Subsurface drip irrigation continues to expand rapidly in West Texas crop production. Expansion brings with it increased irrigation efficiency, and a few new maintenance and fertilization wrinkles for cotton farmers.
“Drip irrigation offers improved efficiency for water and nutrient management compared to center pivots,” says Kevin Bronson, Texas A&M AgriLife Research soil scientist at Lubbock.
But system maintenance and nutrient management are both critical for smooth running and achieving those high yields drip is famous for.
“Chemical maintenance of the system is critical to keep emitters clear,” Bronson said during the recent West Texas Agricultural Chemicals Conference in Lubbock. Acidification is necessary for the hard water in West Texas. Water contains calcium bicarbonate and “needs to be buffered down.” Typical water pH for the area is 7.7. That needs to come down to 6.5 to 6.8 — all the way down to 6 in some cases.
“We recommend an automated system to acidify the water,” Bronson said. “We want to bring pH down one unit in most cases. Bicarbonate in our well water converts to calcium carbonate which will clog emitters that are only 1 millimeter in diameter. I prefer sulfuric acid.”
He said one product, N-Phuric, is relatively safe and adds nitrogen to the soil. He doesn’t recommend phosphoric acid. “Some farmers use it to get phosphorus in the soil, but it’s more risky.
“An alternative to acidification is Kleen Flo, a product that prevents emitters from plugging due to iron, calcium and magnesium. Just 2 to 4 parts per million of Kleen Flo is adequate for maintenance, but the product will not unclog emitters. It’s much safer than acid.”
Bronson says an automated system, a pH meter with a programmable pump, costs about $500.
He said chlorination is useful for biologicals, such as slime and wastes. “Make certain to maintain a 6-foot space between acid and chlorine. They can create chlorine gas, which is hazardous.”
Bronson said adding chlorine once or twice a month in-season at 5 to 15 parts per million is adequate. A 20 parts per million rate could be necessary as a “shock treatment” or when using waste water or effluent as your irrigation source.
“Don’t turn off the acid,” Bronson said, as “chlorine will push pH up and could begin to precipitate carbonate again.” He said low level, continuous chlorination might be necessary when irrigating with waste water. Iron and other metals greater than 0.3 parts per million will oxidize and must be cleaned out physically.”
Bronson said Agri-Tec offers an alternative to chlorination and controls biological growths and slime. “It’s not as hazardous as chlorine.”
He said manganese plugging can be corrected with hydrogen peroxide. “Prevention is best.”
Injecting nutrients through drip systems may improve fertilizer efficiency. Bronson prefers 32-0-0 and does not recommend 10-34-0, which precipitates out. N-Phuric is a good nitrogen source. He said a 15-49-0 or a 0-55-0 analysis should be supplemented with 32-0-0.
He said 0-55-0 also is expensive and may form calcium phosphate if injected.
“Inject from the first square to mid bloom,” Bronson said. “Avoid pre-plant and at-plant nitrogen except for maybe a little starter fertilizer in heavy residue or heavy rains. Use the Texas A&M calculator to determine the total amount of nutrients to inject (Lubbock.tamu.edu).
Bronson said a good starting point for fertility injection rate is 50 pounds of nitrogen for every bale of target yield. “A three-bale yield goal means 150 pounds of available nitrogen,” he said.
But growers may not need to apply that much in-season. Bronson said they pick up nitrate-nitrogen from irrigation, as much as 20 pounds per acre, and from deep residual soil nitrate-nitrogen, as much as 30 pounds per acre. Soil testing is critical. And he recommends testing the entire soil profile, down to at least 2 feet.
Bronson recommends applying phosphorus in a band close to planting. “The greatest benefit from phosphorus comes early, when rooting begins. It’s a starter fertilizer effect.”
Bronson said farmers can use drip systems to help germinate cotton seed in some soils and under some tillage systems. “With a stale seedbed, we can water up with some soil types. Loamy and clay soils in conservation till systems work.” Watering up with drip irrigation is not as successful in sandy soils.
He said injecting fertilizer through drip irrigation systems can provide “good nutrient efficiency. Use adequate water with close to 100 percent replacement for the first eight weeks of the season and then switch to more deficit irrigation for the last two months.”
He recommends care with plant growth regulator applications.
“Injection efficiency is high with drip irrigation. Knifing in phosphorus is as effective as injection.”