Feb 26, 2016
Good fertilizer programs enhanced by drip methods

Many blueberry fields are irrigated by drip. A major advantage of drip is the ability to fertigate. Fertigation is the practice of applying soluble fertilizers to the plants directly through the irrigation water.

“It is often a very efficient way to apply fertilizers because most roots in drip irrigated fields are located near the drip emitters,” said David Bryla, USDA-ARS research horticulturalist in Corvallis, Oregon. He spoke at the recent Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids, Michigan.

Some advantages of fertigation include reduced delivery costs (no need for tractors or spreaders), greater control of where and when the fertilizers are placed, the ability to target application of specific nutrients during particular stages of crop development, and the potential to reduce fertilizer losses by supplying only small amounts of fertilizer to the plants as needed, Bryla said. Disadvantages include costs associated with the need for higher fertilizer quality and the capital costs of the equipment required to inject the fertilizer through the irrigation system.

While nitrate-N is very mobile and moves readily in moist soil to plant roots, Bryla said, ammonium-N moves much more slowly. Therefore, N in ammonium fertilizers will only be available to the plants when it is applied close to the roots. Any ammonium-N applied away from the roots will likely be nitrified and converted to nitrate-N and eventually will be leached from the field by rain or irrigation.

To fertigate, drip lines and emitters should be located near the base of the plants during the first year or two after planting and later repositioned 6-12 inches on each side of the plants, using two lines per row, as the root system develops. Only one drip line per row is needed for adequate irrigation of blueberry on most soil types, but two lines per row are suggested in order to encourage a larger root system and thereby increase plant access to soil nutrients. The drip lines should be installed under weed mat or buried under sawdust mulch to secure the lines and reduce water runoff on raised beds.

Since only a fraction of the soil is wet by the drip emitters, most of the N applied to the plants during fertigation is added directly to the roots, Bryla said. As a result, extra N is not required with fertigation when sawdust or pine bark is incorporated into the soil prior to planting or used as mulch.

Liquid fertilizers should be injected in small and frequent applications (e.g., once a week), starting at leaf emergence and finishing in late July or early August, Bryla said. Fertigation is not recommended for the entire growing season (April–September), because N applications in late summer reduce fruit bud set in blueberry and increase the potential for freeze damage over the winter, particularly in colder regions such as eastern Oregon and Washington.

“Many growers using drip apply granular fertilizers in March or April and then switch to fertigation in May, once irrigation is required on a regular basis,” he said. “The use of granular fertilizer in the spring is less expensive than fertigation and practical for mature plants. However, it may cause fertilizer ‘burn’ (salt damage to shoots and roots from the fertilizer) in new plantings and, in severe cases, can kill young plants. Even small applications of ammonium sulfate applied at a rate of 20 lb/acre of N in the spring prior to fertigation reduced shoot growth and caused root damage in young Draper plants.”

Recently, Bryla said, fertigation and granular fertilization using different sources of N fertilizer during the first five years of fruit production were compared in Bluecrop.

Soil pH was slightly lower with granular fertilizers than with fertigation; however, leaf N was also lower with granular fertilizer, whereas yield was greatest when plants were fertigated using ammonium sulfate or urea sulfuric acid. The results indicate that blueberry is well suited to fertigation. Larger applications of N fertilizer also increased plant growth in the study but did not improve yield in any year, and was even detrimental to berry size during the first three years of fruit production and to yield during the third year when granular fertilizers were applied. Whether N was applied by fertigation or as granular fertilizer, only 65 to 90 lb N/acre per year was required to optimize fruit production.

In addition to applying N solutions, a number of blueberry growers in the Pacific Northwest (and elsewhere) are incorporating humic acids into their fertigation programs, Bryla said. Humic and fulvic acids are commonly used as soil supplements and have been found to stimulate plant growth in a number of crops, including blueberry. Root growth of Draper was particularly enhanced by humic acids during the first two years in a new planting. However, more work is needed to determine if humic acids have any benefits on growth and fruit production in mature plants.

“In organic production,” Bryla said, “we have successfully fertigated fish emulsion and used this product as the sole nutrient source for eight years to date in a certified organic field. Fish emulsion generally has about 4 percent N. The total amount of N in a gallon of fish is then 4 pounds (one gallon weighs about 10 pounds).

The total amount of N to apply was divided into seven equal portions and applied from mid-April (bloom) through mid-July every two weeks. Fish fertilizer also contains significant amounts of P, K and Mg. When fertigating with organic products, it is important to dilute the product so the viscosity is suited to fertigating, Bryla said.

Also, pressurize the system prior to injecting the fertilizer and run the irrigation after injection to ensure the system flushes well.

“While we have had good success fertigating with fish emulsion for several years, our research does show a reduction in emitter performance over time – this may be due to accumulation of organic material from the fertilizer but could also have been caused by iron bacteria,” Bryla said. “Good maintenance of a drip irrigation system is important for maintaining performance over time.”

Gary Pullano, associate editor




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