How much water a soil can hold is very important for plant growth. Soils that can hold a lot of water support more plant growth and are less susceptible to leaching losses of nutrients and pesticides. All of the water held by soil is not available for plant growth. Two separate laboratory tests are required to determine how much plant available water a soil can hold. The first test run on the soil determines the amount of water the soil can hold at field capacity. The second test determines how much water the soil holds when plant roots can no longer extract water (wilting point). The water available for plant growth is the difference between field capacity water content and wilting point water content (see equation below).
Field capacity water content – wilting point water content = plant available water
Field Capacity Water Content (1/3 Bar Water Content)
The first step in determining the field capacity water content of a soil is to place a dry pulverized soil sample on a ceramic plate (see picture). The sample is then saturated with water and left to equilibrate overnight. The next day, the porous ceramic plate is placed into a container that is pressurized with 1/3 atmosphere of pressure (about 5 psi). The slight pressure in the container pushes excess water out of the soil sample and through the ceramic plate. After 24 hours in this chamber, the moisture content in the soil sample is said to be at field capacity. The soil samples are then weighed, placed in an oven at 105oC for two hours and then weighed again.
Wilting Point Water Content (15 Bar Water Content)
The next step is to determine how much water the soil holds when it is so dry that plant roots can no longer remove water (wilting point). First a dry pulverized soil sample is placed on a ceramic plate and saturated with water overnight. The next day the ceramic plate is placed into a container that is pressurized with 15 atmospheres of pressure (about 225 psi). This pressure pushes most of the water out of the soil sample and through the ceramic plate. The samples must be left in this pressurized container for 48 hours. The samples are then weighed before they are placed in an oven at 105oC for two hours to remove the remaining water. The amount of water that is left in the soil is held too tightly for plants to extract (hygroscopic water). Once this step is completed the amount of plant available water in the soil can be calculated as shown in an example below.
Field Capacity Water Content (1/3 Bar Water Content) = 14.9%
Wilting Point Water Content (15 Bar Water Content) = 4. 1 %
14.9% – 4. 1 % = 10.8% available water
In an irrigation situation, this information, along with soil bulk density and soil texture, would be used to determine how many inches of available water a soil profile would hold. This information is also useful in determining how much water can be applied to a soil before leaching of nutrients and pesticides may occur. If you have any other questions on how soil water holding capacity is determined, our technical staff would be happy to answer them.