Zinc for crop production

Zinc (Zn) is an essential micronutrient for plant life. In Minnesota, while some soils are capable of supplying adequate amounts for crop production, addition of zinc fertilizers is needed for others. Zinc is a recommended micronutrient in fertilizer programs for production of corn, sweet corn, and edible beans. Several research projects have focused on the use of this nutrient, and much of the following information is based on the results of that research.

Natural sources of zinc

Zinc exits naturally in rocks. The amount of zinc present in the soil depends on the parent materials of that soil. Sandy and highly leached acid soils generally have low plant available zinc. Mineral soils with low soil organic matter also exhibit zinc deficiency. In contrast, soils originating from igneous rocks are higher in zinc. Plants take up zinc as the divalent ionic form (Zn2+) and chelated-zinc.

The role of zinc in the plant

Zinc is an important component of various enzymes that are responsible for driving many metabolic reactions in all crops. Growth and development would stop if specific enzymes were not present in plant tissue. Carbohydrate, protein, and chlorophyll formation is significantly reduced in zinc-deficient plants. Therefore, a constant and continuous supply of zinc is needed for optimum growth and maximum yield.

Zinc deficiency

Research at the University of Minnesota as well as other universities has identified soil conditions where a response to zinc fertilizers is expected. These conditions are:

Soil Temperature: Cool soil temperatures in early spring can intensify the need for zinc. When soil temperature is low, mineralization of soil organic matter slows down resulting in less zinc being released in the soil solution. Cool temperatures stunt root growth and reduce the plant’s ability to find new sources of zinc in the soil profile.

Soil texture

In Minnesota, crop response to fertilizer zinc happens mostly on fine-textured soils. Recent studies indicate a response to zinc can occur when high-yielding crops are grown on sandy soils with low organic matter content. However, the measured response to zinc fertilization in these situations has been small and has not occurred every year.

Soil tests for zinc are recommended to determine if zinc is needed in a fertilizer program.

Topsoil removal

The probability of a response to zinc fertilization increases where topsoil has been removed or eroded. When soils are eroded, the amount of free calcium carbonate on the soil surface increases. The need for zinc in a fertilizer program increases as the percentage of free calcium carbonate increases.

Previous crop

The probability of a response to zinc fertilization increases if either corn or dry edible beans follow a crop of sugar beets (a non-mycorrhizal plant). This condition is called “fallow syndrome” and is a result of poor colonization of the plant root with arbuscular mycorrhizal fungus which increases the plant’s ability to take up phosphorus and zinc.

Phosphorus level

There is a known relationship between phosphorus and zinc in the soil. Previous studies in Minnesota have shown excessive application of phosphate fertilizers caused zinc deficiency in corn, which resulted in reduced grain yield. This yield reduction occurred mainly in calcareous soil with high pH (pH > 8.3) and low soil test phosphorus and zinc.

A phosphorus-induced zinc deficiency is a concern and may occur only if very high rates of phosphate fertilizer (more than 200 lb P2O5/acre) are used and the soil test for zinc is in the range between Low and Very Low.

Crops that respond to zinc

Crops vary in how much zinc is required to complete their life cycles.

• Large response to zinc: apple, dry edible beans, corn, onion, snap bean, sweet corn
• Moderate response to zinc: grape, lettuce, potato, soybean, tomato
• Small response to zinc: alfalfa, asparagus, barley, canola, carrot, clovers, grass pasture, oat, peas, rye, sugar beet, sunflower, wheat

Deficiency symptoms

Plants fail to develop normally when they are deficient in zinc and certain characteristic deficiency symptoms will appear. Symptoms usually appear in corn in the first two or three weeks of the growing season. If the zinc deficiency is severe, these symptoms may last throughout the entire season.

A zinc deficiency in corn is characterized by broad bands of striped tissue on each side of the leaf midrib. These stripes begin on the part of the leaf closest to the stalk and appear first on the upper part of the plant. A zinc-deficient corn plant also looks stunted.

Zinc deficiency in edible beans first appears as a yellowing of the lower leaves. As the season progresses, this yellowing becomes bronze or brown. The leaves have a rusty appearance. For this crop, however, care must be taken to avoid confusing sunburned leaves with zinc deficiency.

Zinc deficiency in soybean is not common in Minnesota. Symptoms of zinc deficiency in soybean include interveinal mottling or chlorosis similar to symptoms in dry edible beans. Zinc deficiency should not be confused with iron deficiency chlorosis which is more common in soybean in Minnesota.

For both corn and edible beans, suspected zinc deficiency symptoms should be confirmed with plant tissue analysis.

Phosphorus-induced zinc deficiency might be a concern when high rates of manure are applied to cropland. The manure, however, also contains zinc that can be used for crop growth. Therefore, phosphorus supplied from manure should not create a zinc deficiency for crop production in Minnesota.

Tissue and soil tests for zinc

The need for zinc in a fertilizer program can be determined through soil tests and plant analyses. Plant analyses can confirm a suspected zinc deficiency during the growing season. However, plant analysis should be used with soil tests before arriving at firm recommendations for using zinc in a fertilizer program.

Tissue zinc concentration varies between growth stages. Crops must be sampled at the growth stage listed if the interpretation of plant analysis information is to be accurate.