Efficiency of Different Phosphate Fertilizer Sources in Two Hawaiian Soils With Contrasting Phosphate Fixing Tendencies

The efficiency of fused magnesium phosphate, treble superphosphate, and treble superphosphate with calcium silicate was studied in a Gibbsihumox, and the efficiency of fused magnesium phosphate and treble superphosphate was studied in a Chromustert. Sudax was the indicator crop in this pot experiment. Also, the efficency of three granule sizes (normal size, coarse fraction, and fine fraction) of fused magnesium phosphate in a Gibbsihumox was studied in the same experiment. In both studies the plant and first ratoon crops of Sudax were analyzed for P,Si, Mg, Ca, K, and Mn. The two soils were analyzed for extractable P, sorbed P, extractable Si, Mg, Ca, K, Mn, Fe, Cu, Zn, and soil pH. Samples from the Halii soil were also analyzed for extractable Al. The dry matter yields of the plant and first ratoon crops of Sudax increased significantly with increasing levels of applied P in both soils. Treble superphosphate gave significantly higher average dry matter yields than fused magnesium phosphate in the two soils in the plant crop. However, in the ratoon crops average dry matter yields were nearly identical for the two phosphate sources. The application of calcium silicate with treble superphosphate to the Halii soil increased dry matter yields of both plant and ratoon crops, but the increase was not significant. The three granule sizes of fused magnesium phosphate applied to the Halii soil produced similar dry matter yields. Plant P concentrations were similar for fused magnesium phosphate, treble superphosphate and treble superphosphate with calcium silicate in the Halii soil; and for fused magnesium phosphate and treble superphosphate in the Lualualei soil. Plant Mg concentrations in the Halii soil were significantly higher for fused magnesium phosphate than for treble superphosphate with and without calcium silicate. In the Lualualei soil, however, plant Mg levels were similar with the two phsophate fertilizers. Plant Si concentrations were highest with fused magnesium phosphate in the Halii soil. Extractable soil P was significantly higher for treble superphosphate than fused magnesium phosphate in both soils. The addition of calcium silicate with treble superphosphate to the Halii soil increased soil P significantly. The phosphorus requirement of the soil was decreased by the application of calcium silicate. The amount of soil P extracted from the Halii soil for the three granule sizes of fused magnesium phosphate increased in the order fine fraction < normal size < coarse fraction. Phosphorus sorption studies showed that P sorbed by the Halii soil with the three phosphate sources was in the order treble superphosphate with silicate < treble superphosphate < fused magnesium phosphate. In the Lualualei soil the order was treble superphospate < fused magnesium phosphate. Fused magnesium phosphate increased soil pH from 4.1 to 5.6 in the Halii soil with the application of 800 Kg P/ha, and from 7.5 to 7.7 in the Lualualei soil with the application of 200 Kg P/ha. The same amount of fertilizer material reduced the level of soil Al in the Halii soil from 0.7 to 0.2 me/100 g soil. Fused magnesium phosphate appears to be a suitable source of P for highly weathered soils with high P fixing capacity and low pH, Ca, Mg and Si