Phosphorus mass balances for successive crops of fertilised rainfed rice on a sandy lowland soil

Raising and sustaining rice yields in the rainfed lowlands requires an understanding of nutrient inputs and outputs. On sandy lowland rice soils, managing phosphorus (P) supply is a key factor in achieving increased yields and sustainable production. Phosphorus inputs, rice yields, and crop P uptake were used to quantify P requirements of rice: together with results on soil P fractions, P balance sheets were constructed over five consecutive cropping seasons on a sandy Plinthustalf near Phnom Penh, Cambodia. Grain yields ranged from 665 to 1557 kg ha−1 with no added P. Average yields increased significantly with P fertiliser application over five consecutive crops by 117, 139 and 140% when the phosphate fertiliser was applied at 8.25, 16.5 and 33 kg P ha−1, respectively. Without added P fertiliser, a net loss of 1.2 kg P ha−1 per crop was estimated with straw return and 2.0 kg P ha−1 per crop with straw removed from the field, whereas, with added P fertiliser, there was a net P gain in the soil of 5.6 or 9.5 kg ha−1 per crop when straw was removed and returned to the soil, respectively. After one crop, the addition of P fertiliser significantly (P < 0.01) increased recovery in all soil P fractions. Across five successive crops, repeated application of 16.5 and 33 kg P ha−1 rates resulted in progressive P accumulation in the soil, especially a labile NaOH–Po pool, but had no effect on yields and P uptake of rice. By contrast, 8.25 kg P ha−1 per rice crop was generally adequate for grain yields of 2.5–3.0 t ha−1 and to maintain soil P pools