Are you paying enough attention to your crop’s micronutrient (trace element) needs today?

The uptake of the trace elements iron (Fe), zinc (Zn), manganese (Mn) and copper (Cu) by roots is governed by three basic principles.

A soil solution that is acidic will dissolve more metallic micronutrients from the surrounding soil than a soil solution that is alkaline. It is a chemical principle that acids dissolve minerals. But metallic trace elements are not the only essential elements we want in the soil solution, so we mustn’t get too acidic or other nutrients will be less available. Conversely, if the soil solution is alkaline our copper and zinc ions get tied up and are unavailable.

Typically, it is in alkaline soils that trace element nutrition is a problem. Several strategies are used to correct this:

Enter the plant and its rhizosphere: A plant, especially a vigorous growing plant, has a tremendous ability to change and even transform its root environment. Sometimes it does this directly by exuding siderophore molecules to scrape an iron molecule off the mineral fraction of the soil and deliver it to the plant. Indirectly, plants exude various organic acids to lower the pH of the soil solution around the root hairs, making the soil water much more effective at dissolving trace elements in the soil.

An excellent example of this was observed on a field of wheat next to a field of potatoes. When the soil was probed, the native soil tested had a pH of 8.4. When the soil was probed in the potato row where polyphosphate was banded, pH was 6.7. A hundred feet away in wheat that only received preplant fertilizer the pH in the root zone measured 7.1. It was impressive what the polyphosphate fertilizer alone did, but even more impressive was what the wheat did all by itself by naturally exuding organic acids and respiring carbon dioxide.

The most important acidifier, in terms of total amount of nutrients put into solution, is good old weak carbonic acid. Carbon dioxide respired from roots and microorganisms dissolves into the soil water and makes it more acidic. And that makes the soil solution a better solvent and puts more minerals into solution. Both the plant roots and the microbes naturally produce a myriad of different kinds of molecules. These molecules assist in releasing more soil-bound nutrients into solution so they are more available for plant uptake.

It is interesting to note that when a plant has a robust and active rhizosphere, it generally has superior mineral nutrition. Most interestingly, those plants with superior trace mineral nutrition have far fewer problems with yield-robbing and quality-killing diseases.