Use of phosphate in land reclamation for agricultural use [in Indonesia].

Since 1985 sugar yields in Gunung Madu Plantations (Ultisols) have reached the 6 tonnes/ha level and went on to peak at 7.05 tonnes/ha in 1989. Prior to 1983 sugar yields never exceeded 4 tonnes/ha. The main nutritional factor limiting yields initially was inadequate P and prior to 1979 rock phosphate was applied at 120kg/ha. To improve the uptake efficiency of other major nutrients it was essential to build up adequate levels of soil P. Once this was achieved, P input could be reduced to maintenance level while K was increased to boost yield further. The improved uptake efficiency also meant N could be reduced with no detrimental effects to sugar yields. The application of 1 tonne/ha of reactive rock phosphate on legume creeper Mucuna cochinchinensis planted on ultisols significantly doubled the dry matter biomass production at G. Madu (highest yield 7.2 tonnes/ha) and Terbanggi (highest yield 13.6 tonnes/ha). This trend is also similarly reflected in the chemical composition of Mucuna spp at Terbanggi where without P (control) it was 0.12percent P and with North Carolina Rock Phosphate it was 0.25percent P. In terms of kg N or O/ha at Terbanggi, nitrogen was more than doubled and phosphate was nearly quadrupled in the P-treated plots as compared to control. Where alley cropping was practised, the dry mater yield of Flemingia congesta used as the alley crop was improved significantly where reactive rock phosphate was applied. A top yield of 3.61 tonnes/ha was achieved compared to control. Grain yield (expressed in tonnes cal/ha) of the first season's interrop at G. Madu (i.e. maize and rice) were about five times greater than the control (3.2 tonnes cal/ha) plots where no reactive rock phosphate was applied. At both sites, the yields of P-treated plots varied between 16.0-17.8 tonnes cal/ha. Except for G. Madu, yields between TSP and reactive rock phosphate were not significanly different. In the second season the yields of inctercropped maize and soybean in the P-treated plots were also significantly higher than control at both sites. However, yields at Terbanggi were nearly double those achieved at G. Madu. The highest yield at Terbanggi was 9.9 tonnes cal/ha while that at G. Madu was 6.3 tonnes cal/ha, both were MPR (Moroccan phosphate rock) plots. The control plots at G. Madu was a total failure with zero yield. It is hoped that the initial high P-input coupled with the fast growing legume Mucuna after clearing can subsequently create a viable and sustainable cropping system while rehabilitating the abandoned alang-alang land. Subsequent crop yield would only require maintenance fertilizer dosage to sustain economic yield.