Particle-size distributions: Comparing texture systems, adding rock, and predicting soil properties

Conventional soil texture classification systems use different definitions of particle-size distributions (PSDs), For example, sand in the Internation al Soil Science Society (ISSS) system equals the combined separate limits o f coarse silt and sand in the USDA system. Because relationships between te xture and other soil properties are affected by these differences, the abil ity to merge survey data in environmental studies is limited. Previous rese arch calculated two PSD statistics, namely the geometric mean particle diam eter (dg) and its standard deviation (sigmag), which do not depend on separ ate limits. We expanded the development of the PSD statistics dg and ag to compare the USDA and ISSS systems, develop relationships with soil properti es, include rock fragments, and simplify the USDA texture classification to facilitate the use of soil survey data in environmental research. We found that (i) for equal clay and sand fractions, the texture of a soil sample a s described by the USDA system has larger dg and ag values than in ISSS; (i i) for equal clay and sand fractions, soil samples have larger values of ca tion-exchange capacity (CEC) in the ISSS than in the USDA system: (iii) sma ll differences between some of the traditional 12 USDA classes are reflecte d in the dg and og values for samples containing rocks, thereby presenting a rationale for simplification; and (iv) with this rationale, the 12 USDA c lasses were aggregated into five classes.