Alabama Peanuts: Wet Fall Contributes to Low pH and Zinc Toxicity

Split stems are a symptom of zinc toxicity in peanut. Photo: Alabama Cooperative Extension

Several Alabama producers have observed “spotty” areas in peanut fields containing plants which are severely stunted and have split-stems, which are typical symptoms of zinc toxicity. A wet fall and winter may have contributed to this issue.

How does pH affect zinc toxicity?

Maintaining soil pH is arguably the most important management practice for improving soil fertility for crop production.

Most Alabama soils are naturally low in pH and must be limed to create soil conditions which increase plant nutrient availability and decrease aluminum toxicity.

The ideal pH for peanuts is in the range of 6.0 to 6.5. When the pH of a soil falls below a value of 6.0, the availability of most plant macronutrients (such as nitrogen, phosphorous, and potassium) begins to decrease. However, the availability of most plant micronutrients (such as zinc, manganese, and iron) tends to increase. Soil pH levels below the optimum range can increase zinc uptake by peanuts, which are very sensitive to zinc toxicity. This is particularly true in sandy soils.

How did a wet fall and winter contribute to pH
and zinc toxicity issues?

For soil samples collected after periods of prolonged flooding, pH tests can register a “false high” reading. Periods of heavy rainfall last fall and winter likely caused leaching of salts from the topsoil, which can cause soil pH results to read up to 0.5 pH units higher than the actual soil pH. Therefore, fields which were at a borderline critical pH level may not have triggered recommendations for lime.

Low pH has caused increased uptake of zinc by peanut plants, as evidenced in several plant tissue reports from producers who have observed zinc toxicity  symptoms in their fields. The split stems caused by zinc toxicity can also allow secondary plant pathogens to enter the peanut plant.

How can producers correct this issue?

To confirm that zinc toxicity is an issue, producers should submit soil and plant tissue samples from “good” and “bad” areas of the field for lab analysis.

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To collect soil samples, collect at least 10 soil samples from the affected area, combine, and place in a soil sample box/bag to send out for analysis. To collect tissue samples, collect the youngest, fully mature leaves from at least 10 plants in the affected area and place in a paper bag to send out for analysis.

Repeat this same process for a “good” area in the field. Check soil sample reports for low pH values in “bad” areas, and check plant tissue sample reports for high zinc levels. If soil samples indicate low pH, producers should ensure that lime is applied prior to the next crop season.


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