Designing Sustainable Landscapes: Soil available water supply, Soil depth to restrictive layer, and Soil pH settings variables

These three soils variables are among several ecological settings variables that collectively characterize the biophysical setting of each 30 m cell at a given point in time (McGarigal et al 2017). Soils are important drivers of natural communities. We picked three soil attributes that represent the most important factors: depth, chemistry, and water-holding capacity. Depth to resistant layer measures the depth of soils to a restrictive layer (e.g., bedrock) that limits root depth. Areas with shallow soils (usually on steep slopes or ridgetops) can’t support deep-rooted plants. Soil pH strongly affects nutrient uptake by plants. In the east, soils with higher pH (e.g., in areas with limestone bedrock) tend to support a greater diversity of plants, including a number of species that typically grow only in sweet soils. Conversely, some groups of plants (such as members of Ericaceae) are specialized to acidic soils, where generalist species grow poorly at best. Available water supply (AWS) measures the water-holding capacity of soils. It is measured as the total volume of water that available to plants when the soil, inclusive of rock fragments, is at field capacity. Soils with a high AWS are more drought-resistant, supporting plant growth through periods without rain—for instance, good agricultural soils have a high AWS. AWS is calculated as the available water capacity times the thickness of each soil horizon to a specified depth (25 cm in this case). Note that AWS is distinct from our topographic wetness settings variable, which estimates the amount of water delivered to the soil at each point.