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Micronutrient Disorders

by Claudio C. Pasian    

Micronutrient disorders are, perhaps, the most common fertility problem in soilless media floriculture crop production. Micronutrients (from the Greek Micro = small and nutrient = 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. This fact sheet is a brief overview of the principles that control the availability of micronutrients in soilless mixes and how to correct imbalances.

A micronutrient disorder may be a deficiency (when the micronutrient is in deficit) or a toxicity (when the micronutrient is in excess). Deficiencies can occur either because the nutrients are not present in the growing mix or because the nutrient is present but unavailable to the plant. (Occasionally, plants with roots damaged by Pythium or other pathogens may show micronutrient deficiency symptoms.) Some commercially prepared mixes have a fertilizer charger that may include micronutrients. Growers preparing their own mixes should use one of the many commercially available micronutrient complexes to ensure that the micronutrients are present in the growing mix.

Sometimes, the micronutrient present in a growing mix is not available to the plant (the plant cannot take it up). Micronutrient availability is influenced by media pH: except for molybdenum, the availability decreases with increasing media pH and vice versa. Magnesium, a secondary macronutrient, behaves as molybdenum in reference to pH. Water alkalinity is an important factor modifying media pH and hence micronutrient availability. It is important to maintain the pH for soilless media between 5.5 and 6.3. Some crops are more sensitive to media pH than others: hanging petunias and gerberas must be maintained at pH levels of 5.5 in order to avoid micronutrient deficiency symptoms. Other crops are more tolerant to pH changes. Table 1 shows the minimum and maximum levels.

If the deficiency is due to pH imbalance, the approach is to modify the pH of the mix. In this case, adding micronutrients can make matters worse because the level of individual micronutrients may affect the level in the plant of other micronutrients through a process called antagonism. For example, too much iron may produce manganese and zinc deficiencies, while high levels of manganese may result in iron and zinc deficiencies. Copper and zinc are also antagonistic: too much of one may produce deficiency of the other (Table 2).

On the other hand, toxicity can occur when micronutrients are applied in excess (usually more than one application). Common sources of micronutrients are: the charger in the mix, the irrigation water, and fertilizers applied during the crop cycle. 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 much experience can do it) and are usually mistaken by deficiency symptoms by growers.

How do we resolve these problems? First of all, 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. Identify the cause of the problem: is the nutrient not present or is it present but unavailable? Answering these questions will help you (and your Extension agent or consultant) tackle the problem.

If deficiency or 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.

Deficiencies can be corrected by adding the micronutrient that is in deficit or by correcting the factor that makes it unavailable (e.g., high pH). This second course of action is very common among growers who have high alkalinity irrigation water. If only one micronutrient is deficient, DO NOT apply a micronutrient complex fertilizer because, as mentioned above, imbalances can cause antagonism. Apply a salt that contains only the deficient micronutrient. Micronutrients can be:

1) added over time in small amounts with the irrigation water (Table 3);

2) applied once with a concentrated solution during a normal watering (Table 4);

3) applied as a single foliar spray (Table 5).

Toxicities are not easily corrected. The first step is to stop adding the micronutrient that is in excess (switching to a fertilizer without the nutrient causing the problem). Slightly changing (raising, for most micronutrients) 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 as a tool: slightly increase 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).

Micronutrient management is complex and difficult. A more complete treatment of this subject would require more space than available here. This description of the problem should pique your curiosity. At the very least, you should follow this advice: Don't guess. Test!

NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.


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