Soils form distinct layers as biological, physical, and chemical processes develop zones of material accumulation and zones of loss, also called horizons. This evolution is predicable as long as climate and other processes remain constant. This assumption of constancy poses problems for these types of classification systems, especially for those aridic soils that partially developed under humid conditions. It is possible to have more than one classification for the same soil, depending on which soil forming process is assumed to be expressing itself. Due to this and other problems, the USDA abandoned its system based on soil forming processes and developed the system layed out in Soil Taxonomy based on existing quantifiable characteristics.
Engineers also use systems based on existing characteristics, for obvious reasons.
Criteria for distinguishing soils are not necessarily the same for different classification systems. Soil texture is a good example of the problems involved in correlating classifications between two or more systems. Texture describes the proportion of different size classes of the mineral part of the soil. For the same soil, texture-class names can differ depending on the classification system used. Other physical, chemical, and biological characteristics have similar discrepancies between systems which makes one to one correlation between systems nearly impossible unless a detailed soil description is available.