Micronutrient Toxicity

Micronutrient disorders are the fertility problems that I see most often while visiting growers as an Extension Specialist. Micronutrients (from the Greek Micro=small andnutrient=nutritive) are mineral elements needed by plants in small quantities. Even small variations from the optimum level required for plant growth can be damaging. By the same token, levels slightly above the required for good growth can be toxic. It is very important for growers to have a clear understanding about micronutrient management Table 1.

A micronutrient disorder may be a deficiency (when the micronutrient is in deficit) or a toxicity (when the micronutrient is in excess). Toxicity can occur when micronutrients are applied in excess (usually more than one application). Common sources of micronutrients are: the charger in the mix, fertilizers applied during the crop cycle, and the irrigation water. Growers MUST have an idea of how much micronutrient they are adding through each of these sources in order to avoid toxicities. Toxicity symptoms are difficult to recognize visually (only someone with a lot of experience can do it) and are usually mistaken by deficiency symptoms by growers (Photo 1) (Photo 2).

Only a correct diagnosis of the problem will lead to the proper solution. Do you have a micronutrient deficiency or is it an excess? Identify the micronutrient causing the problem. If toxicity is suspected, soil and foliar analysis are recommended for several reasons. First, visual identification of the problem is difficult in the absence of information (made available through analysis). Second, damage may be occurring that is not yet visible and by the time it becomes visible, the damage may be irreversible.

Toxicities are not easily corrected. The first step is stop adding the micronutrient that is in excess (switching to a fertilizer without the nutrient causing the problem). Slightly changing (raising, for most Micronurients) the media pH will decrease the availability of all micronutrients (including the one in excess). Growers trying to correct a micronutrient excess should raise the pH at the maximum level that the species/cultivar can tolerate for normal growth. Lastly, use antagonism Table 2, as a tool: increase slightly the level of a micronutrient that will reduce the availability of another (e.g. if zinc is at high levels, slightly increase the level of copper).