## 3 Biofortification of Crops
### Agronomic Biofortification  
Agronomic method of biofortification of crops with micronutrients is envisaged as a fast and easy way out of the inadequacies of these essential minerals in soils and plants. It involves cultivation of varieties that are rich in minerals and vitamins. This method uses fertilization as a strategy to increase micronutrient content of cultivated crops such as cereals and legumes. It is pertinent to emphasize that the agronomic biofortification method could be more beneficial in developing countries [40].  
To improve micronutrient content of crops using agronomic biofortification, [PERSON] and [PERSON] [41] suggested the use of phytoavailable micronutrient fertilizers, routine correction of the soil alkalinity, crop rotational methods of planting and strategic introduction of symbiotic soil microorganisms. [PERSON] et al. also listed the following as agricultural tools for enhancing nutrient content of crops: fertilizers, cropping systems and soil amendment [42]. Others have stated that micronutrient fertilization, in addition to boosting crop yield, enhances crop nutritional quality, thus addressing the attendant human micronutrient deficiency and health challenge [22; 39; 40; 43; 44].  
It has been observed that micronutrient content of crops decreases even when the yield is high, probably because of continuous mining of these nutrients without replenishment by especially high yielding varieties [14; 45; 42]. Therefore, [PERSON] and [PERSON] advised that the success of any biofortification program will depend on adequate available micronutrients in the soil for plant absorption or supplied externally through micronutrient fertilizer [46]. This is because the complex interaction required in transporting nutrients from the soil to edible portion of the crop need to be surmounted [47]. There is evidence of increasing nutrient and crop yield by application of micronutrient fertilizers [48]. [PERSON] et al. showed increased Fe, Zn and Se content of six cereal plants by growing them on coal combustion residues that was naturally rich in these nutrients [35]. The progress of agronomic fortification is a function of the application methods, fertilizer type and packaging and the crop developmental stage during application [39; 45].  
Conventional fertilizers are readily available for plant uptake but also easily lost through leaching, which is a major challenge. NPK and other agrochemicals have been found to have low use efficiency by plants because of fixation, leaching, microbial degradation, photolysis and volatilization [46; 49]. As such, quantities of these inputs are usually lower than minimum effective doses that reach the crops. Thus, repeated applications are required to attain maximum yield. This pollutes the environment, including underground water sources. Use of fertilizers, which are usually in the form of salts or in ionic form, results in ready availability and also rapid loss from the soil. However, the challenge to sustainably produce crops with high nutritional values amidst the unfavorable biophysical conditions and other limiting factors becomes the focus of the researchers. Along this line, the need for innovative fertilizers is recently presented [50].  
Moreover, arable land and water resources are increasingly limited. Therefore, to continue the development of agricultural sector, use of modern technologies to increase resource use efficiency with the least damage to environment must be pursued [51].