The Importance of Chelates in Plant Nutrition

Chelates are compounds that serve to make a number of nutrients, especially micro-nutrients available to plants. In the absence of chelates in the nutrient solution, plants would be deprived of key micro-nutrients which may lead to deficiencies, inhibited growth and several other undesirable conditions. Growers should therefore, ensure that these compounds are present in the nutrients they use in hydroponics cultivation.

The word chelate is derived from the Greek word “chele” which means “claw”, a rather apt association because chelation is a process somewhat like grasping and holding something with a claw. It would therefore be interesting to see how chelates facilitate absorbtion of nutrients that would otherwise be unavailable to plants. Many trace elements carry a positive charge as ions in solution, while the pores or openings on the roots and leaves of plants are negatively charged. The element is therefore unable to enter the plant because of the fixation of the positive and negative charges. However, with the addition of a chelate, elements such as iron are encapsulated and the positive charge changes into a net negative or neutral charge, which allows the element to pass through the pore into the plant.

Synthetic Chelating Agents

Most commercial fertilizers include one or more chelating agent and higher quality fertilizers incorporate several of these agents. The chelating agent in the fertilizer is identified on the label beside the trace element it serves to make available to plants. If the label on the pack has the letters EDTA beside some trace element, the fertilizer contains Ethylenediaminetetraacetate the most commonly used chelating agent. Higher quality grades of fertilizers also contain DTPA or Diethylenetriaminepentaacetate. Fertilizers that include ethylenediaminedihydroxy-phenylaceticacid, denoted as “EDDHA” beside iron on the label are the highest quality fertilizers.

Chelates have several points of attachment with which they “grasp” the trace element. EDTA has four connecting points to the elements it chelates, while DTPA has five, but the higher number of connection points may not always be an advantage. In some cases the four connection points may hold the element too tightly, while in a different situation these may not hold it tight enough.

When they require the chelated element, plants remove the element, for example iron, from the chelate and it is absorbed into the plant. However, being foreign to the plant, the chelate itself is not absorbed and is released back into solution.

The effectiveness of a chelating agent depends also depends the pH of the solution. EDTA is best suited to slightly lower than neutral pH levels while DTPA is most effective at high pH values. DTPA is more costly than EDTA and less soluble and is found in higher quality fertilizers.

The most effective of the synthetic chelating agents is ethylenediaminedihydroxy-phenylaceticacid (EDDHA). It is found only in select fertilization formulations because of its relatively high cost. It has been demonstrated that plants perform better, even under adverse conditions when the primary source of iron is chelated by EDDHA. In experiments on aeroponically grown chrysanthemums a portion of the plants were inoculated (infected) with a root disease (pythium). Only four percent of the plants that were supplied with EDDHA developed chlorosis (yellowing of leaves), while 35 % of plants supplied with DTPA became chlorotic. Of those supplied with HEDTA, 18% became chlorotic. Additionally, it was found that EDDHA supplied plants absorbed twice the amounts of zinc than plants supplied with HEDTA and DTPA.

Biological Chelating Agents

Apart from the synthetic chelating agents, there are compounds that occur naturally like fulvic acid that function as “natural” chelating agents. Plants growing naturally depend on fulvic acid and other chelating agents found in nature to enable absorption of trace elements. Fulvic acid results from the decomposition of organic matter into humus. The humus is acted upon by microbes to produce humic acids. The humic acids are further processed by micro-organisms into fulvic acids. Like some synthetic chelating agents, Fulvic acid forms four-point bonds with the elements it chelates, but unlike the synthetic agents it can be absorbed into the plant. This adds to the mobility of the nutrients within the plant. The nutrients chelated by fulvic acid can move more freely which prevents a number conditions like localized calcium deficiency which happen due to low mobility of nutrients.
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Fulvic acids can be most effective when the growing environment in the root zone (rhizosphere) is above or below optimal. Unlike synthetic chelating agents fulvic acid retains its effectiveness under conditions like high or low pH. Under such adverse conditions plants supplied with fulvic acid have been found to be remarkably free of signs of stress, deficiency etc. than plants supplied with synthetic chelating agents. Fulvic acid also produces all round improvement of transportability of various nutrients in plant tissue. This is not limited to the fertilizer minerals but also helps improve the transport of other plant fluids.

Amino acids form another category of biological chelating agents. Amino acids can function well as chelating agents due their positive and negative charges that can act as north pole and south pole of a magnet. As chelating agents amino acids form five point bonds with the mineral element.

Conclusion

As chelating agents enable absorption of a variety of nutrients vital for healthy plant growth, growers should look for nutrients that offer a range of chelating compounds. This will ensure nutrient availability over a wide range of conditions, including those above or below optimal.

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