Volcanic fertilization of Balinese rice paddies

Abstract

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.

Introduction

Balinese farmers have long regarded volcanoes as homes of the goddesses who bring fertility to their fields. Nutrient-rich volcanic soils combined with microbial nitrogen fixation and traditional harvest methods that left much of the plants in the fields meant that farmers growing traditional slow-maturing rice varieties could escape the need to fertilize their rice paddies. But following the introduction of fast-growing high-yield ‘Green Revolution’ rice in the 1970s, Indonesian farmers have been encouraged to add all necessary nutrients in the form of chemical fertilizers to their rice paddies. Indonesia has invested heavily in fertilizer production facilities and has provided substantial subsidies for fertilizer use since the 1970s, with maximum retail prices for P and K never more than half the world price. Fertilizer subsidies ‘constitute a significant financial burden to the government’ (Roche, 1994).

Significantly, the recommendations for fertilization were made without taking into account the potential contribution of natural sources (Roche, 1994, p. 63). In 1998, all Indonesian paddy rice farmers (including the Balinese) were urged by the Ministry of Agriculture to add 33 kg phosphorus [added as 125 kg Ca(H₂PO₄)₂, SP-36 ‘super-phosphate,’ equivalent to 101 kg PO₄] and 100 kg potassium (as KCl) per hectare of rice paddies for each rice crop (Indonesian Ministry of Agriculture, 1998). In Bali, usage lags below this target. Actual usage in 1998 was 3 448 000 kg SP-36 on 155 304 ha of paddies, equivalent to 5.9 kg P ha⁻¹ (or 18 kg PO₄ ha⁻¹).

Our research presented here indicates that phosphate and potassium are continuously leached from the volcanic soil by rainfall, and delivered to rice paddies by irrigation systems in quantities sufficient for abundant rice harvests. The superfluous P fertilizer applied by the farmers for the past three decades washes directly into the rivers, representing a significant ecological threat to waterways and perhaps coastal waters, as well as a certain economic burden on the farmers.