Good question. Perhaps we should look at it beginning at the order level. The distinguishing characteristic of Entisols is the lack of pedogenic features. From there, the separations are based upon being underwater (Wassents), wet (Aquents), sandy (Psamments), of fluvial origin (Fluvents), or all the other soils without pedogenic features (Orthents). In the Guy Smith interviews conducted in early 1980s, he discussed some taxa of the Entisols being extremely important agriculturally with more of the worldwide food production on Entisols than any other order.
Entisols occur in a few rather distinct settings. Entisols occur as very shallow soils, usually steep, actively weathering and eroding bedrock on unstable landscapes. Somewhat opposite that, Entisols occur as very young deep alluvial deposits on floodplains. Entisols occur as metastable deposits on uplands as sand dunes often dominated by resistant minerals with little potential to form pedogenic features. Entisols occur on old stable, weathered landscapes such that pedogenic features which had developed are only found at depths below 2 meters - the depth of classification. Finally, Entisols occur as man altered and transported soils.
At the order level, Entisols are distinguished by their lack of pedogenic features. At the suborder level, Entisols are distinguished by their dominant feature affecting agricultural value (submerged, wetness, unstable/poor growing medium, flooding, and others). At the great group level, soil climate (temperature, wetness, aridity) and mineralogy serve as is the main determinant. At the subgroup level, features of pedogenic processes not significant enough for diagnostic criteria are included, as well as soils impacted by man’s activities.
Back to the question, “Why no Fluventic subgroups in Psamments?” Perhaps the agriculturally significant Entisols which occur primarily upon floodplains are not often devoid of silts and clays, thus fit into finer textured Fluvents rather than Psamments. Psamments are often not intensively used for cropping due to their limitations in moisture supplying capacity (droughty), low inherent fertility (or ability to hold and supply nutrients - CEC), low trafficability/stability. Sufficient organic carbon deposited in a fluvial setting (Fluventic) to effect agricultural production in a matrix dominated by sand would likely be a short lived characteristic.
Key to your question regarding the lack of Fluventic great group in Psamments may be the lack of either awareness or concern that the slight increase in organic carbon from fluvial additions has upon the species and production of native vegetation. Until this time, existing taxa were apparently adequate for soil survey and soil classification with the emphasis upon intensive uses of the land. A Fluventic subgroup in Psamments was not needed or perhaps not felt to be useful for to convey the value or management of those soils. As we have expanded the soil survey program to the identification and correlation of specific ecological functions and values, it may be time to entertain the addition of such taxa.
The addition of Fluventic subgroups in Psamments seems consistent with the eight basic attributes of Soil Taxonomy (Chapter 2, Soil Taxonomy, 1999). Seems all that is needed to move this forward is a proposal to amend Soil Taxonomy adding the appropriate subgroups supported with descriptions, documentation, and justification as to why this would improve the science.
