The importance of trace element availability: part 1
NRM
Agriculture
There are fourteen known elements that are essential for healthy crop growth. Divided into macro and micronutrients (or trace elements), they can have a significant impact on plant productivity and crop quality. So, sufficient quantities need to be available in the soil for the plant to take up. Soil pH and texture have a large influence on the availability of nutrients and the subsequent risk of deficiency. But where do these trace elements come from, and what affects nutrient availability?
Sublime soils: the birth of trace elements
The trace mineral content of a soil initially depends on the parent material from which it was formed. While soils derived from basic or sedimentary rock are not usually deficient in trace elements, acid soils derived from igneous rock are generally poor in some trace elements. We can see this in granite, for example, which is commonly poor in cobalt—an element not essential to plants, but to grazing animals.
As the primary materials of sedimentary rocks weather, some of the trace elements now in the soil combine with clay minerals or are adsorbed onto clay particles or organic matter. Soils derived from shales have a higher clay content and tend to contain and retain more trace minerals compared to other soil textures. So, the parent material determines the soil texture which in turn determines trace element content and influences the opportunity plant roots have to take up the nutrients. These soils can contain large reserves of trace elements, but an excess of nutrients does not necessarily lead to high availability or more nutrients being taken up by the plant.
How plants take up nutrients from the soil
The quantity of a trace element available for plant uptake is invariably much less than the total amount in the soil. This is not usually a problem as the required elements are typically required and found in plant tissue in small concentrations. The uptake of plant nutrients is governed by the concentration in the soil being greater than that in the plant. As the roots take up nutrients from the soil solution, they are replenished by diffusion which is a response to the concentration gradient depleting at the root surface.
Plants also access nutrients due to the mutual symbiosis they have with soil fungal mycorrhiza. Most agricultural crops benefit from a symbiosis with endotrophic mycorrhizae of which vesicular arbuscular mycorrhiza (VAM) is the predominant type of fungal infection. Mycorrhiza mycelium physically penetrates the plant root cells so that it can access life-sustaining compounds from the plant, while in exchange these mycelia assist the roots in exploiting the soil for water and nutrients. This relationship does no harm to the plant, and actually allows it to access otherwise unavailable trace nutrients that occur in very small concentrations in the soil. This is because the mycelium network (‘hyphae’) branch extensively throughout soil and rhizosphere enters pore spaces that are too small for root hairs to access.
We can see this benefit in action in plants such as clover. The uptake of zinc and copper, for example, tends to be larger in these plants than those that do not have such mycorrhiza relationships.
What else affects availability?
Chemical form of trace elements and Ionic charge potential
Alongside soil texture, availability also depends on the trace element’s chemical form in the soil. Whether a trace element is in its mineral form and remains in solution, or it is organically complexed, a number of factors come into play to determine the availability of the trace mineral. The principal factor is the ratio of ion charge to radius (in nanometers/nm), often called the nutrient’s ionic potential (IP). Elements with a lower IP value (<30), such as Mn or Cu, form soluble cations but can become trapped by other compounds derived from elements with a greater IP value (between 30 and 95), such as Fe or Mo. This forms trace element complexes.
This is a very dynamic process. The trace element complexes also have strong surface adsorption affinities, meaning they are strongly attracted to clay particles and migrate onto them. These chemical changes in the soil determine the access plant roots have to these trace minerals.
pH
pH also greatly affects the optimal availability of plant nutrients in soil and is strongly linked to the chemical changes already discussed. As there are fourteen nutrients to consider and balance, this could present an issue. Thankfully, there is sufficient overlap in the optimal pH range, ensuring an adequate supply of all nutrients. Soil pH is fundamental to understand and correct if necessary. If, when analysing your soil, the result suggests the pH is acidic, corrective liming products are inexpensive to apply and will ensure the bioavailability of both macro and trace nutrients. If your soil is generally alkaline due to, for example, the concentration of free calcium, then options to reduce the pH are limited. However, applying products where the nutrients are formulated to reduce the likelihood of them being made unavailable for plant uptake can be applied as an alternative.
The above chart shows the relationship between pH and nutrient availability in the soil. The width of the white band shows the likely availability across the pH range. Maintaining the pH between circa 6.2 and 7.0 will ensure the optimum level of nutrient availability possible to support the overall health of the plant.
Measuring copper, molybdenum and boron in the autumn with soil analysis is advised to understand which fields are at potential risk of deficiency, especially if a sensitive crop is being grown—see the below table for more information. Nutrients such as zinc and manganese are best analysed by plant tissue in the spring.
For more information on when specific elements are likely to demonstrate deficiencies, speak to your agronomist or contact @Allison Grundy.
Crop sensitivity
Reference: The Crop Nutrition Directory
I will be continuing this discussion in part two in the coming weeks, so keep an eye out! Meanwhile, today is the perfect time to speak to your agronomist or contact NRM directly to include trace minerals and soil texture when you send us your soil samples for standard analysis [pH, P, K and Mg] this autumn.
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