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PRECISION WITH WHICH SELECTED SOIL PHYSICAL PARAMETERS CAN BE ESTIMATED1

Under field conditions, the validity of many predictions concerning the movement of soil water and solute depends on the inherent variability of the soil parameters used in making the predictions. Data concerning the natural variability of many physical parameters of soils are lacking. A single soil series was studied over a land area 7.3 km in radius. Estimates were made of how some physical properties of soil vary with depth and with large and small land areas. Estimates of the number of samples required to achieve a given precision for each soil parameter were also made. The variability arising from location to location was found to be the major influence on the variation of soil-water conductivity. Restricting sampling to a radius of 5.6 km or sampling within morphological horizons reduced the variance of the soil-water conductivity. A range of 1 to 56 locations was required to achieve a standard deviation equal to 10% of the mean for the different physical properties of soil measured.