Effects of condensed phosphates on plant growth and phosphorus uptake

Four condensed phosphates with large and small molecules, including ring and chain structures, were equivalent to orthophosphate in terms of phosphorus uptake and the dry matter yield of ryegrass grown in two soils in pots. This equivalence was maintained at each of 6 cuts during two seasons. Increasing N rate greatly increased uptake of applied phosphorus but there was no differential effect with different sources of phosphorus, nor was there any interaction between source and application rate. Phosphorus was not leached at any time, indicating that the phosphates were rapidly adsorbed by the soils. Fairly rapid hydrolysis of all the condensed phosphates occurred in a soil of neutral pH but a slower hydrolysis seemed to occur in an acid soil. There was a good correlation between the phosphorus uptake by ryegrass and the additional orthophosphate released by an acid hydrolysis of soil extracts.