Zinc in plants

Zinc (Zn) is one of the eight essential micronutrients. It is needed by plants in small amounts, but yet crucial to plant development.

In plants, zinc is a key constituent of many enzymes and proteins. It plays an important role in a wide range of processes, such as growth hormone production and internode elongation.

Zinc Deficiency

Zinc deficiency is probably the most common micronutrient deficiency in crops worldwide, resulting in substantial losses in crop yields and human nutritional health problems.
Deficiency in zinc might result in significant reduction in crop yields and quality. In fact, yield can even be reduced by over 20% before any visual symptoms of the deficiency occur!

The cost to the farmer, associated with loss of production, is by far higher than the cost of testing the soil and plant tissue and applying zinc fertilizers.

The mobility of zinc in plants varies, depending on its availability in the soil or growing media. When zinc availability is adequate, it is easily translocated from older to younger leaves, while when zinc is deficient, movement of zinc from older leaves to younger ones is delayed.

Therefore, zinc deficiency will initially appear in middle leaves.

Symptoms of zinc deficiency include one or some of the following:

•   Stunting - reduced height
•   Interveinal chlorosis
•   Brown spots on upper leaves
•   Distorted leaves

As mentioned above, the visual symptoms usually appear in severely affected plants.  When the deficiency is marginal, crop yields can be reduced by 20% or more without any visible symptoms.

      
                       Zinc deficiency in corn                                           Zinc deficiency in cotton

In order to identify a zinc-deficient soil, the soil and the plant should be tested and diagnosed. Without such tests, the soil might remain deficient in zinc for many years, without the farmer identifying the hidden deficiency, as visual symptoms may not occur.

Zinc deficiency is common in many crops and on a wide range of soil types. It affects the main cereal crops: rice, wheat and maize as well as different fruit crops, vegetables and other types of crops.

Soil conditions that can result in zinc deficiency include:

•   Low total zinc level in the soil (available + unavailable zinc)
•   Low organic matter content or too high organic matter content (e.g. peat soils)
•   Restricted root growth (e.g. due to hardpan, high water table etc.)
•   High soil pH
•   Calcareous soils or limed soils
•   Low soil temperature
•   Anaerobic, waterlogged conditions
•   High phosphorus level in the soil

  

Diagnosing Zinc Disorders

Visual observation can be a quick diagnostic tool to identify zinc deficiencies. However, it requires knowledge and expertise, as symptoms may be confusing. In addition, once visual symptoms appear, yield loss has already occurred.

Regular soil or plant testing is the best practice to determine if zinc application is required and to ensure that zinc does not accumulate in the soil to undesirable high levels.

DTPA-extraction is the most commonly used soil test to determine available zinc levels in soils.

Zinc toxicity is quite rare and under normal conditions, most soils will have either normal or deficient level of zinc.

Crop	Plant Part	Sampling Time	Low	Adequate	High	Excessive
			-----------------ppm---------------------
Alfalfa	Top 6 inches	1/10 loom	<20	20-70	71-120	>120
Corn	Earleaf	Silking	<20	20-75	76-150	>150
Wheat	Flag leaves	Maturity of flag leaves	<18	19-70	71-100	>100
Rice	Youngest fully developed leaves	Mid-tillering	<20	21-160	161-250	>250
Cotton	Youngest mature leaves	Late bloom / maturity	<50	51-300	300-400	>400

Interpretation of zinc levels in plant tissue of various crops

Correcting zinc deficiency

Zinc fertilizers can be applied to zinc-deficient soils, once deficiency is identified. The most common fertilizer sources of Zinc are Zinc chelates (contain approximately 14% zinc), Zinc Sulfate (25-36% zinc) and zinc oxide (70-80% Zinc), where Zinc Sulfate is the most commonly used source of zinc.

Foliar zinc applications – foliar applications of zinc are not as effective as soil-applied zinc. The foliar application can overcome visual symptoms but it is less effective in increasing the yield.