The importance of trace element availability: part 2

Author: Alli Grundy

1st September 2021

NRM

Agriculture

In my last blog, I discussed where trace elements come from, how plants take up nutrients from the soil, what affects nutrient availability, and identified crop sensitivity to various nutrient deficiencies. Click the link below to read part 1.

Part one

Here we continue the conversation by identifying the need for balance and discussing how to correct deficiencies before they become yield limiting.

A careful balance

While trace element content of a soil generally depends on the parent material, soils can gain trace minerals through dust deposition, water draining into soils from elsewhere, and pollution from human activity. However, trace minerals can be leached out of the soil during periods of heavy rainfall in the winter and spring, and summer droughts can reduce their availability, so it is essential to assess and analyse the soil for trace element supply on a regular basis. This will help farmers and their advisers to adjust crop nutrition plans for optimal yield, crop quality and monitor resources, ensuring soil supplies remain in balance.

Table 1: typical range of essential micro nutrient soil content

Johnston (2004), adapted from Wild (1988)

Correcting deficiency

Foliar applications containing the relevant nutrients can be applied after analysis to correct confirmed deficiency for cereals, oilseeds, pulses and roots. Some deficiencies mean that crops need to be treated before symptoms appear in the crop, such as boron deficiency in oilseed rape. In this case, attempting to correct the deficiency after symptoms appear will not be effective and productivity likely compromised.

• Cereals: Copper and magnesium are often applied together as a foliar application. Zinc is usually applied as a chelated foliar application, but biosolids are a rich alternative source.
• Oilseed rape: Boron foliar applications are effective if applied as soon as early leaf cover allows, but low soil boron levels can be corrected by spraying directly on the soil or by adding a boronated seed dressing.

Note: applications should not be made more frequently than every third year, as it can be toxic to other crops such as cereals and potatoes.

Molydenum applications are most effective when applied directly to the seedbed. Foliar sprays can be applied at the 2-3 leaf stage, but are less effective than treating the soil directly. The best strategy is to manage the soil pH by liming to pH 6.5.

• Pulses: Copper and zinc: see cereals. Boron and Molydenum: see oilseed rape.
• Potatoes: Boron should not be applied to potatoes unless an acute deficiency has been confirmed. Zinc: see cereals.
• Sugar beet: Apply a boron-rich fertiliser (such as sodium borate) before drilling to the seedbed on high risk soils, or routinely apply a foliar chelated product where low levels are known. It is important to do this as early as possible as foliar feeds are not effective when symptoms are apparent. One application in the rotation is sufficient and should reduce the risk of toxicity in susceptible crops.

Copper foliar applications are most common in correcting deficiencies. Applying copper sulphate to the soil can be effective over a number of seasons, but deficiencies must be treated early if yield is not to be compromised.

Regular soil and plant tissue testing are advised to diagnose issues before they become yield limiting. For further advice, speak to your agronomist.

Chelated foliar products

As we mentioned before, inorganic trace minerals are at risk of being locked up by the soil chemistry when applied. Chelated foliar products are a delivery solution for some inorganic trace minerals.  They provide the nutrients with an organic coating which prevent the chemical reactions that naturally occur in the soil when the trace minerals are applied. The organic coating forms weak bonds with the nutrient, allowing the plant roots access to the nutrient(s) before they are organically complexed and locked up.

Chelated nutrients are effective when formulated as a foliar feed. The wax coating of plant leaves repels water to prevent them from drying out, but this action makes it difficult for inorganic nutrients to penetrate the leaf. The organic coating around the nutrient provided by the chelated product allows it to penetrate through the wax, where the chelate releases the nutrient to be used by the plant.

NRM’s checklist for improved farm productivity

• Understand the critical trace nutrients required for your crop.
• Understand the field growing environment, including soil texture, pH and macronutrient status, to determine if a combination of factors may lead to a deficiency.
• Sample your soil for trace minerals to understand the risk of deficiency in time for correction. If you don’t measure, you can’t manage.
• Don’t wait to sample until you see an issue in the crop. Be proactive and prepared to avoid issues before they become yield limiting.

Speak to your agronomist or contact NRM directly to arrange for your soil or plant tissue samples to be analysed today. You can also consider including trace minerals and soil texture analysis when you send us your soil samples for standard analysis [pH, P, K and Mg].

References

Johnston, A. E (2004), Micronutrients in soil and agrosystems: occurrence and availability. International Fertiliser Society, Proceedings No. 544, pp 32.

The Crop Nutrition Directory: Your Easy to Use Guide Helping You Diagnose and Treat Nutrition Deficiencies (2003). Context Products Limited, Publications Division.