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Application of micro-nutrients: Methods, pros and cons and opportunities

By Attie Haasbroek (Agronomist: Western Cape)

According to criteria recommended by Arnon and Stout in 1939, there are 18 nutritional elements needed by plants for growth. However, these elements need to be available in the soil for the plant to take up. If the available levels of a nutrient in the soil are insufficient, additional amounts have to be applied as a supplement. This is where Omnia can assist the producer.

The three most important elements (carbon, hydrogen and oxygen) sometimes get the least attention. The nutrients we usually focus on are the macro-nutrients nitrogen, phosphates and potassium. These elements are applied through fertilization. Secondary nutrients (calcium, magnesium and sulphur) are either found in sufficient quantities in the soil, or they are supplemented by applying lime or gypsum. The last group of elements essential for growth (iron, boron, copper, manganese, zinc, molybdenum, cobalt, vanadium, silicon, sodium and chlorine) is known as micro-nutrients, because these are only used in very small quantities by the plant. Figure 1 shows the composition of soil.

Figure 1: The composition of soil (Klei-mineralogie en grondvrugbaarheid (''Clay mineralogy and soil fertility''), Cornie van Huysteen UVS).

In this article, only the last group of nutrients needed for optimal plant growth, namely the micro-nutrients (Zn, Fe, Mn, Cu, Bo, Mo en Cl), will be discussed.

Determination of micro-nutrient content in soil

Determining whether micro-nutrient deficiencies are present should not be expensive. Soil should be analysed for macro- and micro-nutrient deficiencies. However, the use of soil analyses should be complimented with leaf analyses during the growing season because, even though a soil analysis can determine the correct levels of micro-nutrients, the elements might not be available for uptake by the plant. A soil analysis, without confirmation through a leaf analysis, can therefore be inaccurate. A soil analysis gives an indication of the potentially available micro-nutrients. There can even be greater variations in micro-nutrients than in macro-nutrients in different soil types.

In some instances, the soil analysis indicates nutrient deficiencies, even though the leaf analysis shows no deficiency. Recommendations of micro-nutrient adjustments are therefore based on soil and leaf analyses.

Method of application

The influence of micro-nutrient deficiencies on yields can be overcome by the correct and timely application of micro-nutrients. If you know how specific micro-nutrients react in soil and in plants, you can make a decision regarding which method of application will be the most efficient. Soil applications are usually a more effective method, but if the deficiency symptoms are visible after emergence, a foliar application will be more efficient.

Band placement

The efficacy of soil application can be improved by band placement of the products. Lower application rates can be used to alleviate deficiencies. This supplement can be applied with the planting action, which will eliminate the cost of additional operations. If a liquid product is used, the composition can be adjusted to alleviate individual deficiencies on different soils. These products can be band placed in the root zone of the plant, which will overcome the chemical limitation of the soil.

When standard micro-nutrient granular products are used, the composition of the product is pre-determined, which makes it difficult to address specific deficiencies. If standard products are used and some micro-nutrients are sufficient in the soil, this can lead to the over-application of these nutrients.

The addition of micro-nutrient granules lowers the concentration of micro-elements in granular fertilizers. It can also have a negative influence on the physical quality of the fertilizer granules. The use of micro-nutrient granules can lead to inefficient distribution, since the actual amount of nutrients in a tonne of fertilizer will be very low.

Figure 2: Planter mounted application tank used for band placing liquid micro-nutrient products.

Broadcast application

Broadcast soil application of micro-nutrients is easy as it can be done through a sprayer or an irrigation system. The product’s composition can be adapted easily according to the varying soil chemistry of within a field.

Large amounts of micro-nutrients have to be used when broadcast application takes place. Incompatibility of micro-nutrients with herbicides and fungicides can influence its functioning, which can require an additional cultivation. With broadcast application, micro-nutrients can also be fixed in the soil.

Omnia has done much research on the coating of fertilizer granules with micro-nutrients so that it can be applied during the planting action. Figure 3 clearly shows the effect of these products on the growth of wheat plants.

Figure 3: Visual influence of band placing micro-nutrients and K-humate on fertilizer granules.

During the 2008 season, a yield increase of 261 kg/ha was obtained in the Western Cape. The following season also saw a positive reaction of 181 kg/ha. The increased yield did not affect the wheat grain quality at all.

Figure 4: Seed yield increases achieved with the band placement of granular fertilizer coated with micro-nutrients and K-humate. (From an Omnia trial conducted on wheat in the Western Cape.)

Foliar application

Micro-nutrient deficiencies are sometimes visible on crops during the growing season despite supplemental applications before planting. These are possibly induced deficiencies due to climatic conditions, for example during cold, wet conditions. When uptake through the roots is restricted due to climatic or soil conditions, an adjustment using foliar application will be the most effective method.

When products are foliar applied, the following has to always be kept in mind:

  1. Check the water quality to make sure that the applications are effective. Water with a high pH or total dissolved solids can fix some of the micro-nutrients and render them ineffective. Therefore, you should know the quality of your water. Chelated micro-nutrients that are still effective at high pH can also be used.
  2. When agrochemical products are mixed in a tank, the miscibility of the added products needs to be confirmed. The efficacy of herbicides could be negatively affected.
  3. The timing of application is important if corrections are done due to deficiency symptoms. If the deficiency occurs during a critical growth stage, it needs to be rectified immediately.

The latest trend is to band place micro-nutrients in liquid form. Products are formulated to address specific micro-nutrient deficiencies, but other products such as Rhizovator™ can also be added. The application of micro-nutrients through a planter allows more options for optimal plant nutrition.

A balanced nutritional programme is critically important for optimal production. It is therefore imperative to apply the correct product, using the correct method at the right time to ensure maximum uptake and production (Jones et al., 2011).

Reference
  • ARNON DI, STOUT PR. 1939. The essentiality of certain elements in minute quantity for plant with special reference to copper. Plant Physiol. (April 1939). 14(2):371-375.
  • Jones C., Olson-Rutz K., Pariera Dinkins C. (2011) Nutrient Uptake Timing by Crops to assist with fertilizing decisions.Book of Soil Science, Montana State University. pp. 67-78