The number seven is a lucky and essential number – not only for the western alfalfa industry but also for human existence.

On the human front, the pH of human blood is seven, and that is a good thing.

“If the pH number of blood changed by just 0.2, people would die,” says Kurt Nolte, University of Arizona (UA) Cooperative Extension agent based in Yuma.

Nolte’s lucky-seven comparison kicked off his alfalfa presentation during the UA-sponsored agronomic workshop held in Parker, Ariz. in May. The workshop included multiple presentations on desert-area alfalfa production in Arizona and southern California.

Nolte says the low desert soil pH level of seven helps plants produce higher yielding quality hay for dairy cows and other uses. This is important since most desert soils in these areas are in the 8-8.5 pH range (too high). Producers should add a sulfur-water mix to the soil to lower the pH to seven.

The pH level in pure water is also seven which means what is good for water is good for alfalfa. If soil pH levels are too high (above seven), adding liquid phosphorus in alfalfa can be a solution.

“The pH level is an indicator of overall fertility and goes hand in hand with making many nutrients available to the alfalfa plant,” Nolte says. “Adjusting the soil pH to seven means the plant can take up essential nutrients in the soil.”

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Nolte advocates biannual soil tests to determine the pH level. Avoid buying pH kits at improvement stores and instead send soil samples to a professional laboratory. Costs usually vary from $25-$30 per sample.

Phosphorus

Of the nutrients needed for optimum alfalfa development and yield, phosphorus (P) is the key. It is the most commonly deficient nutrient in desert soils. Once added, it can extend stand longevity in established stands. Applications of 11-52-0 and 10-34-0 fertilizer liquid can improve yields.

Nolte calls P deficiency a “ticking time bomb” since deficiency is usually hard to see in the plant.

Potassium

Added synthetic potassium (K) is usually not needed in desert alfalfa. In many cases, the Colorado River - the major water source in alfalfa production - contains sufficient K for good yields.

Yet growers should be on alert for visual symptoms of K deficiency, including pinhead-size white or yellow spots on the leaf margins. A soil test can determine a K shortage.

If K is limited, good K-based fertilizer choices include a 0-0-60 potash mixture, or potassium sulfate (0-0-52, 18% sulfur), applied prior to seeding. Single K applications should not exceed 200-300 pounds of K₂O per acre.

Plant response from applied K can take 60 days.

Nitrogen

Alfalfa growers also need not worry about nitrogen (N) levels since alfalfa is a legume crop. In other words, the alfalfa plant can fixate N from the air and microbial root activity releases the essential element to the plant.

When buying alfalfa seed, Nolte suggests purchasing seed inoculated with nitrogen-fixed bacteria for the same reason. Some alfalfa specialists recommend adding 20-30 pounds of N per acre at planting to boost the new crop.

Mike Matheron, UA research specialist, discussed several soil pathogen-based diseases in western alfalfa which can reduce quality and yield.

One disease is Phymatotrichum root rot (PRR); a.k.a Phymatotrichopsis root rot, cotton root rot, and Texas root rot.

The pathogen attacks about 2,300 species of dicotyledonous plants, including alfalfa and cotton. Monocotyledonous plants, including grasses, are not affected. PPR susceptibility occurs only during the summer months. 

Globally, PRR is found only in the Southwest from the Colorado River basin east to Arkansas, plus northern and central Mexico. Matheron is unaware of confirmed PRR cases in California.

Largest losses

The greatest plant-PRR losses are sustained in cotton grown from Arizona to Texas, plus northern Mexico. The fungus survives as seed-like sclerotia at soil depths up to six feet deep.

The microscopic fungus kills plant roots and the plants die shortly after infection. The only way to tentatively identify the disease is by searching for fungus strands on the roots.

PPR management tools include not moving PPR-infected soil from one field to another. Planting rotation crops - corn, wheat, or sorghum (all grasses) – can also help break the PPR cycle. Most fungicides have limited success.

Verticillim wilt

A second key disease is Verticillium wilt found in California’s Central Valley; not in the aforementioned desert areas. The fungal pathogen is Verticillium albo-atrum. In severe cases, wilt can cut yields by up to 50 percent by the second harvest year and severely reduce stand life.

The fungus grows inside the plant stem and discolors tissue. The fungus plugs the plant’s “plumbing” in the xylem causing wilted, yellowed leaves. If the grower finds a ring of discolored tissue in the stem and other symptoms, odds are its verticillium.

Disease development can occur between 59-86 degrees Fahrenheit. Arizona growers can send suspect samples to Matheron at the Yuma Agricultural Center - (928) 782-5863.

The best verticillium control measure is plant resistant varieties. Some can be found on the National Alfalfa & Forage Alliance website. Common sense strategies include sanitation practices, crop rotation, and weed control.

Rhizoctonia

A third group of diseases are caused by the soil-borne pathogen Rhizoctonia solani. Each disease has a different name depending on where the pathogen attacks the plant.

Rhizoctonia damping-off disease is found primarily in the California Central Valley and less frequently in Arizona mostly at post-emergence but it is occasionally found during the pre-emergence stage.

Symptoms include seedlings which never emerge plus plant death shortly after emergence.

Root canker disease is common during the summer in the two-state desert valleys. Areas of the plant root may become infected but the plant will grow fine.

Crown and bud rot disease can involve Rhizoctonia and other fungal pathogens. Extensive cankers are the trademark sign.

Stem and foliage blight occurs mostly in hot and humid growing areas. Common traits with all Rhizoctonia solani include: brown-colored mycelium; brown or black coloration, and irregularly shaped sclerotia up to 0.5 millimeter in diameter. The fungus stops growing around 95 degrees F.

In addition to hot temperatures, disease development is spurred by soil moisture and organic matter. There are no economically effective disease management tools available.

Rotary injection

Changing gears to equipment, UA Engineer Mark Siemens discussed rotary soil injection approaches for side dress fertilizer applications in agriculture; used primarily to date in corn. The device is similar to a wagon wheel and applies liquid to the hub. As the device rotates, liquid material is emitted through hollowed spokes into the soil surface.

In corn, sugar beet, and wheat, the device has improved nutrient uptake and crop yield.

Siemens believes rotary point injection could have its best fit yet in alfalfa for applying phosphoric acid to plant stands.

Siemens and others built a four-bed rotary injection system at the Yuma Agricultural Center. The device was tested in 2013-2014 YAC winter vegetable trials and then tested in commercial vegetable fields.

Trial results were good while commercial use produced mixed results. Siemens says another year is needed to work the bugs out of the system.

The UA point injection system is available for growers to test. Contact Siemens at (928) 782-3836 and [email protected].