Soil holds a great amount of water, but your orchard must work increasingly harder to pick up the water as the soil becomes drier. In the spring and fall, trees use a relatively low amount of water per day, so they have little difficulty keeping up with their water needs, even when the soil is moderately dry. As the weather becomes warmer and dryer, it is better for the trees and fruit if the roots have easy access to soil water.
If you replace the orchard soil water when about half of the total has been used by the trees, the trees will come under minimum stress and the soil oxygen will be replaced periodically. The key question is: How do you know when about half of the soil water is going to be used?
First, a few basic concepts:
When the soil if completely wetted, then drained by the pull of gravity, the soil is considered at "field capacity". If your orchard pulled all of the water it could out of the soil, then died, the soil would be at "permanent wilting point". There would still be water in the soil at this point, but it would look dusty dry. The amount of water the trees could pull out of the soil between field capacity and permanent wilting point is the soils moisture holding capacity. This is often measured in inches of water per foot of depth.
The first 50 percent of that water used by the trees is the amount you should count on. I like to call this amount the "Usable Water".
The amount of "usable water" held in your trees root zone when your soil is at field capacity depends on the depth of the soil and root zone, the texture of the soil, its percentage of rock and gravel, textural layers, and compaction. Because of these factors, determining the "usable water" holding capacity of your soil is complicated and difficult to do acurately.
It may not be practical to spend much time getting the soil moisture holding capacity too finely defined, as it varies across most orchards. It is important to make your best estimation of the amount of usable water in the irrigation block, then "tweak" this estimate by observation of soil moisture prior to calculated irrigation time.
To come up with the preliminary estimate, you should estimate tree rooting depth, the soil texture, and the percentage of the soil volume made up of rocks.
I assume two feet of root depth in young or dwarf rootstock orchards, and about 3 feet of root depth in older, vigorous rooted orchards. You might stretch it to 3.5 or even 4 feet in very high quality soil and old trees. I rarely do this depth, because we don't have much very-high-quality soil in Northcentral Washington. Root studies have determined that 2/3 of the root volume of large, old trees is in the top two feet of soil. About 80 percent of the roots are in the top three feet. Why count those few deep roots as equal to those in the top layers?
Good estmates of soil texture can be made by looking at area soil surveys (widely available on various bookshelves) or by sending soil from the first, second, and third foot of soil to local soils labs for a textural analysis.
Below are the common soil textures and the amount of "usable water" held in two or three feet of root zone in those soils.
|soil texture:||-- 2 ft. root zone||-- 3 ft. root zone|
|coarse sand||= .92 inch||= 1.25 inch|
|sand||= 1.25||= 1.74|
|fine sand||= 1.37||= 1.95|
|loamy sand||= 1.41||= 2.03|
|loamy fine sand||= 1.47||= 2.08|
|sandy loam||= 1.54||= 2.20|
|fine sandy loam||= 1.87||= 2.65|
|very fine s. loam||= 2.00||= 2.82|
|loam||= 2.20||= 3.15|
|silt loam||= 2.41||= 3.40|
|sandy clay loam||= 2.20||= 3.11|
|silty clay loam||= 2.03||= 2.91|
|clay loam||= 2.03||= 2.95|
|sitly clay, clay||= 1.87||= 2.81|
Note: the amount of usable water is reduced in very fine textured soils because the tree has difficulty competing with clay particals for water. there is a greater amount of water remaining in the soil at perminant wilting point.
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Washington State University
Tree Fruit Research and Extension Center
1100 N. Western Ave.
Wenatchee WA 98801