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Alfalfa tissue sampling targets plant needs

Todd Rinkenberger for Progressive Forage Published on 26 May 2016

Alfalfa, like any plant system, needs sufficient amounts of nutrients to maintain quality stands and yields. To determine those needs, the Irrigated Alfalfa Management for Mediterranean and Desert Zones book published in 2008 by the University of California Alfalfa Workgroup has a chapter dedicated to fertilization strategies for the arid and semi-arid growing areas of the world, including many parts of California.

The authors state, “A key aspect of designing a fertilization program is evaluating the nutritional status of the alfalfa crop. Nutritional status can be evaluated by visual observation, soil analysis or plant tissue sampling. Using all three in combination provides the best results.”

At the time of publication, all the authors were either retired or actively working as farm advisers or specialists associated with the University of California Cooperative Extension. The writers – Roland Meyer, Daniel Marcum, Steve Orloff and Jerry Schmierer – represent decades of education, research, knowledge and experience working with alfalfa production.

Properly assessing fertilizer needs, they write, “requires complex and often difficult management decisions. The process includes an analysis of which nutrients are needed, selection of the proper fertilizer, application rate, timing and placement, economics, record-keeping and environmental considerations.”

However, with alfalfa prices taking a downward turn this year, some growers acknowledge they are not likely to conduct tissue sampling until prices improve.

“A lot of alfalfa growers don’t tissue sample, and they probably should,” says Dan Putnam with the University of California – Davis. Besides being an extension agronomist and forage specialist, he worked on the UC alfalfa management book as one of its two editors.

“We encourage growers to tissue sample, and in some cases I’ve been a little surprised that a crop that doesn’t look deficient actually is deficient in a nutrient when you do the tissue sampling,” he says, reinforcing the importance of testing.

Putnam also noted that another reason to conduct tissue sampling and apply nutrients when needed is to safeguard against the plants mining the soil for the nutrients and leaving the soil wanting.

Other researchers concur on the value of diagnosing nutrient deficiencies.

“Periods of low prices are not the time to ignore agronomic practices that ensure sustained yields and quality,” says Bruce Roberts, the JG Boswell Chair of Agronomy at California State University – Fresno. “A crop that’s in the field for three or more years, depending on whether the grower is using flood or subsurface drip irrigation, needs to be properly monitored and maintained nutritionally.”

Roberts, who oversees research projects and teaches a variety of classes including forages and range management, cautions that farmers who forgo tissue sampling and ignore alfalfa plants’ nutritional needs risk long-term loss of profitability from short-term cutbacks, especially in a competitive market.

“There’s a lot of people growing alfalfa, so buyers are going to choose to buy the best-quality product on the market,” he says. “Regardless of current market fluctuations, you don’t want to skimp on your inputs and risk losing your market edge of growing quality, high-protein alfalfa hay.”

With 17 essential nutrients of various amounts needed to grow a healthy alfalfa crop, both soil and plant testing works cooperatively to correctly identify deficiencies.

According to the manual, alfalfa obtains carbon, hydrogen and oxygen from water and carbon dioxide from air, but the other 14 elements come from either the soil or fixation of atmospheric nitrogen by bacteria in root nodules. If the plant cannot find the minerals it requires, then growth can slow or cease.

“Thus,” say the contributors of the chapter, “all nutrients must be available to the plant in adequate quantities throughout the production season. The nutrients that are most commonly in short supply for alfalfa production are phosphorus, followed by potassium, molybdenum and boron.”

So despite lower returns, growers should consider developing a plan to test nutrient needs to ensure optimal plant growth.

While the majority of California’s alfalfa is still grown using flood irrigation, some farmers have switched to subsurface drip irrigation.

Francisco Parra is an agronomist and pest control adviser at Burford Ranch in Five Points, California. The ranch grows alfalfa using drip, and he says one of the beneficial side effects of cleaning the drip lines with phosphoric acid is that the plants are able to glean some nutrients from the process.

“You definitely see a response in the crop when we run a phosphoric acid application through the lines,” Parra says about the once-yearly process.

Putnam agreed that not only does running phosphoric acid help clean the lines, but it also provides nutrients. However, he says this should not replace dedicated tissue and soil analysis to find out precisely what the crop needs.

Subsurface drip irrigation can expedite a grower’s ability to get nutrients right where they are needed at the plant’s root zone in a timely manner.

By using tissue samples early in the cutting season, growers can know if there are any deficiencies to address, and the nutrients can be added through the drip lines during the next scheduled irrigation. This will give plants enough time to respond to the necessary inputs to encourage growth before being cut and aid in reducing plant stress during the summer.

While tissue sampling is an important tool, the alfalfa manual warns, “Tissue tests can determine only the single most limiting nutrient affecting plant growth.”

The writers add that the concentration of other nutrients may actually increase due to reduced growth, so it is best to correct the most severe deficiency first and follow up with more tests to determine other deficiencies that need to be resolved.

Correcting weaknesses involves calculating the amount of nutrients that will be removed by alfalfa, the yield potential of the crop, what the most current soil and tissue sample levels are and, if possible, taking into consideration the historic responses to fertilization.

Finally, the authors conclude, keeping concise records is an important part of maintaining a successful alfalfa fertilization program. They encourage growers to keep a record for each field that includes the location of permanent benchmark sampling sites, dates sampled, test results, fertilizers applied with dates, rate and eventual crop yields and any other pertinent information.

Keeping these records and being able to refer to them, they write, helps growers develop a long-term economical and sustainable program.  end mark

Todd Rinkenberger
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