Zinc deficiencies and zinc fertilization

Zinc is an essential micronutrient needed by crops and people. Almost half of the world’s cereal crops are deficient in zinc, leading to poor crop yields. Research has shown that areas with zinc-deficient soils are often regions with widespread zinc deficiency in humans. In fact, one-third of the world population is at risk of zinc deficiency, ranging from 4 to 73% depending on the country. Zinc deficiency is the fifth leading risk factor for disease in the developing world. Providing micronutrients, including zinc, to humans is one of the four quick-win solutions to major global problems identified in the Copenhagen Consensus from an international panel of distinguished economists.

Zinc is essential for human health

Zinc is a key micronutrient needed for a wide range of biochemical, immunological and clinical functions in humans. Zinc deficiency affects:

• physical growth,
• the functioning of the immune system,
• reproductive health and
• neurobehavioral development, among others.

Preventive zinc supplements are important in reducing infant morbidity from diarrhea and pneumonia.

To combat zinc deficiency, five intervention strategies can be used:

• Supplementation using medicines;
• Food fortification through the incorporation of zinc
• additives in food;
• Dietary modification/diversification;
• Genetic biofortification through plant breeding and
• Agronomic biofortification through zinc fertilization.

These five intervention strategies may be used individually or in combination, depending on the setting, target group and degree of zinc deficiency.

The amount of zinc absorbed by the human body is a function of dietary intake of both zinc and phytate (a phosphate storage compound), because the ratio between these two substances affects the bioavailability of zinc. Meeting the needs for absorbed zinc requires an increase in the zinc content and/or a decrease in the phytate content.

Zinc deficiencies in soils

Many agricultural countries around the world are affected by zinc deficiencies.

• In China, zinc deficiency occurs on around half of the agricultural soils, affecting mainly rice and maize.
  
• In India, zinc-deficient soils occupy almost 50% of the agricultural area and are a critical constraint on yield, but crops are highly responsive to zinc fertilization.
  
• In Turkey, major yield and quality benefits in wheat have been obtained with the widespread use of zinc fertilizers, where half of the cereal growing land is zinc-deficient.

Zn deficiency in world crops: major areas of reported problems (adapted from Alloway, 2008a)

Addressing zinc deficiencies through zinc fertilization

Experiments show that soil and foliar application of zinc fertilizer can effectively reduce the phytate:zinc ratio in grain. People who eat bread prepared from zinc enriched wheat show a significant increase in serum zinc, suggesting that the zinc fertilizer strategy is a promising approach to address zinc deficiencies in humans.

Where zinc deficiency is a limiting factor, zinc fertilization can increase crop yields. Balanced crop nutrition supplying all essential nutrients, including zinc, is a cost effective management strategy. Even with zinc-efficient varieties, zinc fertilizers are needed when the available zinc in the topsoil becomes depleted.

The combination of genetic and agronomic approaches offers great promises

Plant breeding, including modern biotechnology, can improve:

• Zinc uptake capacity of plants under soil conditions with low chemical availability of zinc;
• Zinc translocation, thus elevating zinc content in edible crop parts rather than the rest of the plant;
• Zinc bioavailability.

For optimal efficiency, zinc-efficient genotypes should be associated with complementary soil crop management (including fertilization) to ensure adequate zinc uptake by roots and thus enhance zinc nutrition of crops and humans

Links between zinc and plant metabolism are not yet fully understood

The mechanisms of zinc efficiency in plants grown under zinc-limiting conditions are still not fully understood, and more research is needed on this important aspect of zinc physiology.