Agronomy Library > Soil Conservation

Calculate Plant Available Water based on Soil Texture (Modified from Montana Data)
Author: USDA Cooperative Extension Service, modified by Don Wentz
Date Created: January 31, 1984
Last Reviewed: February 16, 2007


Soil Texture Class

Estimated Average Plant (in/foot & mm/30cm)

Sandy Soil        Course Texture


Loamy sands

Loamy fine sand

Loamy very fine sand

Fine sand

Very fine sand

0.5 in/foot     12.7 mm/30cm

1.0 in/foot     25.4 mm/30cm

1.25 in/foot   31.75 mm/30cm

(25.4 mm equals 1 inch)

(30.54 cm equals 1 foot)

Loamy Soils     Medium Coarse Texture

                                             Medium Texture


                           Moderately Fine Texture

Sandy loam

Fine sandy loam

Very fine sandy loam


Silt loam/silt

Clay loam

Sandy clay loam

Silty clay loam

1.5 in/foot     38.1 mm/30cm

2.0 in/foot     50.8 mm/30cm

2.2 in/foot     55.88 mm/30cm

Clayey Soils     Fine Texture

Sandy clay

Silty clay


2.2 in/foot     55.88 mm/30cm

Copied from "Soil Water Guidlines and Precipitation Probabilities for Barley, Spring Wheat and Winter Wheat in Flexivle Cropping Systems in Montana and North Dakota"
Extension Service Bulletin 356, Reprinted January 1988
1. Approved by the Soils Committee, MSU, Plant and Soil Science Planning conference, January 31, 1984
2. Soil depth measured to depth of crop rooting or depth of root limiting layers. 


Soluble salts and gravel will decrease plant available water capacity (PAWC); whereas organic matter, and good soil structure will increase it. The capacity increases about 1.0 inch (25.4mm) for each 1% organic matter (OM). Soils with water restricting layers like compaction sub-soil, shallow bedrock or stratification can increase PAWC of the overlaying layers. Soils that are deep, medium textured, and uniform can have decreased PAW but allow for deeper rooting.