## 6 Nanofertilizers
### Synthesis and Role of Zinc/Zinc Oxide Nanoparticles in Fertilizers  
The primary sources of zinc micronutrients for fertilizer fortification are zinc oxides (ZnO) and zinc sulfates (ZnSO\({}_{4}\)H\({}_{2}\)O or ZnSO\({}_{4}\).7H\({}_{2}\)O). The use of zinc in living organisms cannot be overemphasized as it is contained in most enzymes, necessary for hormones and chlorophyll regulation, and for carbohydrate metabolism. In the nanoparticles form, ZnO can be absorbed, metabolized and accumulated in plant systems. Zinc is a micronutrient and are therefore needed in small quantity; higher concentrations of ZnO NPs can harm the development of plants, leading to inherent aberration in seed germination, root growth and seedling biomass [76]. Nevertheless, its deficiency could hamper the growth and overall development of plants. Figure 1: Negatively charged zeolite platform enhanced by natural polymers and other nutrients for optimum crop performance. Adapted from [74; 75]. When there is deficiency of zinc in the soil, it leads to low or no zinc absorption in the crop, thereby causing malnutrition in food and humans. One third of the human population has been affected by zinc deficiency according to WHO reports due to low zinc content in most foods especially the cereals [77; 78; 79]. Deficiency of zinc in our food together with other micronutrients causes hidden hunger diseases. Since the body cannot produce this micronutrient, it has a required daily amount, which, when not reached for a long time, leads to serious illness. Hence, such could be avoided by fertilizing crops with adequate micronutrients. The mixture of ZnO and zinc sulfate is used as fertilizer, since the oxide is not completely soluble in water. [PERSON] [78] reported that zinc deficiency in soils has been a problem that grossly affects crop productivity, especially in alkaline soils. In the application of these micronutrients to the soil, it is usually difficult to have uniform concentrations due to the amount needed. They are most suitably applied with macronutrients, which act as carriers, although zinc oxide and zinc sulfate can be moderately efficient alone as micronutrient, unlike only zinc [80]. The dissolution and absorption of the micronutrients also depends on the carriers, especially when they are liquids, as 2.0% of zinc could be dissolved in polyphosphate, ammonia, and anhydrous ammonium nitrate (AAN), whereas only 0.05% of zinc is absorbed in orthophosphate [80]. Soil factors and properties such as mineral composition, ionic strength, and organic materials also play important roles in the absorption and bioavailability of the nutrients. Calcareous soils with high pH value and calcium carbonate (CaCO\({}_{3}\)) content affects the solubility and amount of zinc absorbed into the soil [81]. The presence of the calcium carbonate exchanges Zn for calcium and precipitates zinc carbonate. This continues to hamper crop production, even when micronutrients are applied to counter the reaction of the minerals in the soil. Another hindrance to micronutrient absorption is the nature of soils, as sandy soils hardly retain nutrients applied to them because they are easily leached out, therefore, making the micronutrients unavailable for plant absorption. Soil pH is another key factor that determines the availability of micronutrients in the soil. As the soil pH increases, micronutrients availability decreases except for molybdenum. The mechanism in the case of zinc is that, with increasing soil pH, the Zn is adsorbed on the soil surface together with other clay and inorganic materials, and can be precipitated as Zn(OH)\({}_{2}\), ZnCO\({}_{3}\) and Zn\({}_{2}\)SiO\({}_{4}\)[78]. With such behavior, the solubility of the entire system is compromised, hence leading to inefficient desorption of the minerals into the soil for the plant uptake.