Laboratory and field measurement of magnetic susceptibility of Japanese agricultural soils for rapid soil assessment

Measuring volume magnetic susceptibility (kappa) of soil allows rapid soil assessment. In this study, we measured the kappa value of agricultural surface soils in Japan at national and farm scales in order to evaluate the scale-dependent relationship between soil kappa and other soil properties and to examine the repeatability of the kappa value measured in the laboratory and in the field. A handheld field sensor was used to measure two sample sets: (1) 164 samples collected throughout Japan (national scale), and (2) 246 samples collected from paddy fields in Takatsuki and 117 (39 sites x 3 times) samples collected from paddy fields in Mifune (farm scale). Laboratory measurements showed that the coefficients of variation of soil kappa were 112% for the national scale samples, 13% for the Takatsuki samples, and 33-37% for the Mifune samples. The large variation at the national scale was due to several positive outliers, which were classified as Vitric or Silandic Andosols by the WRB classification. Repeated measurement of samples enabled estimation of the within-sample variation in kappa, and the data was considered unreliable if the coefficient of variation exceeded 10%. The correlation analysis using the reliable data indicated that at the national scale, the kappa value was most positively correlated with structural Fe (total Fe minus Fe-d) followed by Al-o and the andic properties (Al-o + 1/2Fe(o)). In contrast, for the Mifune samples collected at the farm scale, the kappa value was most positively correlated with sand content followed by K saturation percentage. Field measurements of soil kappa at the 39 Mifune sites indicated that the coefficients of variation of kappa at the same site were almost all below 10%. The within-site variations were due probably to the incomplete contact between the soil surface and the sensor. Our results suggest that measuring kappa of agricultural surface soils in Japan can help to distinguish Vitric or Silandic Andosols from other soils, and to estimate the kappa-related soil properties at the farm and national scales.

Automated summary

Measuring volume magnetic susceptibility (kappa) of soil allows rapid soil assessment. This study measured the kappa value of agricultural surface soils in Japan at national and farm scales, revealing the scale-dependent relationship between soil kappa and other soil properties.