SPATIAL DISTRIBUTION OF SOIL PARTICLE SIZE AND AGGREGATE STABILITY INDEX IN A CLAY SOIL

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Abstract
We measured aggregate stability index and particle size on the three fields differing in tillage regime. Field I was conventionally tilled, Field II was in transition between conventional and nontillage regimens, and Field III was in native pasture. We studied the spatial distribution of aggregate stability index and particle sizes using variography, kriging, and cokriging. With weighted least squares we fit variogram and cross-variogram mdoels to experimental variograms and then determined the best models by the leaving-one-out method ("jackknifing") of cross-validation. Using the average estimation variance as a criterion for comparison, we found that cokriging is only minimally better than kriging as an interpolator. Cokriging performed better when the convariables were highly correlated. Aggregate stability index was not correlated to texture in field I and was only slighlty correlated with silt and clay in field III.

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