"Fully chelated"? the devil is in the detail"

In today's cost conscious world some growers are attracted by apparently cheaper fertilizer bends. The cost cutting is often down to the supplier using inorganic trace element sources (sulphates) rather than the reliable but dearer chelated (EDTA) versions.

However, fertilizers with inorganic trace elements have serious limitations especially in modern growing systems. Limitations that can end up costing more money in the long run than using top quality, fully chelated formulations. 

Solufeed MD and industry expert Dick Holden explains why. 

Along with Boron (B) and Molybdenum (Mo) the six essential micronutrients include the metals Copper (Cu), Iron (Fe), Manganese (Mn) and Zinc (Zn).

Growers know that soluble iron (Fe) will chemically react with soluble phosphorus (P) supplied as part of the fertilizer. The reaction forms an insoluble precipitate of iron phosphate (FeO4P) that settles out as a deposit in the bottom of the mixing tank. Being insoluble, this material cannot be taken up by the plant, and so the plants quickly become iron deficient. For this reason the iron (Fe) is provided in "chelated" form, usually as Fe EDTA chelate. The chelate binds to the iron and stops it reacting with phosphorus, or anything else, but the chelated iron remains in solution, and it remains available to the plant. 

But what about the other metals, Copper (Cu), Manganese (Mn) and Zinc (Zn). Should they be chelated too…? These metals do not react with phosphorus, so on the face of it there is no reason to chelate these metals. 

However, within the fertilizer solution, the EDTA chelate is not irreversibly attached to the iron (Fe). The chelate "prefers" to bond with the other soluble metals - a process called "preferential chelation". The chelate will attach to the Copper (Cu), Manganese (Mn) and Zinc (Zn) in preference to the Iron (Fe). Once the iron (Fe) is no longer bonded to the EDTA, it is free to react with the phosphorus (P) and is lost from the solution. 

The process of "preferential chelation" is illustrated in the diagram below. This shows all the components of the nutrient solution and the interactions which take place . The iron (Fe) from the FeEDTA is released into the solution which then reacts with phosphorus (P) to form an isoluble iron phosphate precipitate. 

The photographs below show the "preferential chelation" process recreated in the laboratory. Precipitation of iron phosphate first results in a solution with a cloudy appearance which on standing for a short time, develops into a visible layer.

By contrast the beaker on the right shows a clear solution by using fully EDTA chelated sources of copper (Cu), manganese (Mn) and zinc (Zn) along with the usual FeEDTA. No precipitation means no loss of iron.

This process can be readily observed in the fertilizer stock tank solution, where an insoluble deposit will soon form.. The concentrated solution of nutrients react together very quickly. At final dilution rates, the concentration of nutrients is low, and the reaction it therefore much slower - although it still occurs. For this reason it is possible to get away with only chelating the iron (Fe) - and rely on cheap sulphate formulations for the copper (Cu), manganese (Mn) and zinc (Zn) - in situations where the components are held apart and are only mixed together at the low rates of final dilution e.g. by dosing directly into the irrigation system. This only works if, for example, the iron (Fe) chelate is held separate from the other metals - perhaps in mix with the calcium nitrate and so is not applicable for NPK blends where all the trace elements are included together and the iron cannot be separated. 

For a robust product that will remain fully in solution in the stock tank, the answer is to chelate all the metals, not just the iron (Fe). This is a little more costly, but for peace of mind that all the micronutrients remain fully available, it is excellent value.

Below, Solufeed Strawberry Special water soluble fertilizer blend with label expanded to show the trace element content and form. Note that the copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn) are all fully chelated with EDTA.

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