Plant available water is the maximum amount of water a plant can extract from the soil. It can also be an indicator of soil health. Building the water holding capacity of your soil can make your farm more resilient in drought years because the soil can provide more water during dry conditions.

After a big rainfall, when you hear soil squelching under your boot, and it is difficult to walk or drive a tractor through, the soil is at saturation. This means all the air pores in the soil are filled with water (Figure 1, left). Water that drains quickly from macropores (larger pores) after saturation is called gravitational water. The force of gravity is stronger than the bonds between soil particles and water molecules (adhesive bonds) and the bonds between water molecules (cohesive bonds), so the water drains. This can take anywhere from a few hours to days, depending on the soil properties.

Filled to capacity

After draining, the amount of water remaining is the soil moisture at field capacity. Water is held in the soil by cohesive and adhesive bonds that are strong enough to resist the pull of gravity. Macropores drained their water and the mesopores (medium-sized pores) and micropores are full. This is the “upper limit” of plant available water (Figure 1, left dashed line).

The soil moisture held at field capacity is drawn down as water is either taken up by plant roots or evaporated into the air. After time and with no additional replenishment, the only remaining water is held by adhesive bonds between water molecules and soil particles or organic matter. These bonds are so strong that plant roots cannot “pull” the water away from the particle surface, and therefore, this water is not accessible to plants. This is referred to as the permanent wilting point (Figure 1, right dashed line) because plants will wilt beyond rescue in this condition. If you’ve experienced crop loss because of prolonged drought, it is because your soil was at its permanent wilting point.

Crunch the numbers

Plant available water (PAW) is the amount of soil moisture held in the window between field capacity and permanent wilting point (PAW = FC - PWP), shown in the center of Figure 1. It can be expressed as a fraction or percent (volume of water per volume of bulk soil), or as a water depth (such as inches) for a given depth of soil (for example, 0 to 12 inches).

Consider this example: In a plano silt loam soil, the field capacity may be 30% and the permanent wilting point 15%, so PAW = 30 - 15 = 15%. If we know the effective rooting depth of the crop, we can calculate the inches of PAW for that depth. If the effective rooting depth for corn is 36 inches, 0.15 x 36 = 5.4 inches of plant available water.

However, even within the window of PAW, not all water is equally accessible to plant roots. When soil water levels deplete to 50% of PAW, photosynthesis can slow down, and the plant begins to be water stressed. This is because the pores in which water is stored are getting smaller and smaller as plants draw water down. Plants draw water from the larger mesopores first as it is most easily available for passive uptake by roots.

What determines availability?

Soil texture is the single-most important determinant of PAW because it is both the starting point and fundamental limitation over the size and distribution of soil pores. That said, both soil structure and organic matter content can also be determinants of PAW because they influence field capacity and permanent wilting point by changing pore size distribution.

Finer textured soils like clay hold more water at field capacity and permanent wilting point (Figure 2, right side) than coarser textured soils like sands (Figure 2, left side). The distance between soil particles is smaller in clay soils than sandy soils, meaning more water can be held with cohesive and adhesive forces at field capacity. There is more total surface area on all the soil particles combined in clay soils than sandy soils, creating more locations for adhesive bonds between soil particles and water, and thus, a higher permanent wilting point in clays.

Soil structure, or the arrangement of soil particles within the soil, influences PAW through its effect on field capacity. A well-structured soil will have more soil aggregates, creating a greater variety of pore sizes, many of which are the large pores between aggregates that hold a lot of water but do not drain due to gravity. A poorly structured or compacted soil will have more medium to small pores that hold water more tightly and few larger pores that hold more water. Organic matter helps form and stabilize aggregates, creating more pore sizes to store water.

While you can’t change soil texture, you can build soil organic matter to improve aggregation and structure, which could make more water available to plants. This effect is more pronounced in coarser textured soils than in finer textured soils, and increases in PAW are primarily due to enhanced field capacity.

Building soil organic matter can also improve soil functions, like infiltration. Practices like no-till and cover crops help build soil aggregates and improve soil structure, boosting the PAW of your soil.

This article appeared in the November 2025 issue of Journal of Nutrient Management on page 10.

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