SPATIAL VARIABILITY IN PENETRATION RESISTANCE OF A HARDSETTING TROPICAL ALFISOL

Geostatistical techniques were used to analyse the spatial variation o f penetration resistance on an experimental plot intended for root stu dies. Penetration resistance was measured at two soil water conditions . Penetration resistance exhibited spatial structure but the models de scribing the semivariograms were different for the two soil water cond itions. An isotropic linear model provided the best fit for penetratio n resistance in the dry soil while an isotropic spherical model was us ed for penetration in the wet soil. A complementary study of the spati al structure of water content also showed a similar trend. Cross-semiv ariograms were constructed to determine the spatial relationship betwe en penetration resistance and water content. Penetration resistance in the dry soil was negatively correlated with water content. The nugget variances as the percentage of the sill in the wet soil data set sugg est that the topsoil was slightly more variable than the subsoil. The spatial scale of variation in penetration resistance of the wet soil w as 33 m at 7.5 cm depth and 20-27 m at 15-30 cm depth. Punctual krigin g was used to estimate the penetration resistance and water content va lues. The estimated values are presented as contour maps. The pattern of variation and the underlying possible processes for the variation a re discussed. The results suggest that the likely influence of spatial variation of soil properties on crop growth may have to be considered in modelling in order to simulate the real field situation.