Plant Available Water (PAW) defines the portion of water in the soil that a plant can actually absorb and use for growth. It represents the reservoir of moisture that sustains plant life between rainfall or irrigation events. Understanding this range is fundamental to successful gardening and efficient agriculture, as it dictates how frequently and how much water must be supplied to prevent plant stress. PAW is a dynamic range determined by the physical forces that hold water to soil particles.
Defining the Limits of Water Availability
The range of Plant Available Water is defined by two physical boundaries in the soil. The upper boundary is Field Capacity (FC), which represents the maximum amount of water the soil can hold after gravity has pulled all excess water downward. This state is typically reached one to three days after heavy rain or irrigation, once rapid drainage has ceased. At FC, water is held in the soil’s smaller pores but is still loosely held enough for plant roots to access easily.
The lower boundary is the Permanent Wilting Point (PWP), the soil moisture level at which water is held so tightly that a plant cannot extract it quickly enough to prevent permanent wilting. Although water is still present below this point, the force required by the plant to extract it exceeds the plant’s capacity. The total Plant Available Water is the difference between the water content at Field Capacity and the Permanent Wilting Point (PAW = FC – PWP).
These two thresholds are understood through the physics of soil matric potential, which is the suction force or tension with which the soil holds onto water. At Field Capacity, the soil matric potential is relatively low, typically around -33 kilopascals (kPa), meaning the plant exerts little energy to take up water. As the soil dries, water is held in progressively smaller pores, and the suction increases dramatically.
The Permanent Wilting Point occurs when the soil suction reaches approximately -1,500 kPa. At this tension, water molecules are tightly adsorbed to the surface of soil particles, and the plant cannot generate enough suction pressure to overcome this adhesive force. PAW represents the specific moisture range between this low tension (FC) and the high tension (PWP) where water uptake is biologically possible.
How Soil Characteristics Influence PAW
The size and distribution of pores, governed by soil texture, are the primary factors influencing the PAW range. Soil texture is defined by the proportion of sand, silt, and clay particles present. Sandy soils, dominated by large particles and large pores, have a low Field Capacity because water drains rapidly under gravity.
While sandy soils have a low PWP, their low FC results in a narrow PAW range, holding little reserve water for plants. Conversely, clay soils have small particles and many small pores, leading to a high Field Capacity. However, the fine texture of clay causes it to hold water very tightly, resulting in a significantly higher PWP.
This tight hold means a substantial portion of water in clay soil is unavailable to plants, often reducing the overall PAW compared to medium-textured soils. Loam soils, a balanced mix of sand, silt, and clay, generally offer the highest PAW. They possess a good distribution of large pores for drainage and smaller pores for water retention, optimizing both FC and PWP.
The addition of Organic Matter (OM) significantly influences the water-holding capacity of all soil types. Organic matter acts like a sponge, increasing the soil’s surface area and its ability to hold water against gravity, thereby raising the Field Capacity. This effect is especially noticeable in sandy soils, where a small increase in OM greatly improves water retention.
While organic matter slightly increases the Permanent Wilting Point, the increase in Field Capacity is proportionally greater. This widening of the difference means that adding organic matter effectively expands the PAW window, providing a larger buffer of stored water for plants. Furthermore, OM helps stabilize soil structure, promoting a healthy mix of pore sizes for better water infiltration and availability.
Practical Implications for Plant Health and Management
A clear understanding of PAW directly informs efficient water management strategies for farmers and home gardeners. The goal of irrigation is to replenish soil moisture up to Field Capacity without exceeding it, while preventing the moisture from dropping too close to the Permanent Wilting Point. Exceeding FC leads to water waste through drainage and can deprive roots of oxygen, while approaching PWP causes plant stress and yield loss.
Irrigation scheduling relies on targeting a maximum allowable depletion (MAD), a specific percentage of the total PAW that can be used before water must be reapplied. For many crops, the optimal irrigation trigger point is reached when 40 to 60 percent of the PAW has been depleted, depending on the plant’s sensitivity. This approach ensures plants have “readily available water” and avoid undue drought stress.
For monitoring soil moisture, instruments like tensiometers are used to measure the soil matric potential directly. These devices provide a real-time reading of the tension, indicating how hard the plant is working to pull water from the soil. Tensiometers operate most effectively in the low tension range, typically from 0 to about 85 centibars (cb), which is the most readily available portion of the PAW.
This direct measurement allows managers to make precise, data-driven decisions on when to irrigate, rather than relying on visual observation or a set calendar schedule. In drought management, knowing the soil’s PAW helps in selecting water-wise crops or using soil amendments like organic matter to maximize natural water storage. Maximizing PAW is a primary strategy for improving water use efficiency and enhancing a plant’s resilience during dry periods.