Enhanced plant growth by uniform placement of superphosphate with rock phosphate in acidic soils

Partially acidulated rock phosphate or compacted soluble phosphate-rock phosphate mixture has been suggested as an alternative phosphorus (P) source for plants in acidic soils. Interaction between the soluble and the insoluble fractions would be conducive to plant utilization of both P sources in the fertilizer material. Direct evidence supports the beneficial reactions between the two P sources, however, was still insufficient. A pot experiment was hence conducted to evaluate the possible interaction of 32P-labeled single superphosphate (SSP) and a less reactive rock phosphate (RP) from Jinxiang mine, China. In the experiment, SSP and RP were applied to two acidic red soils (typic Hapludults) in a manner that would favor or diminish their interaction either by distributing both P sources homogeneously within the whole volume of the pot soil (uniform placement) or by separating them vertically with each being applied to half of the soil volume (fraction placement). The reference treatments of SSP and RP were arranged in similar manners. Two successive harvests of ryegrass were made during a 2-month period. Results indicated that uniform placement of SSP and RP significantly enhanced plant growth and P uptake, and that P recovery of SSP-P in the higher P-fixing soil was almost twice that of the fraction placement. The data of plant PdfL% (percentage of P derived from labeled-P), which indicated that >80% of plant P was derived from SSP, however, failed to support the idea that soluble P had increased plant utilization of RP. Alternatively, it is considered that the low grade RP had increased plant utilization of both SSP-P and soil-P. Uniform placement of RP and SSP also strikingly improved plant calcium (Ca) and magnesium (Mg) nutrition. The phenomenon suggested that SSP-RP mixture might be a good P source on similar acidic soils in subtropical China.