SHORT COMPARISON OF ZM-GROW SOLUTION AND CHELATED FERTILISERS 

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This white paper describes the difference between foliar sulphate fertiliser ZM-Grow and foliar chelated fertilisers.

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SUMMARY​

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• Chelation is used to protect micronutrients. If micronutrients are mixed with phosphorus or basic soil, they can become bound to solids and unusable;

• If ZM-Grow is sprayed directly onto leaves without touching phosphorus or soil, it is well absorbed to plant leaves, even though it is “unprotected”;

• Chelation is often expensive, and using sulphates can be more cost-efficient;

• Sulphates provide plants with sulphur, whereas many chelates do not.

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CHEMICAL DIFFERENCES

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Sulphate and chelate based fertilisers are commonly used in foliar fertilisation. Both are completely water-soluble and don’t contain solids, therefore spraying nozzle strainers of the sprayers are not affected by blockages.

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In sulphates, the micronutrient cation and sulphate anion are together, as seen in Figure 1 for zinc sulphate (ZnSO4). The micronutrient then exits the structure easily and is absorbed by the plant.

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Figure 1. Zinc sulphate structure

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In chelates, the micronutrient is inserted into a complex organic structure,  like an EDTA molecule. This is seen in Figure 2 as Zn-EDTA chelate (Zn((HO2CCH2)2NCH2CH2N(CH2CO2H)2)), where M = Zn.

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The end structure resembles a lobster claw (Latin chēlē), where the name chelate originates. EDTA is the most common chelation agent, but many other chemicals like DTPA, citric acid etc. can be used for the same purpose.

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The size difference of the structures also has an effect, some crops may better absorb the smaller sized and simple sulphate molecules as opposed to bigger and more complex chelated molecules.

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Figure 2. Zinc-EDTA chelate structure (where M = Zn)

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DIFFERENCES IN APPLICATION 

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Chelates are often used when the micronutrients are mixed with a product that will bind the micronutrients to an unusable form. Typical examples are mixing micronutrients with a phosphorus fertiliser, or adding the micronutrients to highly alkaline soil, where pH is high.

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The claw-like chelate structure protects the micronutrients and gives them more time to become absorbed before they are bound to solids. A downside is that chelating the micronutrients can be expensive, which can reflect on the end price of the fertiliser.

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Chelation is often unnecessary if the fertiliser is applied with foliar spraying instead of applying to the soil, and phosphorus is not added to the same spray application. Due to water solution not having a binding agent, using ZM-Grow for foliar sulphate fertilisation works without problems.

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Sulphate formed products are often cheaper than chelates, and for many crops, Tracegrow’s sulphate based ZM-Grow works more efficiently than chelated alternatives in trail tests. (see figure 1 for citrus). Mixing other agrichemicals like herbicides and fungicides with ZM-Grow has no detrimental effect. 

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Figure 1 . Small field test. Test areas sprayed with backpack mist blower, 900l/ha of water, 4 replicates.

ZM-Grow vs. competing chelate product in a citrus crop test in California in 2018. Leaf metal contents were measured from washed leaves.

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Sulphates contain sulphur, which the chelates usually lack. Therefore, when the plants have sulphur deficiencies, they can be corrected using sulphates, but not necessarily with chelates. In the past sulphur replenishment came from acid rains, but nowadays this does not happen, therefore supplementing has become necessary.

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Using sulphate-based fertilisers can also have a useful side effect since sulphur can act against fungal diseases. In some cases, this will lessen the need for fungicide use.

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ENVIROMENTAL CONSIDERATIONS 

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ZM-Grow is manufactured from recycled micronutrients. Its production has very low energy consumption compared to products that use virgin materials. Chelated versions are more complex, and made from excavated and enriched virgin micronutrients, their energy consumption is higher.

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EDTA is not very biodegradable and can accumulate in the environment. EDTA can also free some earth- bound heavy metals when it dissolves into the ground. It can break the strong bonds that the metals have with soil particles, freeing them in to the environment. Therefore, when manufacturing and using chelates, biodegradable chelation agents are preferred.