Why is chemistry so complex? What is the “key?” Here we go again getting lost in the weeds of chemistry. Maybe not so much. I’ll try my best to give some simple examples of which nutrients to use, where and why. Do I want “che”lated or complexed micronutrients?
First, what’s the difference? Let’s think of chelation like a crescent wrench. The nutrient we are looking to push into the plant, well that’s the nut. When we chelate nutrients, we hold on to those ions (usually metal) on several sides surrounding it. Chelating agents can bind metal ions with several atoms in a molecule, but it’s always with an organic (carbon-based) molecule. That means it can be bound with a nitrogen-based group and even an oxygen-based group in the same larger molecule, like the two sides of an adjustable wrench. They are synthetic, usually very stable and water soluble (hence the grip on a hex nut with a crescent wrench.) It’s held tightly but can easily be released when we need it. We are trying to keep that nut locked in place for a while before it’s released to be absorbed (i.e., solubilized). It is very important in enzymatic reactions.
In medicine for humans, chelating agents are the equivalent of sending lots of crescent wrenches through our body to lock up toxic metal ions. In agriculture, we chelate first to try to get the nutrients into the plant. Glyphosate is one of the best chelators and it works by getting into the plant and tying up the manganese and zinc so the plant can’t use it for its enzymatic reactions. It “systemically” kills the plant from within as it can no longer carry out its normal functions. We release these ions by acidifying them. The most stable chelated nutrient is EDTA iron which is why it is much less effective on high pH soils. EDDHA works much better when the pH goes up. Here in the west, much of our soil is above 7 pH.
Complexing an ion can be multiple binding sites or even one. It can be a single ion, a functional group or even a separate molecule. Think of complexing like an ice cream cone if it’s single (the nutrient is the ice cream, the agent is the cone.) or like a dumbbell if it’s in the middle and binding two ions. The agent is the bar and the nutrients are the weights at the end. In agriculture, lignosulfonates, organic acids like humic and fulvic, gluconic acids, sugars and amino acids are typical complexing agents. They are biodegradable and fairly easy to make. They are attached to carbons and can be easily absorbed. We tend to forget the three main elements of any living creature are carbon, hydrogen and oxygen. Being assimilated with carbon can be very advantageous. Because of their nature, they can solubilize easily but also make precipitates forming less soluble salts in the soil. They’ll have to be acidified later to become available if at all.
Let’s get back to wrenches, ice cream cones and dumbbells. It has been my experience that fertigating with chelates works better than with complexes. It has also been my experience that spraying complexed nutrients foliarly facilitates better nutrient uptake. The caveat is qualifying those statements with, “It has been my experience.” Often, nutrients are applied in conjunction with other chemistries to logistically get to all the necessary functions of an orchard in the short time allocated to grow a crop. Boron often goes out with glyphosate applications. Micronutrients often go out with pesticide or fungicide sprays. Macronutrient blends of NPK get micronutrients blended in with them all the time. Many of these will have some sort of tie-up, so find out first or jar test them. It is important to know what you are applying and how it may tie up other nutrients to make sure it’s effective. After a foliar spray, it’s tough to see a white salt residue on your leaves as certain nutrients tied up and dried out, leaving them fairly useless until it rains again or another spray happens to solubilize them.
A farmer can typically hedge his bet by adding a complexing agent to a nutrient application. Often, sugars are used, organic acids like humic and fulvic, weak acids like gluconic, lacto-bionic, citric and malic acids lend themselves to carrying more nutrients into a plant in a foliar application. The point here is don’t spray nutrients and just hope they are assimilated onto your crop. Do your research, ask your CCA and make a separate, isolated, clean spray if a specific nutrient gets critically low. In most areas, applying nutrients by air can save a lot of time, be more cost effective than the labor and diesel to go through your field again, and be very effective for a specific nutrient.
One last thought: I have been on farms where managers are a little disappointed that certain nutrients are still a little low after a foliar application. Many times, the new growth and larger crop associated with proper nutrition are not taken into consideration. The dilution ratio of larger leaves can play a role in how your nutrients get measured by weight. So, after using that crescent wrench to tighten those nuts where they are needed, lifting those barbells to make yourself stronger, have a little ice cream to top it off. And while you’re at it, do the same for your crops.