A bioassay for comparing phosphorus availability in soils.
Bioassay based on the uptake of 32P from 5 X 10-6 M phosphate solution during a 15 min period was tested on birch (Betula verrucosa Ehrh.) and sycamore Acer pseudoplatanus L.) seedlings grown in (a) sand culture supplied with 1-100 mu g P ml-1, (b) phosphorus deficient soils either unfertilized or fertilized (0.1 g P Kg-1 soil), and (c) a range of twenty-five soils varying from extremely deficient to adequate in available P content. There was a negative exponential relationship (r = -0.95) between phosphorus uptake by birch from the test solution and the concentration of phosphorus previously supplied in sand culture solution. Birch seedlings grown in fertilized soils took up less 32P from the bioassay solution than did those from unfertilized soils. Birch seedlings grown for 2 years in phosphate-deficient soils took up greater amounts of 32P from the bioassay solution than did those grown in soils with adequate phosphorus. Eighty-two per cent of the variation in phosphorus uptake by birch seedlings was accounted for by the extractable P content, the isotopically exchangeable P, the total P content and the phosphatase activity of the soils on which they were previously grown. Ninety-three per cent of the variation was accounted for if the plant phosphorus content and the mycorrhizal development on rootsystems were also included in the regression. The corresponding amounts of variation for sycamore seedlings grown on the same soils were 58 and 83% respectively. The 32P uptake from the bioassay solution by both sycamore and birch was markedly inhibited by 5 X 10-3 M KCN in the bioassay solution, indicating that uptake was metabolically mediated. The results are discussed in relation to the study of phosphorus deficiency in plants, and phosphorus availability in soils.