By Jim Isleib
Even though you didn’t request a special soil test for boron when you sent your samples into the Michigan State University Soil and Plant Nutrient Laboratory, you might have received a recommendation to add 1 or 2 pounds for boron per acre for your selected crop. Like nitrogen, the boron recommendation is not based on chemical analysis of your regular soil sample. Instead, it is based on many years of accumulated research results regarding crop response to boron. Special tests for soil content of nitrogen and boron are available through MSU Soil and Plant Nutrient Laboratory and other reputable labs.
There are two main reasons why boron is routinely recommended:
- Coarse-textured soil low in organic matter.
- Crops sensitive to boron deficiency.
Course-texted soil low in organic matter
Plant-available boron exists in soil mostly as a neutral boric acid molecule. The source for this boron is mostly from decomposing organic matter in the soil. Many, if not most, Michigan agricultural soils contain adequate boron for good plant development. Coarse-textured soils, especially those low in organic material, are the most common places where boron deficiency occurs.
Coarse-textured soils are more prone to drought and, when soils are very dry, decomposition of organic material slows, sometimes causing boron deficiency. Boron is not bound to clay or organic matter particles and leaches through the soil profile readily, similar to nitrogen. In coarse-textured soils, this leaching is likely when wet conditions prevail.
Crops sensitive to boron deficiency
Some crops are much more sensitive to boron deficiency than others. Perennial legume forage crops, including alfalfa, clovers and birdsfoot trefoil are sensitive to low soil boron. Boron addition is often recommended for these crops. Brassicas, including certain vegetables (cauliflower, cabbage, broccoli, turnip, rutabaga, kale and others), canola and forage brassicas like rape and forage turnip are among the crops known to be sensitive to low soil boron levels.
Sugar beets, apples, red beets, celery and sunflowers are also sensitive to boron deficiency. On soils with low cation exchange capacity, your soil test report may include a boron recommendation for these crops.
How can you correct boron deficiency?
With a lot of care. When deficient for a selected crop, application of the proper amount of boron can be a good economical decision. Boron can be blended with dry granular fertilizers, often with potash. Boron can also be mixed with liquid fertilizers or applied to the soil alone and incorporated. However, over-application of boron or the wrong placement of boron can lead to crop damage.
Problems with boron toxicity can occur when crops sensitive to boron toxicity are planted with boron fertilizers present or sprayed with liquids containing boron. Pre-plant broadcast application of boron fertilizer is generally safer for toxicity-sensitive crops than placement in a fertilizer band.
A common problem situation arises when a smaller-scale producer receives a soil test report including a boron recommendation. “How do I do this?” is the usual question. In some cases where the crop is not of high economic importance, such as a wildlife planting or home vegetable garden, you may choose to ignore the boron recommended on your soil test, but not necessarily. The following list provides some ideas for boron application for the market gardener or small scale grower.
- Boron may be blended into dry fertilizers such as 0-0-60 or 0-14-42.
- Boron fertilizers include borax (11 percent boron) and borate granular (14 percent boron). Solubor (20 percent boron liquid) is foliar applied and must be applied at recommended rate for specific crops.
- Application of 9 pounds borax per acre will supply 1 pound boron per acre.
- For gardeners, about 4 teaspoons borax per 1,000 square feet is equivalent to 1 pound boron per acre.
- Dry boron fertilizers should be broadcast along with other fertilizers and worked into soil.
- Boron fertilizer should not be applied if grasses including hay, pasture, turf, small grains or corn are sown immediately following application.
- Manure generally contains 0.03–0.08 pounds boron per ton, more if composted.