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.”