Vertical distribution of magnetic susceptibility in permeable sandy soils

Magnetic susceptibility represents very useful proxy tool to assess the content of ferrimagnetic forms of iron oxides in soils. However, magnetic susceptibility of highly-permeable, sandy soils is very rarely described. The aim of this study is to report on measurements of magnetic susceptibility on the surface and along shallow vertical profiles of sandy soils distinguished in the Polish classification system as rusty soils, ochrous (ochre) soils and Podzols. According to the International soil classification, the first two correspond best to Brunic Arenosols and Rhodic/Chromic Arenosols (Tsitelic), respectively. In total, ten sites were investigated. At each site, surface measurements were performed following a regular rectangular grid with 5-m spacing and total size of 30x30 m. At selected points, vertical soil cores down to depth of maximum 30 cm were collected. Our results show that vertical distribution of magnetic susceptibility is characterized by very different patterns and allows their assignment to two groups. While two sites with rusty soils and all sites with Podzols show distinct, narrow magnetically enhanced layer just below the surface, all four sites with ochrous soils and one site with rusty soil exhibit thick magnetically enhanced layer. In case of ochrous soils, this layer is of much higher magnetic susceptibility than at the other sites. In addition, spatial scatter of magnetic susceptibility in individual depth layers is much bigger in the latter group of soils. Finally, spatial variability of model values of susceptibility, determined as weighted sums of vertical profiles, correlate well with the surface measurements in the case of ochrous soils, but lack of correlation is observed in the rusty soils and Podzols. This study provides the first comprehensive data on magnetic susceptibility of ochrous soils. Finally, our results point to the considerations related with the proper interpretation of magnetic susceptibility measured on the soil surface.