An evaluation of the phosphate potential, Truog, Olsen, and Morgan methods for measuring the availability of soil phosphate

Abstract

The equilibrium potentialsP was compared with three of the better conventional extractants, namely, Olsen's (0.5M NaHCO3), Truog's (0.002M H2SO4), and Morgan's (0.5M HOAc/NaOAc buffer), on a range of soils of uniform calcium status, using lucerne (Medicago sativa L.), and on soils of variable calcium status, using Phaseolus lathvoides L. Relative yield and phosphorus uptake correlated better with soil test than did absolute yield response to phosphorus, for all methods. On the soils of uniform calcium status, the equilibrium potentialSP and Olsen methods were comparable and were superior to the Truog and Morgan methods. There was little to choose between the four methods on the soils of variable calcium status, when a very acid soil containing a high level of exchangeable aluminium was excluded from the correlations. Contrary to previous findings, the predictive value of the equilibrium potentialSP was high. The discrepancy may be explained by differences in the method of measuring the potential. Of the conventional extractants, one such as Olsen's bicarbonate, which reflects primarily the intensity of the phosphate supply of a soil and does not attack phosphate unavailable to plants, is to be preferred to those that reflect quantity rather than intensity.