Zinc (Zn) is an essential nutrient for all organisms, with potential roles in 1000s of proteins in plants and humans. Crops respond to Zn fertilizers on many soil types. For example, increases in wheat yield and production at a national scale have been reported in Turkey following the adoption of Zn fertilizers.
Factors affecting phyto-availabiliy of zn
However, Zn fertilizers remain little utilized globally, and approximately half of all soils used for cereal production are likely to be Zn deficient. These soils include widespread areas of the Indo-Gangetic Plains in South Asia, where intensive rice-wheat cropping systems are practiced. For example, soils of the Indus Plains of Pakistan are mostly derived from calcareous parent material from the Himalayas, which is deposited as alluvial material by the Indus River and its tributaries, or as loess deposits in the northern parts of the Indus Plains.
These calcareous soils support the majority of crop production in Pakistan, which covers 21.4 Mha. These soils generally have low organic matter (0.4–0.7 %) and free calcium carbonate (CaCO3), which buffers the pH within the range of 7.5–8.4 with 100 % base saturation, and have a cation exchange capacity (CEC) dominated by Ca. These factors tend to restrict the phyto-availability of Zn and other elements such as boron (B), thereby limiting crop yields in the absence of their fertilizers.
Many field studies have reported crop yield increases in response to Zn fertilizers. Using these studies, it is straightforward to monetize Benefit-Cost-Ratios (BCRs) based on increased crop output per additional input of Zn. For example, in a review of field studies in Pakistan, an application of 5 kg ha−1 (i.e. ZnSO4.H2O equivalents, containing ~33 % Zn) increased grain yields of wheat by >10 %, at a mean BCR of 7:1 (range 1.3–11:1; NFDC 1998). In addition to yield increases in the year of Zn fertilizer application, there may be beneficial residual effects of Zn fertilizers for subsequent crops for three or more years.
Reasons behind limited usage of Zn fertilizer in Pakistan
However, despite these potential financial returns, Zn fertilizers remain little utilized in Pakistan and elsewhere, for several reasons. These include a lack of quality product availability/access and farmer-awareness.
There is also evidence of an unwillingness to pay for Zn fertilizers due to mistrust of product quality and labeling. The effects of subsidies that focus primarily on the supply of macronutrient fertilizers, and a lack of farmer access to longer-term credit, can also discourage longer term soil fertility-building and lead to imbalanced fertilizer-use at a farm level.
Furthermore, there are scientific knowledge gaps in terms of deploying balanced fertilizer applications in soils with multiple macro- and micro-nutrient stresses including B deficiency and saline/sodic soil systems, which are prevalent in Pakistan.
The use of Zn fertilizers can increase Zn concentration in the endosperm of cereal-grains, there by reducing risks to consumers of dietary Zn deficiency.
The impact of increased dietary Zn intake and subsequent reductions in Zn deficiency within populations can be quantified using a Disability-adjusted life years (DALYs) framework.
Bio-fortification of high-Zn rice and wheat varieties through breeding could save up to 55 % of the 2.8 M DALYs lost annually due to Zn deficiency in India at a cost of US$ 0.68–8.80 per DALY saved.
Fortifying maize meal with a premix containing Zn at large-scale mills in Zambia could save 5657 DALYs annually, of which 1757 were due to Zn deficiency, at a cost of US$ 401 per DALY saved.
While the cost-effectiveness of these interventions varies considerably, it is also necessary to take into account the socioeconomic realities in each target country. The Commission on Macroeconomics and Health of the World Health Organization (WHO 2001) suggested valuing each DALY lost at the national per capita income (or even at three times the per capita income). Under this criterion the above (bio) fortification interventions are likely to be worth implementing in their respective settings.