Some phosphorus test values and fractions of inorganic phosphorus in soils

Authors

• Armi Kaila University of Helsinki, Department of Agricultural Chemistry

DOI: https://doi.org/10.23986/afsci.71636

Abstract

The relation between the P test values obtained by four methods and the inorganic phosphorus fractions of soil was studied on the basis of a material consisting of 346 samples of mineral soils originating from the surface layer and from the deeper layers. Fairly large differences were found in the results between the sand and finesand soils and the clay soils. The Bray 1 test gave higher average values in the former soils than in the latter group, and the contrary was true with the acetic acid test. On the average, the Bray 1 test and the Olsen test extracted equal amounts of phosphorus, the values of the acetate test were very low, and the acetic acid test values were between these except in the deeper layers of loam and silt soils and clay soils in which the acetic acid values tended to be higher than the other ones. The Bray 1 test values and the Olsen test values were closely correlated with each other, and somewhat less closely correlated with the acetate values. The acetic acid values were correlated with the other test values only in the sand and fine sand soils. The variation in the Bray 1 test values was most closely connected with the variation in the NH4F-soluble P, and only in the clay soils the NH4CI-soluble and the alkali-soluble fractions appeared to be of importance. In the Olsen test both the NH4F-soluble and the alkali-soluble fractions had to be taken into account, in clay soils also the NH4CI-soluble fraction. Adding the acid-soluble fraction did not increase the part of the variation in these two test values which could be explained. The acetate test value was most closely associated with the NH4CI-soluble fraction, except in the sand and finesand soils where the alkali-soluble P appeared to be of importance. Taking into account the acid-soluble fraction increased to some extent the part of the variation in the acetate values which could be explained by the variation in the three other fractions. The variation in the acetic acid test value, on the other hand, appeared to depend only on the acid-soluble fraction in other groups than the sand and finesand soils. Less than one half of the variation in it could be explained by the variation in these four fractions or soil pH. It is suggested that the Bray 1 test and the Olsen test would be recommendable for the measurement of the capacity factor of the soil P status.