Zinc

Zinc may help you prevent or reduce the effects of a bad cold, but your plants need it, too!

Zinc (Zn) is a heavy metal micronutrient. It is called a micronutrient because only a tiny bit is needed. The recommended soil levels of zinc range from 1.0 to 7.6 parts per million (ppm). For comparison, a macronutrient, such as calcium, has a recommended range of 1000 to 1500 ppm. But don’t let the small numbers fool you!

How do plants use zinc?

Plants use zinc to break down carbohydrates and to regulate sugar consumption. You can think of it as a plant’s insulin. Zinc also activates certain enzymes. In a study by Juliane Clause, et al, we learn that zinc is moved around in a plant, partly through transpiration, and that zinc is used to facilitate dozens of important chemical reactions within a plant. That study also state that plants use zinc to counteract oxidative stress and as intercellular messengers! Some food plants, such as dandelion, contain a lot of zinc. Other plants, such as avocado, need a lot of zinc. Zinc is commonly applied to grapes, but only before the vines bloom.

Severe zinc deficiency in foreground corn, with healthy plants (planted at the same time) in the background (Alandmanson) CC BY-SA 4.0

Zinc absorption

Unlike many other plant nutrients, which are freely absorbed in solution, zinc is only available to plants in a positively charged form (Zn2+) that must be carried in on specialized transporter proteins. Zinc, on its own, cannot cross cell membranes. Once a plant absorbs a zinc molecule, it is mostly immobile. This means that signs of zinc deficiency usually show up on newer growth.

Symptoms of zinc deficiency

Zinc deficiencies are very rare, east of the Rocky Mountains. Alkaline soil and too much phosphorus, such as we have in the Bay Area, can make it difficult for your plants to absorb zinc and copper. Zinc deficiencies are more commonly seen in containerized plants. Symptoms of zinc deficiency include bronzing, twig dieback, and chlorosis. That chlorosis often presents as yellowing between young leaf veins and general bleaching that does not reach leaf edges (margins) or midribs. The bleaching may also look like yellow or white stripes between the veins of upper (newer) leaves. Zinc deficient leaves may also roll or curl, and may be smaller than normal.

Macadamia shoots exhibiting zinc deficiency (Alandmanson) CC BY-SA 4.0

Copper deficiencies look very much the same, to the untrained eye, so the only way to really know what is wrong is to submit a soil test to a local, reputable lab for analysis. [If you thought your DNA report was fascinating, you’re going to love reading your soil test results!]

Too much zinc

Just as too much phosphorus can make iron and copper unreachable for your plants, too much zinc can bind with iron, making nearly all the other nutrients unavailable. Signs of zinc toxicity, no surprise, look identical to iron deficiency. Zinc toxicity is normally seen in areas with widespread mining or industry and is characterized by severely stunted growth, transplant failure, and wilting. Dark blotches may be seen on older leaves and red areas on vines, petioles, and along margins (edges) and veins.

Before adding fertilizer, you really need to have your soil tested. And those cute little over-the-counter kits are not able to give you the information you need.

Zinc sulfate

Zinc sulfate is a compound used by commercial almond growers in autumn to force trees to drop all of their leaves. These leaves are then collected and destroyed. This is done to reduce the chance of bacterial spot on almonds. This is not something for home orchardists to use without training.

Preventing zinc problems

Soils with insufficient organic matter are more likely to have nutrient imbalances. This means heavy clay soil, improper soil pH, too much calcium carbonate, and poor soil structure can all contribute to zinc and other plant nutrient problems. The best way to prevent or counteract these problems is to develop healthy soil by composting, mulching, and sprinkling some coffee grounds around.

Research is showing us that malnourished soil results in foods with lowered nutrient values. When you start talking about feeding 7 billion people, that can be a real problem.

Feed the soil in your garden and landscape properly, so that it can feed you!