Liquid Trace Elements: How to choose between the different forms. Sulphate, oxide or chelate?

Let’s say you’ve decided your crop looks nutrient deficient, and you’re going to apply a foliar Zinc trace element. That’s all good. But what form of liquid zinc are you going to use? Zinc sulphate, zinc oxide or a chelated product? With so many different products on the market, it can be overwhelming trying to decide which one will give you the best results for your crop. On top of that, there is confusion around what products can and can’t be tank mixed.

It’s important to understand that every product has a different chemical structure. Understanding these structures and how they react on the surface of plants is essential to making the right product choice. There are three main forms you will come across when choosing liquid trace elements: Sulphates, Oxides and Chelates.
We will go over the main features of each of these, so you know the difference.

Note: The choice to apply trace elements as a foliar spray rather than granules is because plants absorb nutrients faster through its leaves than its roots. Nutrients pass through ‘stomata’ which are pores on the plant leaves.

Sulphates:

First up, let’s look at sulphates. These are absorbed the quickest out of the three forms, but nutrients are generally also lost quickest over time. Sulphates are excellent at providing a quick hit of nutrients to plants. But if you’re looking to provide nutrients over an extended time period, there might be better options. Sulphates contain a lower drum analysis than oxides. For example, Zinc Sulphate 160g/l in 110lt or 1000lt drums. Out of the three forms, sulphates are generally applied at the highest rates to maximise uptake. Sulphates are a low-cost option compared to other products on the market.
Sulphates also have a high salt index. This is something to be mindful of in harsh conditions or high temperatures, as it can cause burn on sensitive or soft plants. This also means sulphates are not a good choice if you’re looking to mix in the tank. They’re highly reactive, so our advice is not to try to combine these with other chemicals without consulting your agronomist first.

Oxides:

Oxides are generally are taken up the slowest by plants, and release nutrients over a longer period of time. You’ll come across these as suspension products with the highest nutrient analysis in the drum — for example, Activist Max Zinc at 70% and Rutec Zn 7000 at 70% zinc.
Unlike sulphates, oxides don’t contain salts. Meaning they are much safer to use on softer crops and when weather conditions are extreme. This also means oxides might be a better choice in tank mixes as they aren’t as reactive, and are more compatible than sulphates.

Chelates:

In terms of speed of plant uptake, Chelates sit somewhere between sulphates and oxides, ensuring safe and fairly fast delivery of nutrients. They’re a great form of trace element. They’re also the best choice if you want to tank mix with other chemicals. This is because the chelate chemical structure wraps around the actual trace element molecule, helping to protect it. Chelates work well in tank mixes with other chemicals because they don’t react with the other elements in the mix or in the soil. They also aren’t affected by the pH of the soil or any residual anions. However, as you might have surmised, the technology and benefits of chelated products make them the most expensive out of the three.
Synthetic chelates are very large structures. This is why chelated products have a low drum analysis, for example Rutec Zn740 with Zinc at 7.4%. If you see a chelate that claims to be more than 7.5% in microelement analysis, be wary – it potentially might not be fully chelated and may react in tank mixes.

There are some other factors to take into consideration when deciding on liquid fertilisers, such as plant moisture content and particle size.
Moisture content and pH of the plant will both influence how products are taken up through the plant. Soil moisture can also affect the uptake of nutrients in a variety of ways. For example, it impacts the solubility of ions, and how nutrients interact with other compounds such as sodium and chloride. Drying soils, or soil where root growth is restricted, slows down or stop soluble ions moving, and the ability of plants to take up trace elements can be restricted.

When it comes to trace elements, size does matter. And this is where spending the extra dollar does ensure better quality and better performance.
The size of the particles in a liquid suspension of trace element products will also have a big impact on how well they are taken up. Cheaper generic products generally have unrefined and large sized particles. These will restrict the uptake of micronutrients and be slow and inefficient. This is because the particles on the surface of the plant leaf are generally smaller and fewer. Bigger particles can be wasted, and some particles may end up being too large to stay on the surface of the leaves. Products with smaller particle size are the best choice for uptake and distribution. Quality suppliers and brands usually have invested the time and money to ensure this. The smaller the particle, the faster the uptake. Also, the number of particles per unit volume is much higher. These products will provide the highest nutrient uptake, so make sure the products you are using are from a reputable brand to ensure the best results possible.

If you have any questions regarding formulations or trace elements, please contact our agronomy team.