University of Illinois assistant professor of crop sciences Fabián Fernández has noticed visible signs of boron deficiency in alfalfa.
In Illinois, alfalfa is the crop most sensitive to boron deficiency. Corn and soybean have very low boron requirements, and boron toxicity can be a problem for corn if large amounts of the nutrient were applied to an alfalfa stand in the year prior to planting the corn.
Fernández said that boron deficiency in alfalfa is most common in sandy soils that are low in organic matter or in soils with 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,” explained Fernández. As a result, signs of the deficiency are yellow-reddish leaves in the top portion (new growth) of the alfalfa plant while the older leaves remain green. For this reason, the deficiency is often refer to as “yellow top.”
The symptoms are similar to those of leafhopper damage, which occurs later in the season and is often misdiagnosed as boron deficiency. “An easy way to distinguish between the two problems is to look for flowers,” said Fernández. “Alfalfa will flower if the problem is caused by leafhoppers, but probably will not if it is due to boron deficiency.”
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 may die.
Fernández noted that it is unusual to see boron-deficiency symptoms this early in the growing season. 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 said.
If boron deficiency is suspected, there are several ways to investigate the problem. First, it is important to realize that dry conditions may be causing the symptoms, which could disappear as the soil receives more moisture.
However, it is also possible that boron levels in the soil are low. Fernández recommends taking samples for analysis. 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 the plants are 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. “Because deficiencies typically appear in patches in the field, I would suggest taking samples from the affected area and the unaffected area and analyzing the samples separately,” said Fernández
He added, “I also would recommend taking a look at the roots of the crop to determine whether other problems may be causing the symptoms.” 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 investigations show 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 advised. 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.
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