Zinc - Alphabetically Last, But Not Least

If your last name starts with “Z”, you typically were the last one called for roll call in school. Alphabetically, zinc (Zn) comes in last of the 17 essential plant nutrients, but it is definitely not the least. Widespread and chronic Zn deficiencies in crops are frequently observed in many areas.

Zinc’s role in plant nutrition is not easily explained, but generally revolves around metabolic and enzymatic processes. Plant enzymes activated by Zn are involved in carbohydrate metabolism, maintenance and integrity of cellular membranes, protein synthesis, regulation of auxin synthesis, and pollen formation.(1) Additionally, Zn helps to regulate the gene expression required for tolerance to environmental stresses and short periods of heat and salinity stress.

Deficiency symptoms of Zn include stunted growth, interveinal chlorosis, bunched and smaller leaves, shortened internodes, and sterility. Plant tissue sampling is recommended to identify or confirm Zn deficiency as some deficiency symptoms may be easily confused with other micronutrient deficiency symptoms. Plant tissue levels below 20 ppm Zn are considered deficient, and visible deficiency symptoms often manifest themselves when Zn tissue levels drop below 17 ppm. The ideal range for Zn in plant tissue typically ranges 25-100 ppm. Excessive Zn tissue levels, above 200 ppm Zn, may depress iron uptake and aggravate iron chlorosis symptoms.

All soil types may exhibit crop Zn deficiency when Zn levels are unusually low. Pay close attention to acid, leached, and sandy soils which may be low in total Zn. Availability of Zn in alkaline soils is often reduced due to decreased solubility of Zn compounds. Soils derived from parent materials such as granite, gneiss, sandstone, or limestone may also be Zn deficient. Soils with unusually high organic matter or excessive phosphorus levels may antagonize crop uptake of Zn. Soil analysis can assist in identifying soils low in available Zn. Preferred soil test levels of DTPA-extractable Zn usually fall in the 1.0 – 3.0 ppm range. Soils testing less than 0.5 ppm DTPA-extractable Zn are considered very deficient.

Correction of chronic and severe Zn deficiency usually requires regular and frequent Zn applications, both soil and foliar applied. Typical pre-plant broadcast rates of soil applied zinc sulfate fertilizers range from 5- 25 lbs Zn/acre. Chelated Zn fertilizers are effectively used in both soil and foliar applications, and offer increased fertilizer uptake and use efficiency. Using the tools of soil and plant analysis will help to identify soils and crops which will be responsive to Zn application.

1.  Hafeez, B., Y.M. Khanif and M. Saleem, Role of Zinc in Plant Nutrition-A Review, American Journal of Experimental Agriculture 3(2): 374-391, 2013.

By Michael Larkin, NW Division Agronomist