Glasshouse comparison of North Carolina phosphate rock, alkali-treated Christmas Island phosphate rock and single superphosphate as P fertilizers on an acidic soil

Abstract

Three fertilizers were compared for their P release to pasture species growing on an acidic soil under glasshouse conditions. The fertilizers were a reactive apatitic phosphate rock (North Carolina, NCPR), a KOH-treated crandallite/millisite/goethite phosphate rock (Christmas Island C-grade ore, KC) and single superphosphate (SSP). Four successive crops were grown after an initial application of five rates of each fertilizer. The effect of P recycling on the yield responses to fertilizers was assessed in one set of treatments by replacing P removed in successive herbage harvests with P in sheep dung. NCPR and KC were not as effective as SSP for the first crop, and effectiveness decreased as fertilizer application rate increased. With subsequent crops (24) and for total dry matter production data (crops 1-4 combined) the relative effectiveness of NCPR depended on P recycling treatment. When P was removed at each harvest, NCPR was just as effective as SSP, but KC was less than one third as effective as SSP. When P was recycled after each harvest, NCPR was less effective than SSP, and KC was at best only one-third as effective as SSP. After three crops, the residual value of the three fertilizers was assessed against crop response to a fresh application of SSP. Residual value of all fertilizers was significantly increased by recycling P to the pots. KC had a significantly lower residual value than the other fertilizers, but there was no significant difference between the residual value of initially applied SSP and NCPR. Solubility of P in the SSP and KC fertilizers as measured by water and neutral ammonium citrate was well correlated with plant response to the fertilizers, while the performance of NCPR was underestimated by these methods.