A greenhouse pot experiment was conducted with paddy rice (Oryza sativa L. var. ‘Nato’) grown on Mountview silt loam at its initial low P level and on this soil receiving preplant unlabelled P applications (medium and high soil P levels). Labelled granular CSP (concentrated superphosphate) alone, a 2/1 mixture of CSP and DCP (dicalcium phosphate) and a 1/2 CSP-DCP mixture were evaluated.

Dry-matter yields and tillering increased markedly with amount of applied P on the low P soil, and slightly on the medium P soil; there was no response on the high P soil. Response decreased with decrease in water solubility of the applied P at the low and medium soil P levels. Total uptake of P by rice showed similar trends, except that uptake increased significantly with amount of P applied at the medium soil P level.

Percentages of plant P from the labelled fertilizer and uptake of fertilizer P increased in all situations with amount of applied P, decreased with decrease in water solubility of the applied P, and decreased with increase in soil P level. Uptake of soil P increased markedly with increase in soil P level. Soil labelling was much less satisfactory than fertilizer labelling for evaluating the effectiveness of P in different fertilizers.

A solution of labelled CSP was applied in a second experiment at time of transplanting and 3 or 6 weeks later. On the low P soil, yields, P uptake and tillering increased with rate of applied P but decreased with later date of application. On the medium P soil, yields, tillering and uptake of P by the grain were rather similar among treatments. Uptake of P in the foliage and mature straw, however, increased with later date of application. These results indicate that late-applied P is readily absorbed by rice but is not effective for increasing yields.