Comparing Conceptual Models Used to Map Soils In Arid and Semiarid Vs. Humid Soil Landscapes.

Soil scientists use the Soil-Landscape models to conduct soil survey. They observe the landscape, look for differences in topography, vegetation, and parent material, describe soil profiles, and group pedons into map units. To delineate soil map units, they use field observations and remote sensing information. Then, they relate the soil morphology and the spatial variability to climate and time factors, and develop a conceptual model to predict soil occurrence.

In arid and semi-arid areas of the Western United States, water is the most limiting factor in soil development. Soil scientists observe soil landscapes looking for water-receiving positions where soils may be more likely to approach field capacity for short periods of time. Vegetation is a good field indicator to predict soil occurrence. Vegetation type and density are strongly influenced by soil moisture, and small variations in available water content significantly affect composition of plant communities. Elevation, slope gradients, and slope aspect affect both soil temperature and evaporation. Dark reddish-brown or redder soil matrix colors are often associated with argillic horizons of Late Pleistocene or older ages and may be buried or exhumed soils.

Soil scientists mapping soils in humid areas also use soil-landscape models but with a different emphasis. While the mapping approach does include observations of soil landscapes and use of field indicators to predict soil catenas, the excess of water is often a significant factor. Soil scientists in humid areas look for flooding and ponding, drainage class, water tables, and leaching environments, and associate these conditions with soil attributes such as gley matrix color, texture, and redox features.