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MICROSCALE PH SPATIAL DISTRIBUTION IN THE AP HORIZON OF MEXICO SILT LOAM

Conventional pH measurements treat soil pH as a random, independent variable for providing the mean pH of soil samples. This is assumed to represent the unsampled neighborhood. However the measurement will be inadequate if spatially dependent heterogeneity of the soil property exists among the samples. A laboratory study was conducted to determine microscale pH spatial dependence by the semivariogram function. Four soil cores, 13.5 cm in length and 10.5-cm in diameter, were taken from the Ap horizon of a Mexico silt loam (fine montmorillonitic, mesic Udollic Ochraqualfs). Soil pH was measured at 1-cm horizontal and vertical intervals by the pH-sensitive glass microelectrode, and the spatial variability of soil pH within the cores was examined. The pH decreased and the variability increased with depth. The homogeneity among samples was larger near the surface. Semivariograms indicated that semivariances depended on both distance and direction. In all cases, spatial variances in the vertical were larger than in the horizontal. Spatially dependent structures were exhibited by all samples, but only two cores expressed an identical range, and the nugget effect of these cores accounted for a large portion of the sample variance. Extent of pH spatial dependence in the other cores could not be determined because of unbounded variances. This study illustrated that soil heterogeneity and spatial dependence of pH in microscale exist and vary with sampling site and scale. Bulk soil pH measured by conventional procedures does not reveal variability that could have important impacts on plant root and microbial growth.