Availability to Plants

Zinc is found in very small amounts in soil, usually from 10 – 300 ppm (parts per million), and mostly in rock minerals. The Zn²⁺ ion can in fact replace Fe²⁺ and Mg²⁺ ions in minerals.

Zinc can also be found on the exchange sites of clay minerals and organic matter, and adsorbed to solid surfaces in the form of Zn²⁺, zinc hydroxide ions (ZnOH⁺) and zinc chloride ions (ZnCl⁺).

Zinc adsorption rises as pH rises, making this micronutrient less available in solution to plants. Zinc solubility is especially reduced in high pH soils and in the presence of calcium carbonate.

Zinc in Biochemistry

The zinc requirement of plants is low, but important nonetheless. Zn²⁺ is similar in function to Mg²⁺ and Mn²⁺, in that it is an essential cofactor in enzyme reactions. Many enzymes cannot function if these ions are not present.

Zinc is required for enzymes involved with photosynthesis and for those which detoxify free radicals. It also has roles in DNA and RNA synthesis as well as stabilising those molecules once formed. Zinc’s role in DNA and RNA synthesis in turn has effects on cell reproduction and overall plant growth.

Zinc incorporates into plant tissue and increases a plant’s ability to withstand cold temperatures. Zinc is also important for the formation of auxins — plant growth hormones which aid in growth regulation and stem elongation.

Zinc Deficiency Symptoms

Zinc is not very mobile in plants, and a zinc deficiency will appear in new growth first. Because of zinc’s role in DNA and RNA synthesis, a deficiency appears as stunted growth, with small leaves and shortened lengths between the stem nodes. The nodes are the parts on a stem from which leaves and buds develop, thus a characteristic of a zinc deficiency is small, stiff leaves clustered in a rosette or whorl shape at the top of a young shoot. The leaves may also be narrower than normal, and with wavy edges.

Bud development is also poor, which leads to reduced branching or flowering.

A deficiency can also show as interveinal chlorosis (insufficient production of chlorophyll between the veins). These chlorotic regions could be pale green, yellow, or white in an extreme case. Chlorotic areas may also have necrotic (dead) patches or turn a bronze colour.

Zinc Toxicity Symptoms

Zinc toxicity is not common, as it isn’t usually present in toxic amounts. But it is more likely in acid soils, where zinc is less likely to adsorb to particles and more likely to be in a soluble form in soil water. Toxicity is also possible when there are high amounts of magnesium. Another common source of zinc toxicity is from water in contact with galvanised metal surfaces, new or old — even old galvanised rainwater tanks can present a problem.

A zinc toxicity can resemble a zinc deficiency, which can be confusing. Typical signs are reduced root growth and leaf expansion, followed by chlorosis, but can also include smaller leaves, clorotic new leaves, necrotic leaf tips, and overall stunted growth.

High amounts of zinc can also reduce the uptake of phosphorus, iron, manganese or copper and cause deficiencies in those.