Zinc (Zn) is an essential micronutrient for both plants and humans.
It is the most deficient micronutrient in the soil worldwide. It has been estimated that 49% of the world’s agriculturally important soils have inadequate levels of zinc and that one third of the worlds human population suffers from zinc deficiency (Sillanpää, 1990; Cakmak et al., 2010).
As with other cereals, zinc deficiency limits rice growth and yield (Chaudhary et al. 2007).
The main factors responsible for causing zinc deficiency in staple food crops, such as rice are soil related. They include; low zinc total value in the soil, high pH, high calcite content, high concentration of bicarbonate ions and salts and high levels of available phosphorous (Figure 2).
The most important factors affecting zinc availability to crops can be noted as follows (Chang et al., 2007; Alloway, 2008; Sillanpaa, 1990):
- Zinc total value may be very low in highly acidic soils due to the intense soil leaching.
- Zinc availability decreases with increasing soil pH. This is because the solubility of minerals, which help zinc uptake is reduced. These ‘uptake facilitators’ include coil colloidal particles such as clay minerals, iron and aluminium oxides, organic matter and calcium carbonate.
- Zinc usability decreases with temperature and light intensity due to limited root development.
- Zinc usability is decreased by high levels of phosphorous in the soil.
- Zinc uptake by plants is inhibited by some metal cations such as Cu²+ and Fe²+ because they all use the same transport mechanisms to enter the plant roots.
The sources of phosphorous and cations limiting the availability of zinc to the crop are; excessive phosphorous fertilizer applications, inherent soil minerals and irrigation water.
Availability of zinc in submerged soils is decreased due to the formation of complexes and compounds such as insoluble franklinite (ZnFe2O4), insoluble ZnS, insoluble ZnCO3, and insoluble Zn (OH)2 (Brar and Sekon, 1976).
These findings explain why rice grown in flooded conditions have
higher requirements for zinc. Foliar applications of zinc fertiliser
provide an appropriate solution to zinc solubility issues, as it avoids the complex soil interactions and other soil related issues.
Figure 2: Major soil chemical and physical factors effecting availability of zinc to roots (Cakmak, 2008)
Zinc is an important micronutrient for normal human growth and development. Widespread and extensive zinc deficiency has been reported in many rice producing areas.
One of the contributing factors, among many, is the high phytate content of rice and other cereal crops, which makes zinc non-bioavailable in the human digestive tract. Studies have suggested a simple way to overcome zinc deficiency in humans is by applications of zinc to crops like rice.
Various independent studies have shown that foliar fertilization is a more effective approach than base fertilisers for the correction of zinc deficiency in rice (Zhang et al., 2018; Jaksomsak et al., 2018) and at the same time fortifies the plant with zinc that is bio available for human nutrition.
Role of Zinc in Rice Plants
The importance of zinc in regulating several plant biological and physiological processes cannot be over emphasized. Zinc is involved
in more than 300 plant enzyme activities explaining why its deficiency will limit rice growth and yield (Ur et al., 2012).
These enzymes are involved in carbohydrate metabolism, maintenance of cell turgidity, protein synthesis, auxin regulation and pollen formation (Ur et al., 2012).
Regulation of gene expression linked to stress defence responses in plants is also zinc dependent; deficient plants are hence susceptible to injuries caused by excessive light, extreme temperatures and fungal pathogen invasion (Hafeez et al., 2013 and Lefevre et al., 2014).
Moreover, zinc deficiency in rice reduces pollen viability leading to reduced grain set and severe yield penalties (Yoshida et al., 1970).
Rice zinc deficiency can also occur at very early stages of development, as reported by several studies done with seedling in nurseries. Therefore, zinc deficiency in rice is a major cause of yield losses worldwide (Gao et al., 2006).
Below are conclusions of some studies that were carried out on rice production in various parts of the world especially in the areas where zinc deficiency is moderate to high in soil and the human population.
Rice cultivation benefiting from Zinc application:
- Soil zinc application led to an increase in zinc concentration of 7% in rice grains while foliar application led to increases of 25%. (Edward et al. 2015).
- Two-year field experiment indicated that soil zinc application has no significant effect on grain zinc concentration. In contrast, foliar zinc application increased grain zinc concentration by an average of 61%. (Ai-qing et al. 2013).
- Amauri et al. 2014 concluded in their study that treating rice seed with zinc increased the grain yield by 5.8% and foliar application increase the 7% compared to the untreated.
- Treating rice seed with zinc did not negatively change the germination rate or seedling emergence (Amauri et al. 2014).
- Zinc application increases the crop growth rate of rice (Mustafa et al. 2011)
- Application of zinc reduced phytate concentration and Phytate: zinc molar ratio in cereal (Yang et al. 2011).
- Denre et al. 2017 concluded that the application of zinc in staple foods like rice may play a role in reducing zinc deficiency in human beings and animals.
- Muthukumararaja and Sriramachandrasekharan (2012) observed that rice yield significantly improved with zinc fertilisation in zinc deficient soil. More so, they observed that the zinc content, uptake and zinc use efficiency had significant positive linear relationship with grain yield of rice.
OMEX Zinc Application
For the best results to enhance crop development and increase grain zinc content, apply OMEX Kingfol Zinc at 0.5-1.0 l/ha during the tillering stage of rice.
Ai-qing Zhao Xiao-hong Tian Yu-xian Cao Xin-chun Lu Ting Liu (2013) Comparison of soil and foliar zinc application for enhancing grain zinc content of wheat when grown on potentially zinc-deficient calcareous soils.
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