Healthy alfalfa’s do not turn reddish-yellow, as with a boron-deficient crop.
In Illinois, alfalfa plants are the targets of a problem that might be deceiving to a farmer’s first glance. The alfalfa leaves are turning a reddish-yellow in the top new growth of the plants while the older leaves remain green. At a first look, it appears the plants have leafhopper damage, but that occurs later in the season. What the alfalfa plants are actually suffering from, according to Fabián Fernández, assistant professor of crop sciences at University of Illinois, is boron deficiency.
“An easy way to distinguish between the two problems is to look for flowers,” Fernández says. “Alfalfa will flower if the problem is caused by leafhoppers, but probably will not if it is due to boron deficiency.”
However, it’s unusual to see symptoms of boron deficiency this early in the growing season, Fernández notes. The problem usually appears on the second or third cuttings of alfalfa, especially during drought periods.
“I suspect that the reason we are seeing boron deficiency in some fields so early this growing season, especially in sandy soils that have lower capacity to retain water, is that it has been unusually dry,” he says.
Also referred to as “yellow top,” alfalfa boron deficiency occurs most commonly in sandy soils that are low in organic matter or have a high pH. Often, boron deficiency appears during drought conditions because the lack of water reduces the capacity of organic matter to release it. Boron does not translocate easily in the plant, which, Fernández explains, causes the leaf discoloration.
Boron deficiency also affects alfalfa cell development, often causing shorter internodes and bunching of the top leaves. If the deficiency is severe, the growing points might die.
If boron deficiency is suspected in an alfalfa crop, there are several ways to investigate the problem:
- First, try watering the alfalfa crop. It’s possible that dry conditions could be causing the symptoms, which will disappear as the soil receives more moisture.
- If moisture isn’t the issue, take a soil sample. If a hot-water extraction test shows less than 2 pounds of boron per acre, it needs to be applied. Another way to determine whether alfalfa is suffering from boron deficiency is to take samples from the top 6 inches of the plant at early bloom. If the boron concentration in the tissue is less than 25 ppm, a boron application would probably help.
- If boron deficiency doesn’t seem to be the problem after soil or plant analysis, observe the roots of the crop. Sometimes a deficiency observed in the canopy has little to do with the amount of nutrient supply from the soil but has a lot to do with the condition of the roots. Reduced root growth can affect the plant’s ability to tap into nutrients and water.
If the investigation shows that remedial action is needed, boron should be applied. Unless soil tests also show marginal concentrations in areas with no visible deficiency symptoms, the application should be restricted to the affected area, not applied to the entire field. A possible strategy is to apply 30 pounds per acre of household borax, which contains 3.3 pounds of boron.
If the field needs other nutrients, such as phosphorus or potassium, it might be easier to mix this small amount of boron with the other fertilizer.
“In sandy soils for alfalfa production, I suggest yearly applications of 1 to 2 pounds of boron per acre,” Fernández says. For fine-textured soils that have boron deficiency, an application of 3 to 4 pounds of boron per acre in the year of establishment will usually correct the problem for a few years.
The only caution is that if oats accompany alfalfa during establishment, the boron application should be delayed until after the first year to avoid toxicity problems for the oats. Similarly, to avoid boron toxicity in corn, it should not be applied to alfalfa during the year prior to corn planting. Finally, if a foliar treatment is preferred, a rate of 0.1 to 0.3 pounds of boron per acre is normally enough to correct the problem.