Effects of radioactive phosphate fertilizers on yield and phosphorus uptake by ryegrass in pot experiments on calcareous soils from Rothamsted

Abstract

1. Two factorial pot experiments with ryegrass grown on calcareous soils from adjacent long-term experiments on Hoosfield, Rothamsted, are described. The effects of the method of application of phosphate, of the amounts of saP tested and of the level of phosphate applied are discussed with special reference to the manurial history of the soils.2. Yield and total phosphorus uptake by ryegrass were slightly greater in the early stages of growth when superphosphate was applied as a powder than when an equal amount of phosphate was applied in solution, but this effect disappeared in later cuts of grass. Total phosphorus uptake was not significantly altered by the levels of 32P tested, and yields were only significantly decreased at one sampling date in one experiment. Uptake of fertilizer phosphorus decreased and ‘A’ values increased, however, in both experiments at the higher rates of application of 32P.3. The addition of fertilizer phosphorus, as superphosphate or monocalcium phosphate, increased the uptake of soil phosphorus by ryegrass on all soils on which there was a yield response to phosphate. The recovery of fertilizer phosphorus, estimated radiochemically, was less, therefore, than the increase in phosphorus uptake by the crop on the soils on which there was a yield response to phosphate fertilizers.4. ‘A’ values were determined on all soils and were shown to be almost independent of two- and five-fold increases in the amount of labelled phosphate tested. ‘A’ values were related to the previous phosphate manuring of the soils and increased by about one-third of the difference in phosphate content on soils that had received heavy applications of superphosphate or farmyard manure over 50 years ago. The ‘A’ values of soils that had recently received superphosphate in the field decreased in 3 years by more than the amount of phosphate taken up by the crops. ‘A’ values of soils that received rock phosphate in the field were lower and did not decrease with time.