Zinc is an essential plant micronutrient. It is important for production of plant growth hormones and proteins and is involved in sugar consumption. Good root development as well as carbohydrate and chlorophyll formation are also dependent on zinc. Maintaining adequate zinc levels is important for enabling plants to withstand low air temperatures. Zinc is also involved in the synthesis of auxin, a plant hormone that helps plants determine whether to focus on growing tall or becoming bushy.
The amount of zinc and its availability in soil can be affected by:
- pH ‑ increased pH decreases zinc availability
- Phosphorus ‑ high levels of phosphorus decrease zinc availability
- Organic matter ‑ organic matter adds zinc and can increase zinc availability
- Nitrogen ‑ low levels of nitrogen can reduce a plant's ability to uptake zinc
- Moisture ‑ excessively wet soil reduces the ability of plants to uptake zinc
- Copper ‑ zinc and copper appear to be taken up by plants via the same mechanism so when one is in excess, plants don't absorb enough of the other
- Magnesium ‑ magnesium may assist with the uptake of zinc
- Arsenic ‑ high levels of arsenic can inhibit zinc uptake
Ideally, for healthy and productive soil the concentration of zinc should be 1‑200 mg/kg. If the concentration of nickel in your soil is greater than 7400 mg/kg it would be a good idea to test the food grown in your soil and seek expert advice about whether you need to take special corrective action in the levels are high.
Zinc deficiency is most commonly seen on alkaline and sandy soil especially if the soil is boggy. Excess levels of phosphorus and copper as well as low levels of nitrogen in the soil can also increase the chances of zinc deficiency. Plants may also find it difficult to take up zinc if the soil is contaminated with high levels of arsenic.
Symptoms of Zinc Deficiency
Zinc deficiency may result in interveinal chlorosis and stunted leaves. As zinc is only moderately mobile in plants, symptoms may occur on either old or new leaves though if the size of the leaves is affected, it's more likely to be older leaves that show the symptoms. Some species may develop necrotic spots though this is certainly not a universal symptom.
Treating Zinc Deficiency
Treating zinc deficiency can be quite complicated. Firstly, if your soil's pH is too high be sure to lower it (sulphur will lower soil pH but there are other options too). Zinc is most available to plants if the soil pH is between 5.5 and 6.5 although most plants will uptake sufficient zinc if the pH is between 4.5 and 7.5 provided the soil is not deficient and there aren't any other problems with the soil.
If soil tests reveal that the soil contains excess levels of
If soil tests reveal low nitrogen levels, applying a nitrogen containing fertiliser can help ‑ just be sure it's not going to upset the balance of other nutrients by following the instructions on the packaging. Likewise, if the magnesium level in the soil is low, adding additional magnesium may also boost your plants' uptake of zinc. Only after all of these steps have been taken care of should the addition of zinc be considered.
If you decide to add zinc to your soil, it is crucial that you take the above actions and then test your soil zinc levels. I would recommend using a soil testing company that will provide guidance on how to correct any identified issues as they will be able to tell you exactly how much zinc to add to your soil and thus you'll avoid over correction. If the deficiency is mild, animal manure may be all that is required to correct the problem. Another fertiliser that may be suitable for organic gardens and which may useful if the deficiency is severe, is zinc chelate (check the packaging to confirm that the brand produces certified organic zinc chelate). If you wish to use a synthetic zinc containing fertiliser, you might choose zinc sulfate, zinc oxide or zinc nitrate. The latter is particularly useful if you find yourself having to constantly apply nitrogen to your soil.
Zinc toxicity is most common when plants are grown in acidic soil and when there is excess magnesium in the soil.
Symptoms of Zinc Toxicity
Zinc toxicity is hard to detect because the most common symptom is darker than normal leaves. Excess zinc may however induce
Treating Zinc Toxicity
Treating an excess of zinc in the soil can also be rather complicated; primarily, you need to focus on treating deficiencies in other elements. Firstly, soil pH should be monitored to ensure it doesn't drop too low (lime can raise the pH if it drops below 6 for most plants and it is also useful for providing calcium if the soil is deficient). If