Can micronutrients become toxic to plants?

Plant micronutrients can become toxic if they're applied in excess. In drip-irrigated orchards, boron toxicity shows up as burned brown edges on an apple leaf. Farmers apply too much iron to soybeans, causing a bronze discoloration of the leaves. I have resuscitated citrus groves. The primary problem was that the farmer doubled the zinc rate to try to protect the crop's health, but the result was significantly stunting root development that would remain permanently damaged.

Soil pH emphasizes poison risk. Acidic soils that are less than 5.5 in pH can release toxic aluminum and manganese into the solution. Alkaline soils that are greater than 7.5 in pH can produce toxic molybdenum accumulation in legumes. One client had established an over-limited alfalfa field that reached 12 ppm molybdenum, which is three times more than the safe limit. This forced the client to remediate the soil for one full year with sulfur.

Split applications can help with the risk of overdosing on the plant. Apply half the zinc sulfate pre-planting and then the half at flowering. For foliar applications, the foliar sprays are mixed with chelated iron at a 0.25% concentration, since higher doses tend to scorch the leaves. Before spraying the entire field, I check every batch of ten plants.

Tissue testing can reveal toxicity in its early stages. In regards to iron overload in rice paddies, tissue tests show levels above 400 ppm, with normal levels around 150 ppm. Immediately flush fields and replace the soil with iron-free nutrients. In the case of excessive boron, leach fields with 6+ inches of water. Never rely solely on soil tests; the root's absorption of toxins is unpredictable, particularly during wet seasons.

Read the full article: 7 Essential Micronutrients for Plants: Complete Guide