Question: Why is zinc important for crops?

Zinc is an essential mineral to all living organisms (animals and plants) and is a co-factor in all enzyme-based reactions. It is required only in small but critical concentrations to allow key plant physiological pathways to function normally.

Crops that can be adversely affected by zinc deficiency include: cereals (rice, wheat and maize), fodder crops (sorghum), pulses (beans, chickpeas and soybeans) and tree fruits (apples, citrus and peaches), vegetables (potatoes, onions and sugar cane) and also non-food crops like cotton and tobacco.

Many plant and crop types grown in southern Africa can be affected by a deficiency in zinc. These include citrus, apples, guavas, nuts, tea, coffee, potatoes, tomatoes, wheat, rice and maize. Other non-food crops like cotton and flax are also adversely affected by a deficiency in zinc.

In developing countries, most people live on cereals like maize, rice or wheat, but one third of the world’s population is at risk of zinc deficiency. In humans, zinc is an especially essential mineral and deficiencies can cause impaired growth and serious damage to the body’s immune system, leaving us open to attack by disease.

Zinc is also especially important for many of the biochemical reactions taking place in plants, such as the establishment and maintenance of disease defence mechanisms, photosynthesis/sugar formation, growth regulation, seed production and general fertility. A deficiency of zinc can result in unhealthy crops, lower growth rates and ultimately poor yields or even in crop failure.

Plants require many different micronutrients, which are essential for health, growth and reproduction. While micronutrients are usually required in smaller quantities than nitrogen, potassium and phosphorus (NPK), they are all very important to plant growth and general health.

FULL YIELD POTENTIAL

• Even when optimum quantities of NPK nutrients and water requirements have been satisfied, a crop will not achieve full yield potential if its zinc supply is insufficient.
• When crops are deficient in zinc, reduced yields and crop quality will be experienced.
• Losses as high as 30% in yields of cereal grains in crops such as maize, wheat and rice can occur as a result of latent deficiencies without the appearance of any obvious visible symptoms of stress.

The main causes of zinc deficiency in crops are:

• Low total zinc concentrations in soils (especially sandy, calcareous and sodic soils).
• Low availability (high pH, calcareous and sodic soils).
• High levels of phosphate and nitrogen and restricted root zones due to high soil compaction or a high water table, particularly in soils of marginal zinc status.

In many parts of the world, where deficient soils have been treated with zinc additions, subsequent soil testing has shown that many had sufficient zinc levels for several years after treatment. High-yielding crops have a greater zinc requirement than those grown less intensively and so soils will require regular soil testing.

ZINC TREATMENTS

• Zinc treatments have given yield responses of up to 4 tons/ha in wheat and rice and up to 2 mt/ha in maize.
• Due to their high water solubility (a key factor required for fertiliser efficiency) inorganic salts such as zinc sulphate are most commonly used to correct zinc deficiencies, and are a cost-effective form of soil adjustment.
• Zinc sulphate is available in dry (powder or crystalline) form and is also fairly cheap.
• Zinc sulphate can also be used as a foliar treatment for crops (normally with calcium hydroxide to neutralize acidity).
• As with all acidic products, care must be exercised to avoid zinc burn.
• Application rates will vary depending on the form of zinc applied, soil conditions, the crop and the method of application.
• They can range from 2.5 kg Zn/ha to 22 kg Zn/ha for inorganic forms such as zinc sulphates.
• Generally, local agronomists will be able to determine the required levels and should be consulted.

There are various methods available to anyone wishing to supplement the soil for their crops with zinc:
1. Treatment of seeds.
2. Fertigation – uptake supplied via an irrigation system.
3. Soil application – systemic uptake via the roots.
4. Foliar spray – uptake by spraying or dusting the leaves of the crop.

COST-EFFECTIVE

Zinc may be supplemented as a single nutrient fertiliser (simply by using a zinc sulphate solution, a highly water-soluble form of available zinc) or as an addition level to an NPK fertiliser.

This treatment is highly cost-effective when the costs of the zinc application and the value of the increased yields derived from it are taken into account:

The value to cost ration (V:CR) calculation depicts how improvement in yields compares to the cost incurred to achieve the improvement. At a V:CR of 1.0 the improved value in yield covers the cost fully. At a V:CR of 10, the cost incurred is paid back tenfold.

Essentially, in low-margin/low-profitability crops, this ratio is very important. If a crop achieves a very low net profit margin, any improvement in yield will make a dramatic improvement in overall profitability.

The table below gives illustrative numbers of some crops.

What it shows is that, in many cases, less than 1% improvement in yield (t/ha) is required to fully recover the cost of the zinc supplementation. In the case of potatoes for example, market value per hectare may be about R3 200/ha.

The zinc addition may cost R200/ha, and so it will require a mere 1.3% improvement in yield to cover the cost. A 5% increase in yield would result in an additional R1 600/ha in the farmer’s hands and a 15% increase in yield, R1 800/ha.

• This article first appeared in Farming SA.