Relationships between labile‐P, NAHCO3‐soluble P, and mobilisation of non‐labile P in soils treated with inorganic phosphate fertilisers and organic manures

Weighted‐mean L‐values and uptakes of P by plants were determined using ryegrass grown in a controlled environment on soils obtained from long‐term field experiments at Rothamsted (Batcombe and Winchester soil series), Woburn (Stackyard and Cottenham series) and Saxmundham (Beccles series) given different organic and inorganic manurial treatments. Linear regression analysis showed that, under the experimental conditions used, (small soil volume, weekly additions of all nutrients other than P) initial soil NaHCO3‐soluble P accounted for more than 95% of the variance of plant‐P uptake and weighted‐mean L‐values. The relationships between NaHCO3‐soluble P, plant‐P uptakes and weighted‐mean L‐values (labile P) were all independent of soil type and form of P fertiliser applied. In most soils, L‐values increased with increasing cut number, and the rate of increase was linearly related to initial soil NaHCO3‐soluble P, suggesting that NaHCO3‐P measurements may provide useful information on the availability of initially non‐isotopically exchangeable soil‐P reserves. The plants removed about 68% more P from the soils than was measured as initially NaHCO3‐soluble and about one‐half the labile soil‐P. Labile soil‐P was 3.5 times greater than initial NaHCO3‐soluble P. The initial soil NaHCO3‐soluble P required to provide at least 95% of maximum yield increased with increasing cut number (from 37 mg P kg−1 soil at cut 1 to 160 mg P kg−1 soil at cut 4), probably because the NaHCO3‐P pool was replenished more slowly than P was removed by the plants.