Since the beginning of agriculture, people have recognized that a sufficient water supply is essential for crop production. The capacity of soil to store and supply water for plant growth is a fundamental agroecosystem service. It also is a soil health indicator.
How well this soil water storage function performs affects
- water use efficiency,
- irrigation management, and
- drought resiliency.
To understand soil water holding capacity, we will answer two questions: (1) What is the capacity of a soil to store water and have it available for plant growth? (2) Are all soils the same in this capacity or do they behave differently?
When water, either rainfall or irrigation, touches the soil surface, part of the water is lost through surface runoff, and part is lost by evaporation. The remaining water enters the soil profile via infiltration and becomes soil water.
Soil is a mixture of mineral particles, organic matter and pore space. Depending on the size, number, and continuity of the pores, soil water is retained in or moves downward through the soil and recharges groundwater through the pore space.
This shows how high water will move up into soil with large pores versus small pores through capillary action. A healthy farm soil will have different sized pores, allowing for water flow and retention to be balanced. Adapted from Weil & Brady 2017 Credit: Xi Zhang
Capillary force is important for soil to retain water in pores. The capillary phenomenon can be seen when the tip of a paper towel is dipped into water and the water seems to climb up through the fibers of the paper towel. If we place capillary tubes with one end in the water, the water will rise into each tube. The height of the water in each tube depends on the size of the opening of the tube. The smaller the tube, the higher the rise. Soil pores can be simplified as a bundle of capillary tubes with different diameters. Therefore, smaller pores hold water more strongly than larger pores.
From the perspective of plant growth, soil water can be classified into three types: (1) excess water or gravitational water, (2) available water, and (3) unavailable water. When all the pores are filled with water, the soil is water-saturated and at its maximum capacity to hold water. This situation usually occurs for a very short period when water is being added. Water will soon drain out of large pores mainly under the gravitational force.
After the free drainage of large pores, medium and fine pores are still filled with water as gravity can be offset by capillary force. Moisture in the soil will reach a nearly constant value for a certain depth. The soil is at field capacity. Water held between saturation and field capacity is subject to free drainage over a short time period and is generally considered unavailable to plants.
Water held at field capacity can supply plants with needed water. As water content further decreases, soil particles hold water more strongly as a film coating on the mineral particles. Plant roots can still draw up water from the smaller pores, but it takes them more energy.
Although plants can take up water over a wide range of soil moisture conditions, the drier the soil gets, the harder and more stressful it becomes for the plant to take up water because of this natural attraction of water to soil mineral particles. The plant must invest more energy to overcome the force with which the soil pores hold the water. This means that the energy that the plant spends for water uptake is not available for other important physiological functions, like growth or creation of flowers and crops.
In short-term situations where plants are water stressed, they start to wilt. If this is a long-term situation, the plant may not recover. Soil water content at this point is called permanent wilting point. There still can be water in the soil, but it’s not available for use by most plants. If we have a sponge, we can easily squeeze water from the sponge when it is saturated with water, but we need to use more energy to squeeze water when the sponge becomes drier and drier.
Soil water holding capacity is influenced by soil pore space and pore-size distribution, which are controlled by texture and structure. Texture is determined by the relative amounts of sand, silt, and clay particles in the soil.
Farmers often amend their soil with compost and other materials to help their soil hold water well. At home, can add compost to a sandy or clay soil can help with soil structure and soil health, in addition to increasing water-holding capacity. Credit: Clay Robinson
Although the individual pores are larger in the coarse soil, they have limited ability to retain water. Water can be held tighter in small pores than in large ones. Medium and fine textured soils have smaller pores but many more pores than a coarse soil. Silty and clayey soils can hold more water than sandy soil.
However, total water holding capacity cannot tell us how much water is available to plants, all because of the attraction of water to soil mineral particles discussed above.
Although fine texture soil has high total water holding capacity, it has moderate plant available water. Most of the water is held too tightly by soil particles in tiny pores for plant uptake. In contrast, medium texture soil has moderate total water holding capacity, but it has high plant available water because the soil has a significant portion of medium and fine pores.
Soil structure can increase plant available water by increasing porosity. Besides soil texture and structure, soil compaction can influence soil water holding capacity as well. Compaction can decrease the total pore space and destroy large and medium pores making them into smaller pores. This means the water is less available to plants. Farmers try to avoid doing field management when the soil is wet to decrease the possibility of compaction.
While it is impossible to change soil texture to improve soil water holding capacity, soil structure can be improved by adding organic matter to promote pore development and thus increase water holding capacity. For example, if you have a home garden, you can add compost to your soil. This can increase the water holding capacity and availability for your plants.