(News article for July 17, 2021. This article is part of a series. Click to see the introductory article and ones about nitrogen fertilizers; phosphorus and potassium fertilizers; and calcium, magnesium, and sulfur fertilizers.)
Micronutrients – including iron, manganese, boron, zinc, copper, nickel, molybdenum, and chlorine – are no less important than macronutrients, but plants need them in smaller quantities. In most cases, the amounts naturally available in soil are sufficient, but there are situations in which micronutrient fertilization is needed.
It’s critical not to overapply micronutrients. They can be toxic to plants at concentrations much lower than what would be normal for macronutrients.
If a micronutrient deficiency is suspected, it’s generally a good idea to take both a plant tissue sample and a soil sample before attempting to address the problem.
Soil pH plays an important role in micronutrient availability. In some cases, the long-term solution to a micronutrient deficiency or toxicity issue is to adjust the soil pH to an appropriate level for that plant.
For example, iron deficiency is occasionally observed in blueberry plants. Symptoms include young leaves that are light green or yellow. This tends to occur when soil pH is too high for blueberries, which are native to acidic soils. If a soil test confirms that pH is high, this can be addressed, and iron deficiency will likely cease to be a problem.
Micronutrient availability is affected not just by soil pH but also by the concentrations of other nutrients in the soil. Excessively high levels of certain nutrients can impede uptake of others.
If a plant tissue sample report confirms that the concentration of a micronutrient is low, options for addressing the deficiency depend on the nutrient, the crop, and, in some cases, soil pH.
Pecan trees sometimes experience zinc deficiency. Symptoms include wavy leaf margins and yellowing between veins of leaves near the ends of branches. In Louisiana, commercial pecan growers are advised to send in leaf samples for plant tissue analysis between July 7 and August 7.
If a tissue sample shows low zinc in pecan leaves and a soil test shows that soil is relatively acidic (below approximately pH 6, which would be lower than ideal for pecans), a soil application of a zinc-containing fertilizer, such as zinc sulfate, can be effective. However, applications of zinc sulfate dissolved in water to the leafy canopy during the spring and summer are expected to be more effective, especially when soil pH is high.
The topic of micronutrient fertility is a broad one. If you have questions about specific situations, let me or your local agent know.
Contact Mary Helen Ferguson.
High soil pH often leads to symptoms of iron deficiency on blueberry plants. Yellowing between the veins is seen on young leaves. (Photo: John Hartman, University of Kentucky, Bugwood.org)
In pecan, zinc deficiency symptoms include wavy leaf margins and yellowing between veins of leaves near the ends of branches. (Jonas Janner Hamann, Universidade Federal de Santa Maria (UFSM), Bugwood.org)
Discoloration in turnip root suggesting boron deficiency. Note that discoloration will eventually occur even in healthy roots, after a cut is made. If boron deficiency is the cause, symptoms are observable immediately after cutting. (Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org)
Discoloration of melon leaves after copper fungicide application, suggesting copper toxicity. (Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org)