Drip irrigation systems are designed to deliver water precisely and uniformly to the crop roots and often results in water savings. Water is applied in low volumes at frequent intervals.
There are two types of drip systems: a rigid drip tube that can be used as a permanent system and a less durable, collapsible drip tape that is often installed in orchards just for the establishment phase.
The drip tube is made of rigid polyethylene and is designed to last 10 to 15 years, or possibly the life of the orchard. It has emitter flows of 0.4 to 2.0 gallons per minute and usually has pressure-compensating emitters embedded in it. Bob Mittelstadt, general manager of Clearwater Supply, Inc., in Othello, Washington, said pressure-compensating emitters can make a huge difference in the uniformity of water application in orchards on slopes. Emitters can be spaced from 12 to 60 inches apart. The fewer emitters, the longer the hose can be, which reduces the cost. However, the closer the emitters, the more consistent the soil moisture is along the line of tubing.
Mittelstadt said emitters do not necessarily follow the tree spacing. “If you put your emitters on a tree-by-tree basis, you may not have a big enough wetted area to promote the root growth,” he said.
The hard drip tube is often used above groundeither on the ground or attached to the trellis wire. It can be ordered with clips already installed so it can easily be attached to the lower wire.
Drip tape is thinner than drip tubeusually 5 to 15 mil (thousandths of an inch) thick. The flow rate ranges from 0.07 to 0.27 gallons per hour. Emitters are usually the nonpressure-compensating kind and can be spaced from 6 to 26 inches apart. Drip tape is usually buried, which improves the wetting pattern in the soil.
Filtration is absolutely necessary with any kind of drip system to maintain its longevity, Mittelstadt said. When surface water is used, the smallest particle in the water must be only a tenth the size of the diameter of the emitter to make sure it goes through. Drip tape requires more filtration than drip tube. Ponds help reduce the silt content of the water because the particles settle to the bottom.
Drip lines need to be maintained with flushing and injection of chlorine and sulfuric acid as necessary to maintain the desired pH level in the soil. Check soil moisture with a screw augur to be sure that the irrigation is not going too far below the root zone, he advised, as it’s easy to overirrigate in the spring.
When using a drip system for fertigation, it’s best to have an experienced dealer set up the system, Mittelstadt said. Before beginning, have the water tested for electrical conductivity, bicarbonates, sodium, and magnesium, etc., and continue to monitor the pH, total salts, and total nitrogen.
Fertigation should not begin until the buds have pushed about an inch and the roots are growing. Most growers fertigate once a week, applying up to 25 pounds of nitrogen per acre per week. The grower should back off when the total salts in the soil reach 1.0 millimhos per 100 grams of soil or the total nitrogen reaches 400 pounds per acre. High salinity can affect the tree’s ability to draw moisture from the soil and cause moisture stress.
There are three types of liquid nitrogen that can be applied:
–UN-32 (urea-ammonium nitrate). This will aggressively lower the pH unless the soil has a high buffering capacity.
–CAN-21 or CAN-17 (ammonium nitrate and calcium nitrate), lowers pH moderately.
–CN-9 (calcium nitrate) does not lower the pH.
Mittelstadt noted that trees that are high in nitrogen are more prone to crown rot, and more vulnerable to fireblight as the bacteria will run down the phloem to the roots more quickly. Vigorously growing trees are also more likely to have deficiencies of nutrients, such as zinc. He recommends taking tissue samples at 12 inches and 24 inches of growth to assess nutrient levels in the tree.
Growers should work with someone with experience of fertigation and be cautious when mixing different fertilizers, especially with calcium or phosphorus, as they can react with each other and turn into something resembling cottage cheese.