What it is: The amount of water in soil is based on rainfall amount, what proportion of rain infiltrates into the soil, and the soil's storage capacity. Available water capacity is the maximum amount of plant available water a soil can provide. It is an indicator of a soil’s ability to retain water and make it sufficiently available for plant use.
Available water capacity is the water held in soil between its field capacity and permanent wilting point. Field capacity is the water remaining in a soil after it has been thoroughly saturated and allowed to drain freely, usually for one to two days. Permanent wilting point is the moisture content of a soil at which plants wilt and fail to recover when supplied with sufficient moisture. Water capacity is usually expressed as a volume fraction or percentage, or as a depth (in or cm).
Why it is important: Soil is a major storage reservoir for water. Water availability is an important indicator because plant growth and soil biological activity depend on water for hydration and delivery of nutrients in solution. Runoff and leaching volumes are also determined by storage capacity and pore size distribution.
In areas where rain falls daily and supplies the soil with as much or more water than is removed by plants, available water capacity may be of little importance. However, in areas where plants remove more water than is supplied by precipitation, the amount of water held by the soil may be critical. Water held in the soil may be necessary to sustain plants between rainfall or irrigation events. By holding water for future use, soil buffers the plant – root environment against periods of water deficit.
Available water capacity is used to develop water budgets, predict droughtiness, design and operate irrigation systems, design drainage systems, protect water resources, and predict yields.
Specific problems that might be caused by poor function: Lack of available water reduces root and plant growth, and it can lead to plant death if sufficient moisture is not provided before a plant permanently wilts.
Agricultural land management practices that lead to poor available water capacity include those that prevent accumulation of soil organic matter and/or result in soil compaction and reduced pore volume and size:
What you can do: Farmers can grow high residue crops, perennial sod and cover crops, reduce soil disturbing activities, and manage residue to protect and increase soil organic matter to make improvements in a soil’s available water capacity. When feasible, tillage, harvest, and other farming operations requiring heavy equipment can be avoided when the soil is wet to minimize compaction; and compacted layers can be ripped to break them and expand the depth of the soil available for root growth.
Soils with high salt concentration tend to have reduced available water capacity because more water is retained at the permanent wilting point than if water was held by physical factors alone. These effects are most pronounced in soils in dry regions where salts accumulate because of irrigation or natural processes. For soil high in soluble salts, management activities that maintain salts below the root zone can be used. These include irrigation to leach salts below the root zone and practices that promote infiltration, reduce evaporation, minimize disturbance, manage residue, and prevent mixing of salt-laden lower soil layers with surface layers.
Conservation practices resulting in available water capacity favorable to soil function include:
For more information go to Soil Management Practices.
Measuring available water capacity:
U.S. Department of Agriculture, Natural Resources Conservation Service, 2005. National Soil Survey Handbook, title 430-VI. Soil Properties and Qualities (Part 618), Available Water Capacity (618.05). Online at: http://soils.usda.gov/technical/handbook/