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Agricultural nutrient profile: micronutrients-part 2

Posted in: Agronomy and Soil Health

Key micronutrient challenges

While they are required in much lower quantities than macronutrients, micronutrients are every bit as important to the successful development of crops.

We have chosen to focus on zinc, boron, copper and manganese because they are potentially the four biggest micronutrient challenges growers deal with. Below, we will look at the role of each, review common signs of deficiency, review availability and touch upon strategies to address deficiencies.

Zinc (Zn): why do plants need it?

Zinc’s role is quite complex. It is responsible for numerous plant functions and ultimately has a significant impact on crop quality and yield.

Zinc:

  • Is a major metal component of many enzymes
  • Facilitates enzyme reactions
  • Is critical to leaf and bud development
  • Plays an important role in fruit/ tuber early development (necessary for the production and synthesis of IAA (Indoleacetic Acid)
  • Aids in cell differentiation (contributing to larger/denser fruit)
  • Aids in the absorption of calcium

A plant deficient in zinc will have small, inefficient leaves (which affects photosynthesis). As a result, a zinc deficiency will have a significant negative impact on both yield and crop quality.

Zinc availability and timing.

Zinc is predominantly absorbed as a cation (Zn2+). The presence of other nutrients can inhibit the intake of zinc. This can occur with high concentrations of calcium and phosphorus in the soil.

An important relationship exists between phosphorus (P) and zinc. In soil or tissues with low zinc levels, a high P level can cause the problem to worsen, and lead to a Zn deficiency. For this reason, the P:Zn ratio is key to predicting the availability of zinc to the plant, more so than the actual concentration.

Soil temperature is another factor that can affect Zn availability in the spring. A zinc deficiency affects the plant’s ability to produce IAA (Indoleacetic Acid), which is necessary to the development of roots and shoots. The cooler the soil, the harder it is for the plant to uptake zinc required for this important growth stage – even with adequate Zn concentrations. The earlier a crop is planted, the greater the need for row-applied zinc.

Zinc can also benefit the plant when applied as part of an early post-emergent nutrient treatment, while the plants are still in the early growth stages.

Boron (B): Why do plants need it?

Boron wears many hats when it comes to its role in plant development. This includes the promotion of:

  • Root growth
  • Pollen production and viability
  • Sugar transport
  • Cell wall syntheses and structure
  • Lignification (strengthening of cell walls)
  • Phosphorus and potassium uptake
  • Metabolism of carbohydrates, RNA, IAA and phenol
  • Mycorrhizae colonization
  • Respiration
  • Membrane function

Signs of a boron deficiency include inhibited root growth (roots appear stubby and bushy). Cell walls will be altered, which can lead to disorders such as cracked stem, stem corkiness, hollow stem disorder, black heart and more. It also results in pollen tube elongation. Low boron levels can also affect root tip and meristematic growth.

Boron availability and timing.

The movement of boron within a plant is restricted to xylem, which depends on respiration. In dry growing conditions, inhibited respiration will reduce boron uptake.

Boron is typically applied as a general pre-plant or pre-emergent fertilizer. Because of its importance to pollen production and viability, plants can benefit from the addition of a pollen aid containing boron.

Copper (Cu)

Copper is required for several stages of plant development. Chief among these are:

  • Enzyme function
  • Cell wall formation
  • Electron transport
  • Oxidation reactions
  • photosynthesis

Wheat, barley, oats, canary seed and flax are particularly sensitive to a copper deficiency. Deficiencies are known to occur in sandy soils in the black or grey soil zone, or in peaty soils.

Copper is not transferred from old leaves to the new ones, so the older leaves can remain green and appear healthy while new leaves show signs of damage. If there is a deficiency, newer leaves will yellow, wilt, and pigtail at the tips. Other symptoms include excessive tillering, aborted heads, delayed maturity, extended flowering period and poor grain fill.

Proper field scouting (followed by soil and tissue testing) is important, because symptoms are usually not uniform across the crop, and may appear in concentrated patches.  High levels of phosphorus can also limit Cu absorption by plant roots creating a deficiency.

Copper availability and timing.

Copper is not considered to be mobile in the soil. Plant availability and solubility is pH dependent – improving the more acidic the soil. In fact, solubility increases by a factor of ten with each unit decrease in soil pH. For example, a soil with a pH of 6 will be ten-times more soluble than a soil with a pH of 7. Source: Government of Saskatchewan.

The most effective method to remedy copper levels is by broadcast and incorporation with copper sulfate. In zero till systems, it is best to broadcast and incorporate in a year when strategic tillage takes place. Blending with other fertilizers is not recommended as the blend readily absorbs moisture.

It can also be addressed in-crop as part of a liquid foliar application. Multiple applications may be required depending on the severity of the problem.

Manganese (Mn)

Manganese is another important micronutrient. It plays a role in:

  • Enzyme function
  • Metabolic activation
  • Photosynthesis
  • Germination rate
  • Maturity
  • Phosphorus and calcium availability
  • Resistance to root pathogens

Deficiency symptoms are very close to those of iron deficiency- appearing on the young leaves as yellowing with green veins.  If the manganese deficiency progresses tan areas develop between the veins, differing from iron deficiency symptoms (white appearance of leaves).

Manganese availability and timing.

Organic and high pH mineral soils are prone to manganese deficiency. Organic soils can tie up manganese, while plant available manganese isn’t easily absorbed through the roots in basic soils.  Manganese competes with iron for plant uptake and to a lesser extent Zn, Cu, Mg, and Ca.

The most efficient method of fertilization involving manganese is foliar.

A foliar program should be used as a supplemental treatment particularly in extreme Mn deficient soils or highly sensitive crops. If soil is consistently deficient, an amendment program is advised.

Next: Law of the Minimum

How important are micronutrients in crop production? It only takes one nutrient deficiency to set back plant growth. Learn why the Law of the Minimum dictates that micronutrients are no less important than macronutrients.

Learn More

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