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Alfalfa nutrient requirements and deficiency symptoms

Alfalfa nutrient requirements and deficiency symptoms
“Many people talk about suspected nutrient deficiencies in alfalfa, but usually phosphorus is the only deficient nutrient,” Mike Ottman said at the 2010 California Alfalfa and Forage Symposium in Visalia, Calif. Spiking fertilizer prices in recent years have led more cost-conscious farmers to ask more nutrient questions and more closely pencil the numbers.

University of Arizona Extension agronomist Mike Ottman can be a man of few words, but his knowledge of alfalfa nutrients and deficiency symptoms could fill volumes.

Speaking at the 2010 California Alfalfa and Forage Symposium in Visalia, Calif., in December, Ottman summarized his scheduled 20-minute presentation on nutrient requirements and deficiency symptoms in Western-grown alfalfa in a mere five seconds.
“Many people talk about suspected nutrient deficiencies in alfalfa, but usually phosphorus is the only deficient nutrient,” Ottman concluded.

After a good-hearted chuckle from the 400-plus in attendance, Ottman expounded on nutrient requirements and deficiency symptoms in Western alfalfa. He said spiking fertilizer prices in recent years have led more cost-conscious farmers to ask more nutrient questions and more closely pencil the numbers.

According to Ottman, 16 to 18 nutrients are required to produce high-quality alfalfa with good yields in the West. The nutrient list includes the primary minerals nitrogen (N), phosphorus (P), and potassium (K).

“Alfalfa yields of 8 tons per acre in the West require about 450 pounds of added nitrogen, 300-pounds-plus of potassium, and under 50 pounds of phosphorus per acre per year,” said Ottman.

Secondary nutrients include calcium, magnesium, and sulfur (S). The list of required micronutrients include iron, manganese, chloride, boron (B), zinc, copper (Cu), and molybdenum (Mo). Less than five pounds of micronutrients per acre are generally required to achieve 8-ton/acre alfalfa yields annually.

Alfalfa also requires carbon, hydrogen, and oxygen fixed through the photosynthesis process from carbon dioxide and water.

The metal cobalt is not required directly by the alfalfa plant but is required to assist rhizobium bacteria in fixing N in the root nodules. A question often posed to Ottman is, should rhizobium bacteria be applied at seeding for new alfalfa fields? The general rule of thumb, he said, is to add bacteria to new fields never planted in alfalfa.

Alfalfa seeds are commonly coated with rhizobium bacteria which can provide the needed bacteria for nodule development. Some growers also mix live bacteria into the drill box at planting. That is fine, Ottman says, but refrigerate the additional bacteria until planting or store according to the manufacturer specifications.

Phosphorus – A phosphorus deficiency in alfalfa typically occurs during the first several cuttings of the year when the soil is cooler in the early growing season. P deficiency symptoms include stunted plants; similar to the effects of water stress.

Good diagnostic tools include soil and tissue tests. An eyeball detection of P deficiency is difficult. Ottman suggests laying suspected plants next to normal plants to compare. P-deficit plants are bluish green in color.

A common P fertilizer for alfalfa is mono-ammonium phosphate (11-52-0) applied at planting or top dressed annually.

Tests conducted by Ottman at the Maricopa Agricultural Center in Maricopa, Ariz., evaluated whether liquid- or dry-applied P fertilizer was more effective in curing P deficiency while achieving maximum hay yields.

“The general rule of thumb is any form of phosphorus fertilizer provides pretty much the same response,” Ottman said. “The tested fertilizers did not provide much difference in liquid or dried forms.”

Nitrogen deficiency rare in Western alfalfa

Nitrogen - N deficiency in Western alfalfa is rare. Plant symptoms include poor nodule development on the roots. Check the roots by digging up a plant with a shovel, Ottman suggests, and washing away the soil followed by an up-close nodule inspection.

“Cut the nodules open with a thumb nail to see if they are reddish or pinkish; if they are then the nodules are functioning,” Ottman said. “Greenish-colored nodules are non-functioning. Some nodules are shed after each cutting, and then new ones are formed.”

Conditions which can favor an N deficiency in seedling alfalfa include: cold soil, waterlogged soil, shallow and sandy soils, spring cuttings, high yield levels, alfalfa-grass mixtures, and low pH soils.

For poorly nodulated roots in an established stand, Ottman suggests drilling pre-inoculated seed with two to five times the bacteria at a rate of 3 to 5 pounds/acre.

A N deficiency in a newly-planted alfalfa stand can result in small yellow plants mixed with normal tall green plants. Applying 11-52-0 fertilizer at planting at a rate of 200 pounds/acre provides 22 pounds of N per acre as a starter fertilizer for the seedling stand. To totally provide the N need of the plant without the N provided by nodules, about 50 pounds of N per acre would be required per cutting.

Potassium – K deficiency tends to occur in sandy soil. Symptoms include spotting on the leaf edges, similar to the effects of the blue alfalfa aphid; and from air pollution containing sulfur dioxide.

Soil and tissue tests can determine if insufficient K is the culprit. Fertilizer solutions include muriate of potash and potassium chloride with a maximum rate of 300 pounds of K20/acre.
With K, the issue is often not the soil lacking K, but the uptake availability to the plant.

“There can be hundreds of thousands of pounds of potassium in the mineral structure of the soil in an exchangeable form, but it may not available in the needed soluble form,” Ottman explained.

Sulfur - S deficiency is also rare in alfalfa, yet a growing deficiency is increasing in overall crop production nationally. Today’s fertilizers are purer with less sulfur. Low S levels are typically found in sandy and low organic matter soils. Symptoms include reduced growth and yellow leaves.

S fertilizer options include gypsum, a calcium sulfate containing 15 to 17 percent sulfur. The maximum application rate for Western alfalfa is 15 to 50 pounds of sulfur/acre/year.
Boron – Symptoms of a rare B deficiency resemble the effects of the three-cornered leafhopper with yellowing and some reddish color on the leaves. Low B levels are found mainly in low organic soil. Tissue testing can unveil if B deficiency is the culprit.

Borax (11 percent B) is a source of B along with other boron fertilizers. Ottman suggests applying boron judiciously at the maximum rate of 1 to 3 pounds per acre annually for the stand life.

Molybdenum – Mo deficiency occurs in low pH soil; symptoms include stunted plants with yellow leaves. A tissue test is advised.  Sodium molybdate (40 percent Mo) is one example of a molybdenum fertilizer. The maximum molybdenum application rate is 0.1 to 0.5 pounds Mo/acre/year over the life of the stand.

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