Some Illinois alfalfa fields are showing visible signs of boron deficiency, reports Fabian Fernandez, University of Illinois Extension soil fertility specialist.
The problem is most common in sandy soils that are low in organic matter or in soils with a high pH, he says. 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," Fernandez explains. 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 that 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," says Fernandez. "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 cell development, often causing shorter internodes and bunching of the top leaves. If the deficiency is severe, the growing points may die.
It usually appears in the second or third alfalfa cutting, 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.
If boron deficiency is suspected, it’s important to realize that dry conditions may be causing the symptoms, which could disappear as the moisture situation improves. But it’s also possible that boron levels in the soil are low, so Fernandez recommends having a sample tested. If a hot-water extraction test shows less than 2 lbs of boron per acre, it needs to be applied.
Another way to diagnose boron deficiency is to take samples from the top 6” of plants at early bloom. If the boron concentration is less than 25 ppm, an 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," he says.
"I also would recommend taking a look at the roots of the crop to determine whether other problems may be causing the symptoms," Fernandez adds. 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. A possible strategy is to apply 30 lbs/acre of household borax, which contains 3.3 lbs of boron.
If the field needs phosphorus or potassium, it might be easier to mix boron with the other fertilizer. "In sandy soils for alfalfa production, I suggest yearly applications of 1-2 lbs of boron per acre," Fernandez advises. For fine-textured soils with boron deficiency, 3-4 lbs of boron per acre in the year of establishment will usually correct the problem for a few years.
If oats are planted with the alfalfa, 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. If a foliar treatment is preferred, a rate of 0.1 to 0.3 lbs of boron per acre is normally enough to correct the problem, says Fernandez.