C A L I F O R N I A
Vegetables A SPECIAL GROWERS’ REPORT OF AG ALERT ®
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Improving irrigation efficiency is one key practice farm advisors say could help maintain residual soil nitrate in the active root zone of crops such as lettuce. Most Central Coast farmers already use drip irrigation systems, which farm advisors say can be very efficient.
Advisors discuss ways to meet new fertilizer limits ByBob Johnson
zone and available to the crop. A soil sample from the root zone—12 inches for lettuce or spinach—is dropped into a tube filledwith distilledwater. After the tube is shaken, a color-coded test strip can be dipped into it, to learnhowmuchnitrate-nitrogen already exists in the soil. The test strips and laboratories generally report nitrogen in parts per million; Smith and Cahn advise converting that to pounds per acre. They said nitrogen contained in well water should be included in the nutrient budget andwill be counted by regulators as applied nitrogen. For example, a lettuce crop using 7 inches of irrigation water with 89 parts per mil- lion nitrates receives 25 pounds of nitrogen fromwell water. But well water nitrogen can still be tricky to calculate, the advisors said, because the actual nitrogen contri- bution does not include nitrates in the water used to germinate and start the crop. The contribution of nitrogen in irrigation water depends on the efficiency of the irrigation system and the precision of the amount of water applied, Smith and Cahn said, which is why nitrogen efficiency in cool-season vegetables correlates closely to irrigation efficiency. A large majority of Central Coast farmers have already converted to drip irrigation to grow their crops, and a growing number germinate with single-use drip tape. These
Three key practices in fertilizer efficiency can help Central Coast vegetable grow- ers strive toward strict regulations imposed by a regional water board, according to University of California farm advisors. Richard Smith and Michael Cahn of UC Cooperative Extension have spent years researching how farmers can target nitrogen applications, and outlined practices in a blog posted after the Central Coast Regional Water Quality Control Board adopted the standards, known as Ag Order 4.0. Smith and Cahn describe the key fertilizer-efficiency practices as “first, by measur- ing residual soil nitrate and adjusting fertilizer applications accordingly, second by accounting for the nitrate in irrigation water as part of the nitrogen budget, and third by improving irrigation efficiency to help maintain residual soil nitrate in the active root zone of crops.” The farm advisors said some farmers have taken a proactive approach to the wa- ter-quality regulations. “It is important to make attempts to begin implementing these practices and see what is possible for your operation given the crop mix, soil types, and nitrate levels in the irrigation water,” they said. A soil nitrate quick test offers away to learn howmuch nitrogen is already in the root
See PRACTICES, Page 8
July 7, 2021 Ag Alert 7
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