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Spatial Dependence and Interpolation of Soil Properties in West Sumatra, Indonesia: II. Co-regionalization and Co-kriging¹

ABSTRACT

Co-kriging was used to interpolate values of topsoil 0.5 M NaHCO₃-extractable P at 234 locations in West Sumatra, Indonesia, by exploiting its spatial covariance with a more densely sampled property, 25% HCl-extractable P. NaHCO₃-P and HCl-P had been sampled at 52 and 107 locations, respectively, in a nongeometric pattern across the 106 650 has region. Isotropic semivariograms of NaHCO₃-P and HCl-P and their cross semivariogram showed spatial dependence over 6.3, 4.2, and 10.4 km, respectively, at the sampling scale used. The map of co-kriged values for NaHCO₃-P showed a similar regional pattern but more local detail than that achieved by kriging from NaHCO₃-P samples alone. Co-kriging reduced estimation variances (relative to kriging) by up to 40% in areas where sampling density of NaHCO₃-P was lowest. Co-kriging variances exceeded those of kriging by up to 10% in areas where sampling density of NaHCO₃-P was high. In such areas, the covariate HCl-P had little effect on the interpolated value but still added a component to the estimation variance. Co-kriging could not be used to interpolate values for extreme locations where there were no NaHCO₃-P samples within the radius of the kriging neighborhood. Despite local improvement in estimation precision the full benefits of co-kriging interpolation are not universally obtained from nongeometric variate-covariate sampling patterns.

¹ Journal Series no. 0000000 of the Hawaii Institute of Tropical Agric. and Human Resources.

² Former Graduate Research Assistant, Associate Professor, and Professor of Soil Science, respectively, Dep. of Agronomy and Soil Science, Univ. of Hawaii. The senior author is now Soil Scientist, Soil Bureau, DSIR, Christchurch, New Zealand.

Received for publication November 4, 1985.

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