Let your soil and rotation tell you where to expect boron deficiencies
In our December 2014 installment of Nutrient Navigator (“Plants Need Boron to Thrive”), we outlined how power-packed boron plays a critical role in yield. To review, the micronutrient helps:
- balance sugars and starches in plant cells and translocate them throughout the plant.
- move carbohydrates to the roots and assists in cell division.
- carry out nitrogen metabolism and protein formation.
- build cell walls.
- take in water through the plant’s root systems and then manage water within the plant.
So even though boron only registers 0.5 ppm to 2.0 ppm on most soil tests, you don’t want plants to run short. You don’t want to overapply either, though, because too much boron can be toxic to plants. Besides, applying any unneeded nutrient is a waste of money.
Maintaining the happy medium isn’t difficult if you follow a few basic rules, says Farm Journal Field Agronomist Ken Ferrie. First, remember boron deficiency only shows up under certain conditions.
“Environments that trigger boron deficiency include high-pH soil,” Ferrie says. “Some soils are naturally
alkaline, so you would anticipate boron problems there. Another likely source of boron deficiency is highly leachable soils—if your soils leach nitrogen, they probably also leach boron.”
Deficiencies are more common in sandy, non-irrigated, low-organic matter soils with 0.5% to 1.5% organic matter content. Organic matter is a major source of boron.
“You can run into boron deficiency on any soil if you grow crops such as alfalfa that have high removal rates and don’t restock the soil,” Ferrie says.
Because boron enters plants through a process called mass flow (water moving into plant roots, carrying
nutrients along with it), crops grown in soils with low water-holding capacity and without irrigation might experience boron deficiency.
Rotation is also a factor. If you grow highly responsive crops, such as alfalfa, you’re more likely to discover a boron deficiency than if you grow low-responsive crops, such as corn.
Visual symptoms can alert you to possible boron deficiency. At that point, you must do diagnostic tests to determine the range of limitations because of boron.
“If you see deficiency symptoms, test your soil to see whether boron is in the high, medium or low range,” Ferrie says. “Your soil test tells you the potential for a deficiency. Follow up with tissue analysis to see if boron is getting into your plants.”
One of the challenges with boron is the nutrient is not mobile in plants. “You might sample tissue this week and find adequate boron, but two weeks from now, the new growth might be deficient,” Ferrie says. “So, in suspect areas, you might need to continue sampling plant tissue throughout the growing season.”
Because boron availability is usually related to the amount of water in the soil solution, take growing conditions into account. In a sensitive soil with low boron-supplying power, you can expect to see deficiencies in dry years. But in heavier soils, if a deficiency shows up for only one season, it might have been caused by dry conditions.
Boron deficiency can be hard to notice in corn, but look for white spots or strips between veins.
“For example, in Illinois, during the 2012 drought, early tissue tests showed boron was sufficient,” Ferrie says. “Later, in the middle of the drought, almost every tissue test showed a deficiency, not just in sensitive environments where we expected deficiencies, but also in heavier, higher-organic matter soils where deficiencies are uncommon. If a farmer applied boron based on those tissue analyses, without considering the weather, he would have wasted his money.”
If you’re not sure you have a boron uptake problem, run strip trials, in addition to soil and tissue tests and
visual inspections, he says.
“A response to boron might show up in yield,” he adds. “Also conduct tissue analyses to see if you have changed the plants’ uptake. Strip tests can build your confidence that you’ll get a financial return from boron applications.”
Although boron deficiencies generally occur on sensitive soils, poor liming practices can induce deficiencies anywhere. “Applying too much lime can drive calcium levels too high,” Ferrie says. “Excessive levels of calcium can tie up boron ions in the soil, reducing uptake by the plants. The same thing can happen with excessive potassium levels.”
Avoiding overapplication is especially important in highly leachable sandy soils with low cation exchange capacity and low organic matter content. “Base your lime application rate on your soil test’s buffer pH reading, rather than the water pH reading,” Ferrie explains. “A silty clay loam soil might require four or five tons of lime per acre to neutralize a water pH reading of 4.7. But a sandy soil with the same water pH reading might need only 1,500 lb. of lime. Base lime rates on the buffer pH reading or adjust the water pH reading to account for soil texture changes.”
Varying-textured soils within a field require individual management. “Some fields have pockets of sandy soil, or sand ridges, scattered among loam or silty clay loam soils,” Ferrie says. “It’s important to identify these areas and break them down into their own management zones.
On soybeans lacking boron, look for thicker- and darker-than-normal leaves.
“Variable-rate lime application, adjusting the rate on the sand ridges, is essential to avoid boron problems,” Ferrie says. “You’ll also want to vary your boron application to account for different management zones, just as you vary nitrogen, phosphorus and potassium rates.”
Boron fertilizer can be broadcast because it will move downward with water. But because of the minute amounts required—½ lb. to 1½ lb. per acre—you’ll need to mix it with other fertilizer as a carrier. “Be careful about putting boron in the furrow with your starter fertilizer,” Ferrie warns. “A concentrated band of boron might damage plants.”
Because boron is not mobile in plants, they need to take up the nutrient almost daily. “In crops sensitive to boron uptake through the season, you need to make foliar applications,” Ferrie says. “In this situation, you must treat the plants, not the soil. Early in the season, with small plants and little leaf surface, band the spray application, rather than broadcasting. You might need multiple applications throughout the growing season.”
Typical foliar application rates range from 0.1 lb. to 0.3 lb. per acre. “If you plan to piggyback a foliar boron
application with other nutrients or herbicides, do a jar test to make sure the products are compatible,” Ferrie says.
Foliar applications work best where high pH or leachability make soil building economically inefficient, Ferrie says. “On some soils, you might be able to maintain soil test levels by applying boron every one to three years. But you might need to foliar feed in seasons when the weather goes against you,” he notes.
If you’re applying boron for the first time, check with your supplier to apply the correct rate. “Most boron fertilizers contain 11% to 20% actual boron,” Ferrie says. “If you apply a 20% product at a rate calculated for an 11% product, you will put on twice as much as intended, and that could cause toxicity problems.”
Improving soil health will help with boron uptake. “Eliminating compaction makes it easier for water to flow to plants, carrying boron along with it,” Ferrie says. “Increasing microbial activity will make it easier for the soil to mineralize boron into the soil solution, so plants can pick it up.”
Learn and Profit from Nutrient Navigator
The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher.