Balinese rice paddies

Since the advent of high-yielding Green Revolution rice agriculture in the 1970s. Balinese farmers have been advised to supply all the potassium and phosphate needed by rice plants via chemical fertilizers. This policy neglects the contribution of minerals leached from the volcanic soil and transported via irrigation systems. We documented frequent deposition of volcanic ash deposits to rice producing watersheds. Concentrations of phosphorus in rivers were between 1 and 4 mg l(-1) PO4, increasing downstream. We measured extractable potassium and phosphate levels in the soils of unfertilized Balinese rice paddies, and found them to be indistinguishable from those in fertilized paddies., and sufficient for high grain yields, Field experiments varying phosphorus applications to rice fields from 0 to 100 kg superphosphate per hectare (7-26 kg P ha(-1)) demonstrated small increases in harvest yields only with the smallest additions. Direct measurements of PO4 in irrigation waters indicate that most of the added phosphate flows out of the paddies and into the river systems, accumulating to very high levels before reaching the coast. (C) 2001 Elsevier Science B.V. All rights reserved.