A pedotransfer function to evaluate the soil profile textural heterogeneity using proximally sensed apparent electrical conductivity

Since soil texture is difficult to quantify, its determination is often restricted to the topsoil. To improve its mapping accuracy, observations of the apparent electrical conductivity (ECa) obtained from proximal soil sensors are increasingly being used due to its close relationship with soil texture under non-saline conditions. However, a single ECa measurement does not allow an unambiguous interpretation of both the topsoil texture and its vertical distribution, because of its bulk support over the entire profile. Therefore we selected 88 soil profiles with a ‘pseudo-homogeneous’ textural composition from a database containing 1500 profiles with nine textural fractions and the organic matter content analysed for every horizon. These profiles were revisited and the ECa was measured with an EM38DD sensor. With these data we obtained the following pedotransfer function (PTF): ECa-V = 16.2 + 1.314% clay, with R2 = 0.81. This function allows converting a topsoil clay percentage into a reference ECa value measured with the vertical coil orientation (ECa-V), assuming standard temperature and moisture conditions and a dominance of mica clay minerals. The ratio between the observed ECa-V and this reference ECa-V value is a measure of the soil profile textural heterogeneity: if this ratio is near to one, the profile is quasi-homogeneous; a ratio exceeding one indicates the presence of a more conductive material deeper in the soil profile, and vice versa. To explore this idea we considered an intensively surveyed area of 3000 km2 in Belgium with a wide soil textural variation. Based on our PTF, a regional-scale topsoil clay map was converted into a reference ECa-V map which was found to be a valuable aid in evaluating the soil profile textural heterogeneity by field-measured ECa-V data.