Relationship between the 0.01 M CaCl₂- and AL-soluble soil phosphorus contents

The traditional Hungarian method for determining soil phosphorus (P) status is ammonium-lactate acetic acid (AL) extraction. AL is an acidic solution (buffered at pH 3.75), which is also able to dissolve P reserves, so there is a need for extraction methods that also characterize the mobile P pool. 0.01 M CaCl₂-P is considered to directly describe available P forms, because the dilute salt solution has more or less the same ionic strength as the average salt concentration in many soil solutions. The amount of AL-P may be two orders of magnitude greater than that of CaCl₂-P. Previous studies suggested that the relationship between AL-P and CaCl₂-P was influenced by soil parameters. Regression analysis between AL-P and CaCl₂-P showed medium or strong correlations when using soils with homogeneous soil properties, while there was a weak correlation between them for soils with heterogeneous properties. The objective of this study was to increase the accuracy of the conversion between AL-P and CaCl₂-P, by constructing universal equations that also take soil properties into consideration. The AL-P and CaCl₂-P contents were measured in arable soils (n=622) originating from the Hungarian Soil Information and Monitoring System (SIMS). These soils covered a wide range of soil properties. A weak correlation was found between AL-P and CaCl₂-P in SIMS soils. The amounts and ratio of AL-P and CaCl₂-P depended on soil properties such as CaCO₃ content and texture. The ratio of AL-P to CaCl₂-P changed from 37 in noncalcareous soils to 141 on highly calcareous soils. CaCl₂-P decreased as a function of KA (plasticity index according to Arany), which is related to the clay content, while the highest AL-P content was found on loam soils, probably due to the fact that a high proportion of them were calcareous. The relationships between AL-P, CaCl₂-P and soil properties in the SIMS dataset were evaluated using multiple linear regression analysis. In order to select the best model the Akaike Information Criterion (AIC) was used to compare different models. The soil factors included in the models were pHKCl, humus and CaCO₃ content to describe AL-P, and KA, CaCO₃ content and pHKCₗ to describe CaCl₂-P. AL-P was directly proportional to pHKCₗ, humus and CaCO₃ content, while CaCl₂-P was inversely proportional to KA, CaCO₃ content and pHKCₗ. The explanatory power of the models increased when soil properties were included. The percentage of the explained variance in the AL-P and CaCl₂-P regression models was 56 and 51%, so the accuracy of the conversion between the two extraction methods was still not satisfactory and it does not seem to be possible to prepare a universally applicable equation. Further research is needed to obtain different regression equations for soils with different soil properties, and CaCl₂-P should also be calibrated in long-term P fertilization trials.