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.

cblake@farmpress.com