By John Esslinger
Zinc deficiency in vegetable crops has become more common in the last few years.
High tunnel tomatoes are the crop where I most often see zinc deficiency. That makes sense—tomatoes are a crop that requires high fertility. Tomatoes grown in high tunnels tend to significantly out-yield field-grown tomatoes, increasing demand for nutrients. There is also a tendency to use tomatoes heavily in the high tunnel crop rotation (if crops are rotated at all). Can low levels of zinc reduce yield, fruit set, and fruit quality? Absolutely.
Zinc is immobile in the plant, which means that the plant will not take zinc from older leaves to supply new leaves. Zinc deficiency, therefore, shows up in the new growth, usually as light green to yellow coloring between green veins. Be aware that manganese, magnesium, and iron deficiency symptoms can look similar. In addition to low levels of zinc in the soil, high levels of phosphorus can have a negative impact on zinc mobility in the plant. Excessive phosphorus can inhibit the movement of zinc from the root system to the upper parts of the plant.
So, what is the best way to manage zinc? It is not wise to randomly apply zinc or to apply it in relatively large amounts. Remember zinc is a micronutrient which means that while essential for plant growth, the plant uses only small amounts of this nutrient. Over application will result in zinc toxicity.
Your soil has a reserve of zinc that is available to your vegetables. Does it have enough zinc? Organic soil amendments like manure and compost provide some zinc but are they enough? Early season plant tissue testing (leaf analysis) will determine if plants are getting the zinc they need. If a tissue test indicates that zinc is needed there are a couple of ways to provide it. Zinc chelate is water soluble and is an effective way to get zinc into the plant. Zinc sulfate contains 35.5% zinc and 17.5 % sulfur. It can be sprayed on as a foliar application. Some zinc sulfate products are OMRI approved. Retest after a few weeks to determine if your zinc application was enough to correct the deficiency.
While zinc is only one part of your total plant nutrition program, it is important. A deficiency is easily corrected. Your vegetable crops are too valuable not to tissue test. Don’t let a nutrient deficiency reduce the yield and quality of your crop.