Water Holding Capacity of Soils

Available Water Holding Capacity (AWC) of soil is the amount of water held in the soil for crop growth and yield. Field capacity is the amount of water a soil will hold against gravity at a water tension of 1/3 atmosphere. Permanent wilting point is the point where plants cannot obtain more water and remain wilted (water tension of fifteen atmospheres). The goal in soil health is to improve the soil structure to increase infiltration and improve organic matter to increase water-holding capacity. Available water holding capacity (AWC) is the quantity of total plant available water a soil can provide to a growing crop. This is a soil health test offered at Ward Laboratories.  A recent publication,  Carbon-Sensitive Pedotransfer Functions for Plant Available Water, has developed calculations for field capacity and wilting point. This publication allows us to provide AWC to soil customers quickly.

Soil Health Practices

There are soil characteristics like texture (percent sand, silt, and clay) that influence AWC but do not change with time. However, there is another soil characteristic, organic matter, which influences AWC. Management practices such as No-Till, crop diversity and cover crops will increase soil organic matter, build soil structure, and increase porosity. Tillage and traffic are management practices that negatively affect soil structure and porosity. Thus, these also negatively impact AWC. Minimal soil disturbance increases microbial activity that form glues binding sand, silt and clay together forming water stable aggregates.  Increased water stable aggregates improve water infiltration capturing more rain and irrigation water. Building stable organic carbon is a necessary goal for increasing AWC.

Laboratory Analysis

Laboratory tests needed to calculate AWC:

1. Soil pH and excess lime
2. Soil organic carbon (substitute organic matter when needed)
3. Clay percentage
4. Sand percentage

Field capacity and wilting point are calculated from these four parameters for calcareous and noncalcareous soils. When requesting AWC we will analyze these four parameters. If you are testing your soil for pH, organic matter and plant nutrients, ask for AWC and soil texture by hydrometer to get field capacity and wilting point values for your fields.

There are some interesting calculations that you can develop from Ward Laboratories, Inc reports field capacity and wilting point for the AWC test.

Parameters needed:

1. Weight of water per gallon
2. Pounds of water per cubic foot
3. Square feet per acre
4. Bulk density of the soil in question (assume 1.325 g/cc if bulk density is unknown)
5. Depth of soil sample
6. Field capacity (% by volume)
7. Wilting point, % by volume

The table below is an example and entering bulk density (default value is 1.325), soil depth in inches, % field capacity, and % wilting point all the calculations will be reported.

Table 1. Calculations of available water for crops in gallons and inches of water per acre.

Givens:
Weight of water per gallon	8.34
Pounds of water per cubic foot	62.4
Square feet per acre	43560
Analytical Data:
Bulk Density of Soil, g/cm3	1.325
Depth of Soil Sample, inches	6
Field Capacity, % by vol	34
Wilting point, % by vol	14
Calculations:
Gallons of Water in one acre inch of water	27,160
Gallons of water at Field Capacity/Acre inch of soil	12,235
Gallons of water at Field Capacity/Acre 6 inches of soil	73,413
Gallons of water at Wilting point/Acre inch of soil	5,038
Gallons of water at Wilting point/Acre 6 inches of soil	30,229
Gallons of Available Water for Plants per Acre	43,184
Inches of water per acre for inches of soil depth	1.59

Download Spreadsheet for Water Holding Capacity Calculations