Issue link: http://read.dmtmag.com/i/657340
8 APRIL 1, 2016 Good Fruit Grower www.goodfruit.com COARSE SAND SAND FINE SAND LOAMY SAND LOAMY FINE SAND SANDY LOAM FINE SANDY LOAM VERY FINE SANDY LOAM LOAM SILT LOAM SANDY CLAY LOAM SILTY CLAY LOAM CLAY LOAM SILTY CLAY CLAY 2.5 ACRE INCHES 2.0 ACRE INCHES 1.5 ACRE INCHES 1.0 ACRE INCHES .5 ACRE INCHES How much water does your soil hold? 3.0 ACRE INCHES The amount of "usable water" held in an orchard's root zone can vary signifi cantly depending on the type of soil. This chart shows how some common soil types affect the usable water per acre inch in an orchard with a 3-foot deep root zone, which is typical for an older, vigorous rooted orchard. An acre-inch of water is about 27,150 gallons. SOURCE: WASHINGTON STATE UNIVERSITY JARED JOHNSON/GOOD FRUIT GROWER Help with your irrigation schedule Tim Smith's irrigation needs worksheet is available on the WSU Tree Fruit Research and Extension Center website at bit.ly/1ni9JkC. More irrigation decision aids can be found at WSU's Ag Weather Net at weather.wsu.edu and at WSU's Irrigated Agriculture Research and Extension Center at iarec.wsu.edu. Soil types can be found at the USDA's Web Soil Survey site at 1.usa.gov/1QLNviu and, in bad years, the shifting supply of water. For instance, sandy loam and loamy sand have two different water holding capacities, which is the amount of water left in soil after it has been soaked and allowed to drain but not dry. "I think most growers throw up their hands and sur- render to the complexity of the situation and just try to keep from falling behind," Smith told Good Fruit Grower. But taking the easy way out by using a set irrigation schedule for the entire season will give them the correct amount of water only twice, once in the spring and once in the fall. At all other times, the trees' fl uctuating needs rise and fall. Meanwhile, Smith suspects many well-meaning but over-enthusiastic orchardists over irrigate in the spring when irrigation districts open the gates. He suggests leaving trees alone in April and using the water instead to wash cars, fi ll spray tanks, control frost and top off on-farm reservoirs, postponing irrigation until May. "Most people can't resist putting that fi rst set on," he said. But that may do more harm than good, he said. The fi rst irrigation is usually the time of the fi rst powdery mil- dew infection in cherries and may even spread fi re blight in apples or pears in warm weather. Also, overwatering in April can reduce root growth and fl ush out recently added iron and boron and reduce the trees' uptake of phosphorus. Too much water (as well as too little water and a host of other causes) can lead to iron chlorosis in TJ MULLINAX/GOOD FRUIT GROWER Knowing your soil type can help determine how much you should irrigate.