Soil Water
Soil Water Thresholds
Soil water thresholds are specific values of soil water content indicating water availability for plant consumption. The water content of soil is particularly important, as it influences its moisture, the amount of nutrients available to plants, and the aeration state of the soil. Soil water refers to water that exists naturally in soil as gravitational water, capillary water, and hygroscopic water, depending on the functions of the soil. The term gravitational water refers to the portion of soil water that drains freely under gravity following a saturation event (e.g., rain irrigation) until it reaches field capacity. The upper limit of water storage is often called “field capacity” (FC), while the lower limit is called the “permanent wilting point” (PWP).
Saturation
At saturation, which usually occurs immediately after a heavy rainfall or an irrigation application, all pore spaces in the soil are filled with water. When the soil is at or near saturation, some of the water is free to drain or percolate due to the force of gravity. This excess water is referred to as gravitational water. It is lost to the groundwater table within 1 or 2 days after cessation of rainfall or irrigation. It is of little use for plant growth because of its transitory state.
Field Capacity
When a saturated soil is allowed to drain freely under the force of gravity and if there is no evaporation loss, moisture content of the soil will approach an equilibrium level after a time. Typically, this will occur within one to three days where the rapid downward movement of water will become negligible as matric forces play a greater role in the movement of the remaining water (Section 7.3). The soil then is said to be at its field capacity (FC). Usually, it is the upper limit of moisture content that a soil can retain after it has been thoroughly wetted and allowed to drain freely.
Permanent Wilting Point
Permanent wilting point is the threshold where it becomes impossible for plants to extract water at a rate fast enough to keep up with their water demand. At permanent wilting point, soil particles hold the water so strongly that it becomes difficult for plant roots to extract it. The permanent wilting point is typically determined by measuring the water content of soil samples at different pressures in the lab, and determining the point at which the soil can no longer release water to plants.
Plant Available Water
Plant available water capacity (PAW) is the maximum amount of water stored in a soil profile that can be used by plants. PAW is the difference between soil water content at field capacity and permanent wilting point. Water capacity is usually expressed as a volume fraction or percentage, or as a depth (in or cm). PAW is affected by soil texture, soil depth and restrictive layers, organic matter content, soil compaction, and salt content. Available water capacity increases with increasingly fine textured soil, from sands to loams and silt loams (Table 7.1). Coarse textured soils have lower field capacity since they are high in large pores subject to free drainage. Fine textured soils have a greater occurrence of small pores that hold water against free drainage, resulting in a comparatively higher field capacity.
Management Allowable Depletion
Management allowable depletion (MAD) is the portion of the plant available water that can be depleted before plants experience water stress and potential growth reduction (consequently yield reduction). Although plants can extract water across the entire range of plant available water, the cost is not the same. If plant available water is depleted below the management allowable depletion limit, plants begin to face water stress. The greater the depletion, the greater the water stress until permanent wilting point threshold is reached and a plant’s vital processes cease. Unlike previous thresholds that were mainly a function of soil type, the value of management allowable is a function of stress tolerance, growth stage, and water use of the crop.
Click on the following topics for more information on soil water.
