A pedotransfer function for estimating bulk density of forest soil in Japan affected by volcanic ash

Pedotransfer functions (PTFs) for estimating bulk density (BD) of forest soils in Japan affected by volcanic ash were developed using 3513 stone-free soil data. We found that 29 existing PTFs of five empirical types and one physical type generally overestimated the BD. This occurred because volcanic soil with the unique physical and chemical properties and forest soils with high pore volume resulted in lower BD and higher organic carbon content (OC) compared with other soil groups and produced a much lower bulk density of the ‘pure’ mineral fraction, a parameter of the physical PTF (1.153 in this study), compared with previous studies. Revised PTFs were newly developed by a non-linear least-squares method using the same algorithms of the six types of published PTFs. The best fit revised PTFs were a decimal one, BD=1/(0.882+0.133OC), and a physical one based on organic matter fraction (OM), BD=100/[(OM/0.140)+{(100−OM)/1.152}]. The revised PTFs predicted 37% of the samples to within ±10% and 67% of the samples to within ±20% in relative error for BD and explained 68% of BD variations. The variable mixture proportion of volcanic ash in the soils, the differences of soil textures, and the topographic conditions of the forest soils in Japan may have caused the variation in the observed BD.