Estimated “Useable Water” In Orchards

By Common Area Soil Series

by Timothy J. Smith, WSU Cooperative Extension

It is difficult to determine the “usable water” in an orchard based on soil texture alone. The amount of water held in the soil may be affected by other physical soil factors, such as the percentage of gravel or larger stones, the degree of compaction, soil layers that restrict root growth, or radical changes in soil texture by depth. Each area of soil has its’ unique character, and variable ability to hold water for tree use. There is a strong tendency for finer textured soils to hold more water than those with sandy, coarse texture. However, a deep, uncompacted sand may provide more available water to the tree than a shallow, rocky loam.

The actual amount of water stored in your soil must be determined over time, by observation. Your calculated usable water estimate should results in proper soil moisture remaining in the soil on the day before you calculate the need for irrigation. In most soils, your observation of the soil brought up from the top three feet with a probe will provide an adequate evaluation of it’s readiness for irrigation.

In mature, deep rooted orchards, it’s time to irrigate when the top foot of soil near the tree is moderately dry, the second foot is moderately moist (about 50% field capacity), and the third foot is moist.

What is “dry” and “moist”? Your experience and a soil moisture “look and feel” will be your best guide.