Zinc is essential for the normal healthy growth and reproduction of plants. When the supply of plant-available zinc is inadequate, crop yields are reduced and the quality of crop products is frequently impaired.

In plants, zinc plays a key role as a structural constituent or regulatory co-factor of a wide range of different enzymes in many important biochemical pathways and these are mainly concerned with:

	carbohydrate metabolism, both in photosynthesis and in the conversion of sugars to starch,
	protein metabolism,
	auxin (growth regulator) metabolism,
	pollen formation,
	the maintenance of the integrity of biological membranes,
	the resistance to infection by certain pathogens.

When the supply of zinc to the plant is inadequate, one or more of the many important physiological functions of zinc is unable to operate normally and the growth of the plant is adversely affected. The changes in plant physiological mechanisms brought about by a deficiency of zinc result in the plant developing visible symptoms of stress which might include one or more of the following: interveinal chlorosis (yellowing of the leaves between the veins), bronzing of chlorotic leaves, small and abnormally shaped leaves, stunting and rosetting (leaves form a whorl on shortened stems). These different types of symptoms vary with plant species and are only clearly displayed in cases of severe deficiency. In cases of marginal or moderate deficiency, plants can often have greatly reduced yields (<40% reduction) without obvious visible symptoms. This is often called 'hidden', 'latent' or 'sub - clinical' deficiency. These deficiencies may remain undetected in crop land for many years unless soil or plant diagnostic tests are carried out because there are no obvious signs of stress.

Losses of yield of 40% or more in many zinc deficient soils have a major economic impact on the farmer due to the reduced income as a result of lost yield. Zinc deficient soils can be identified, or diagnosed, by soil testing, or the analysis of the crop plants (usually leaves). The results obtained from soil and/or plant analysis can be compared with critical values for zinc in local soil types for specific crops and a decision made on whether or not zinc fertilizer applications to the soil or crops are required.

Very many plant species are affected by zinc deficiency on a wide range of soil types in most agricultural regions of the world. The major staple crops: rice, wheat, maize and sorghum are all affected by deficiency, together with many different fruit, vegetable and other types of crops including cotton and flax.

	low total zinc content (such as sandy soils with low contents of organic matter)
	neutral or alkaline pH
	high salt concentrations (saline soils)
	high calcium carbonate content (calcareous soils)
	low pH, highly weathered parent materials (e.g. tropical soils)
	peat and muck (organic soils)
	high phosphate status
	prolonged waterlogging or flooding (paddy rice soils)
	high magnesium and/or bicarbonate concentrations (and in irrigation water).

An adequate supply of zinc is essential for obtaining cost effective yields of crops all over the world. The cost to the farmer of lost production is high but the expense of applying zinc fertilizer when crop symptoms, soil tests or plant analysis show that they are required is relatively low. No farmer in areas where soils have been shown to be deficient can afford not to maintain an adequate zinc status in his soils.

Source: International Zinc Association