Zinc

Zinc activates a wide range of enzymes involved in carbohydrate metabolism, protein synthesis, and gene regulation. It is also required for the synthesis of auxin, the plant hormone that drives cell elongation, which is why zinc-deficient plants often show shortened stem internodes.

Zinc availability falls sharply as soil pH rises, so calcareous and heavily limed soils are especially prone to deficiency. Soil organic matter helps hold zinc in plant-available forms, so eroded or organic-matter-depleted soils are also at greater risk.

• High soil pH (calcareous or over-limed soils) is the leading cause of zinc deficiency
• Low soil organic matter reduces the reserve of plant-available zinc
• High soil or fertilizer phosphorus can further reduce zinc uptake
• Flooded conditions in lowland rice paddies can also lower zinc availability

Zinc deficiency produces interveinal chlorosis together with shortened internodes and smaller-than-normal leaves, sometimes called "little leaf" or rosetting. It is widely documented in maize and lowland rice grown on high-pH or eroded soils.

Unlike some micronutrients, zinc excess is directly phytotoxic, causing chlorosis, stunted root growth, and reduced overall vigour. It can occur near zinc-contaminated or historically mined or smelted soils, or from over-application of zinc-containing fertilizers and amendments.

Phosphorus and zinc show a well-established antagonism: high phosphorus availability, whether from soil reserves or fertilizer, can induce or worsen zinc deficiency. Zinc also competes with iron and copper for uptake, so imbalances among these micronutrients can compound one another.