Drought stress can be controlled by irrigation. Scheduling the time and amount of application will depend on the availability of water as well as the soil water status, allowable depletion, and water use rate.

Moisture available to the plant is influenced by the available water holding capacity (AWHC) of the soil. The AWHC is defined as the amount of water that can be held by the soil between the permanent wilting point and field capacity of the soil. Permanent wilting point is the soil water content where plant is no longer able to extract water. Field capacity refers to the water content of the soil after it has been fully wetted, and excess water has drained.

Additional soil information necessary to the irrigation scheduler includes the available water in the soil at the time of irrigation. The available water is that amount of water contained in the soil at the time of interest within the AWHC range. This property may be monitored with soil water sensors. One type of sensor, called a tensiometer is very applicable to sandy soils.

The soil water properties are dependent upon the soil type and texture. Sandy soils have low water-holding capacities (0.5 to 1.0 inches of water per foot depth of soil) as compared to the water-holding capacities of loam or clay soils (2.0 to 3.0 inches of water per foot depth of soil). Therefore, more frequent, smaller amounts of irrigation are required on a sandy soil to avoid movement of water below the plant root zone.

The allowable depletion is the level of water depletion which will be allowed before irrigation will be applied. The level of allowable depletion will vary with the plant characteristics as well as the irrigation system characteristics and capacities.

The plant water use rate may be obtained directly or estimated. However, it is very difficult to obtain direct information, and therefore, most water use data are estimated. Estimation methods can involve the use of field sensors, measured pan evaporation data, published data, or broadcast information.

For more information and charts go to:
Irrigation of Lawns and Gardens. Dorota Z. Haman, Gary A. Clark, Allen G. Smajstrla. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: May 1989.