With heavy rains, a deficiency of this leachable nutrient is possible.
• By Ethan Carter, UF/IFAS Regional Crop IPM Agent, and Michael Mulvaney, UF/IFAS Cropping Systems Specialist •
In early June, peanut fields across the Florida Panhandle ranged in age from freshly planted to nearly 40 days old. Many areas have had consistent rainfall and overcast days, which has left fields saturated and complicated late plantings and spray applications. It also offers the opportunity to discuss boron and potential issues associated with this leachable nutrient.
Boron is highly mobile in the soil and can rapidly leach from the root zone. Along with manganese, it is one of the most commonly deficient micronutrients on sandy soils.
When tissue testing is done to evaluate boron levels, the entire above-ground portion of plants is collected. The desired range is 20 to 60 parts per million. Less than 20 parts per million is a critical level for deficiency and 100 parts per million or more is a critical level for toxicity.
What To Look For
In our region, typically well-drained to excessively-drained sands are most susceptible to boron deficiency. A deficiency can also occur when little or no boron is applied with the crop fertilizer. In peanut, boron deficiency is associated with hollow-heart, which lowers grade.
This condition presents itself as discoloration within the kernel. However, severe boron deficiency can present itself in a manner similar to zinc toxicity with split stems and roots, compacted branch terminals and few developing pods.
Classic deficiency symptoms in the field are compacted branch terminals and cracks on stems and pods, dark areas on internodes, mottled leaves and stubby roots with negligible pod yield. Stem and branch cracking symptoms can remain visible even after harvest to help diagnose boron deficiency.
When these type symptoms are found, a tissue sample and soil test are important to determine if the field has issues with either boron or zinc. Boron can be added to the initial crop fertilizer or applied with early herbicide and fungicide applications.
Foliar applications of one-half pound per acre elemental boron can be made during the mid-bloom stage to correct boron deficiency.
Fertility should always be monitored after prolonged rain events or heavy irrigation. Nutrient leaching can result in unexpected field problems.
Erosion Can Cause Toxicity
Toxicity symptoms first present as leaf-tip chlorosis that extends down the margins and between the veins. With increasing levels of toxicity, the leaf margins die. Excessive boron application can result from slowing the tractor down at the end of the rows rather than maintaining constant speed until cutting the sprayer off before turning.
Rainfall events can cause toxicity as well as deficiency. For example, a localized boron toxicity was caused in a field by a rainfall event of 4 to 5 inches, where sheet erosion puddled water and topsoil run-off in lower areas of the field.
Boron was probably concentrated in those lower areas through sheet erosion rather than leaching through the soil profile. A tissue sample of these plants showed boron concentration was 1,116 parts per million.
Boron and manganese are normally the most deficient micronutrients on sandy soils. If peanuts are to be planted on a sandy soil, it would be advisable to use boron at the rate of 0.5 to 0.75 pounds of elemental boron per acre.