Path Analysis of Phosphorus Retention Capacity in Allophanic and Non-allophanic Andisols

Abstract

Andisols derived from volcanic ash minerals are characterized by a high P retention capacity (PRC). The PRC of soil is often correlated with various soil properties, and a simple correlation analysis alone may not necessarily explain the direct cause and effect relationships between soil properties and PRC values. The objectives of this study were to determine which soil properties best explain the variability in the PRC of allophanic and non-allophanic Andisols using path analysis. A total of 671 Japanese Andisols were used in this study. The PRC values were determined for allophanic and non-allophanic soil samples, along with pH, organic matter (OM) contents, acid ammonium oxalate extractable Al (Alox) and Fe (Feox), and pyrophosphate-extractable Al (Alp) and Fe (Fep). The PRC value of allophanic soils was correlated with Alox (r = 0.72) and Feox (r = 0.56). Path analysis revealed, however, that Alox was the single most important direct effect on PRC, and the correlation between PRC and Feox was mostly partitioned to an indirect effect through Alox. According to path analysis on non-allophanic soils, Alp was the single most important causal factor in predicting PRC, and the correlation between Alox and PRC (r = 0.77) was mostly partitioned to an indirect effect through Alp. The correlation between OM and PRC was attributable to organically bound Al in allophanic soils by the indirect effect of Alp on PRC through OM. Pyrophosphate extraction may be a better indicator than oxalate extraction for predicting PRC in non-allophanic Andisols.