Relating the usda and isss soil particle size distributions




Different systems are in use for texture classification of soils. The two most common systems originated with the International Soil Science Society (ISSS) and the United States Department of Agriculture (USDA). These systems use different particle size cutoff limits to define sand percentages, namely, 0.02mm to 2mm in the ISSS and 0.05mm to 2mm in the USDA. To merge data sets with soil properties reported in the ISSS with those reported in the USDA system, it is necessary to convert the ISSS particle size distribution (PSD) to the USDA PSD or vice versa. Previous research converted sand or silt of a soil sample but allowed the PSD statistics of the whole sample to change. We developed an approach that held the sample PSD statistics, the geometric mean particle diameter and the geometric particle standard deviation, constant in the ISSS and USDA systems. The standard errors of the difference between the two approaches were ±2.21% and ±4.14% for clay and sand conversions, respectively, and the error of conversion in the present method was ±0.2%.