Correction for seed‐phosphorus effects in L‐value determinations

Measurements of the specific activity of plants grown in 32P‐labelled soils have frequently been used to estimate soil labile phosphorus (L‐values). It is usually assumed that the plant‐P uptake is totally derived from soil‐P. The contribution of seed‐P to plant‐P uptake and L‐values is seldom considered. This paper describes results of an experiment which measured the effects of seed‐P on L‐values and how correction for seed‐P effects may be made when only the tops of plants are analysed. L‐values were obtained from three harvests of ryegrass grown in three soils of very low, low and medium P status (soils I, II and III), containing 4, 14 and 37 mg initial NaHCO3‐soluble P kg−1 soil, and supplied with increasing amounts of added P. Ryegrass, supplied with increasing amounts of 32P‐labelled 31P, was simultaneously grown in P‐free sand, under identical experimental conditions to the soil‐grown plants. The seed‐P in the tops of the sand‐grown plants, at increasing dry matter yields, was calculated and used to make appropriate corrections for seed‐P in the tops of soil‐grown plants. After correction for seed‐P effects, L‐values were reduced at the first harvest by 69% in soil I, 18% in soil II and 10% in soil III, at the second harvest by 27, 11 and 5%, and at the third harvest by 18, 6 and 4%, respectively. Increasing the amount of added P to soil‐grown plants also caused some reduction in L‐values in soil I but the effect was much less in soils II and III. The reduction in L‐values at higher rates of added P in soil I was probably a result of increased P uptake diluting the effects of seed‐P. In the other two soils, plant‐P uptake was much greater, at all rates of added P, and this effect was very small.