The spatial variability of the soil characteristics colour, clay content, pH and penetrometer resistance was evaluated in seven areas on which strongly weathered and intensively cropped soils occur, in the Planalto Médio region, State of Rio Grande do Sul, Brazil. Nested analysis of variance was performed with the objective of assessing their homogeneity and optimal soil sampling density. Representative soil profiles were characterized and classified in these areas. Field work was performed from August to November, 1989. Kandiudox, Hapludox and Kandiudult occur in the studied areas. All characteristics evaluated, except soil colour value and soil pH, at 60-80 cm depth, presented high variability. The contribution to the total variance increased slowly from 50 m to 600 m sampling interval. The increase in variance was very large from 600 m to the distance between the areas (> 3,000 m), indicating that sampling intervals of 600 m or more are sufficient to study characteristics related to soil genesis. Variance in chemical and physical properties related to soil management will be present within the areas under investigation by adopting such sampling intervals. Even sampling schemes with intervals of 50 m would not be able to solve such variances.
variability of soil characteristics; spatial dependence; strongly weathered soils; soil classification