Hasaneen et al. (2016) and Abdel‐Aziz et al. (2019) used two types of NPK nanofertilizers: nanochitosan loaded with NPK and carbon nanotubes (CNTs) loaded with NPK for fertilization of French bean plants in greenhouse conditions. They used three different methods of application of nanofertilizers, i.e. seed priming, soil incorporation, and foliar application. The results obtained revealed that, foliar application was the best method of application for both the nanofertilizers. For foliar application treatment, the life cycle of the plant was shortened by 37.5% (80 days) as compared with other nano treatments or nontreated plants (110 days). Also, nanochitosan‐NPK showed better improvement in the crop plants than CNTs‐NPK (Hasaneen et al. 2016; Abdel‐Aziz et al. 2019). Abdel‐Aziz et al. (2016) reported that foliar application of nanochitosan loaded with NPK (10–100%) to wheat plants grown in sandy soil in greenhouse conditions improved the crop productivity and shortened the lifecycle of the plant by 40 days as compared with traditional NPK fertilizers. The lowest concentration of NPK nanofertilizer (10%) recorded to have best effects on wheat growth and productivity. This study lacks comparison of the results of nanochitosan‐NPK with pure nanochitosan. The nanochitosan used in these studies was produced by polymerization of methacrylic acid with chitosan.
Similarly, Ha et al. (2019) produced nanochitosan from ionic gelation of chitosan with tripolyphosphate and then loaded NPK on nanochitosan particles. They applied the nanochitosan‐NPK fertilizer to the leaves of coffee plants in greenhouse conditions. It was observed that application of this developed nanofertilizer enhanced the rate of photosynthesis, increased the rate of uptake of nutrients and growth of coffee plants as compared with untreated plants. The results obtained from the above‐mentioned studies were reported in greenhouse conditions, and field trials are important to validate the approach of using NPK nanofertilizers.
In groundnut, foliar application of nanocalcium oxide (CaO) fertilizer reported to increase Ca accumulation and root development in plants as compared with normal CaO and calcium nitrate (CaNO3) (Deepa et al. 2015). This study showed that CaO nanoparticles were transported through phloem, but the mechanism of its action is still not known. In another study, Mg nanoparticles (500 mg/L) in combination with normal Fe (500 mg/L) were used to treat black‐eyed pea (Vigna unguiculate) and showed an increase in seed mass (10%) as compared with plants treated with normal Fe (Delfani et al. 2014). Nano‐S was also shown to enhance root and shoot growth of tomato and pumpkin plants as compared with untreated plants (Salem et al. 2016a,b). Both these studies showed that the effect of nano‐S was concentration‐dependent and higher concentrations caused deleterious effects on the growth of both plants. This suggested that further studies are needed to adjust the suitable concentrations of applied nano‐S.
1.2.4 Micronutrient Nanofertilizers
Micronutrient nanofertilizers are those fertilizers which provide micronutrients to the plants (the elements which are needed in small amounts by the plant but are essential for plant growth and metabolism) (Adisa et al. 2019; Zulfiqar et al. 2019). The most important micronutrients are zinc (Zn), boron (B), iron (Fe), manganese (Mn), and copper (Cu). It was reported that cucumber seedlings grown in nutrient solution containing Zn nanoparticles (synthesized from waste tire rubber) showed an increase in shoot and fruit yield as compared with other cucumber seedlings grown in commercial zinc sulphate (ZnSO4) solution (Moghaddasi et al. 2013). Adhikari et al. (2015) reported that maize plants treated with zinc oxide (ZnO) nanoparticles showed increased shoot dry weight and shoot height as compared with untreated plants. Also, Subbaiah et al. (2016) applied ZnO nanoparticles (particle size: 25 nm) as foliar spray on maize which resulted in enhancement of maize growth, yield, and Zn content in the produced grains as compared with maize plants treated with normal ZnSO4. Application of Zn nanofertilizer in pearl millet (Pennisetum americanum) caused significant increase in shoot and root growth, increase in chlorophyll content and leaf protein. Moreover, enhanced crop production as compared with control plants over a period of six weeks was also recorded (Tarafdar et al. 2014). Foliar application of micronutrient nanofertilizers of Zn or B to pomegranate trees resulted in an increase in fruit yield (30%). The magnitude of increase was most pronounced with low concentrations of nanofertilizers (34 mg/ tree for B and 636 mg/ tree for Zn) (Davarpanah et al. 2016). Dimkpa et al. (2017a) showed that micronutrient nanocomposites (ZnO, B2O3, and CuO, at 2.8 mg Zn/kg soil, 0.6 mg B/kg soil and 1.3 mg Cu/kg soil, respectively) when applied to soybean plants under drought stress improved the growth and yield of treated plants as compared with untreated plants.
There are few studies performed demonstrating the foliar spray application of iron oxide nanoparticles, results recorded easy uptake of these nanoparticles and improved growth in different plants such as corn (Li et al. 2016), pumpkin (Zhu et al. 2008), and watermelon (Li et al. 2013; Wang et al. 2013). In addition, some of other studies on the use of iron oxide nanoparticles on crops plants such as wheat (Ghafari and Razmjoo 2013), lettuce (Zahra et al. 2015), soybeans (Alidoust and Isoda 2013; Ghafariyan et al. 2013), clover (Feng et al. 2013), peanut (Rui et al. 2016), rice (Alidoust and Isoda 2014), and Citrus maxima (Hu et al. 2017) showed that Fe nanofertilizer improved several traits of these plants including increased biomass, grain yield, Fe fortification, and improved biochemical parameters such as nutrients uptake, chlorophyll content, and photosynthesis (Raliya et al. 2018). But it should be mentioned that the effect of Fe nanoparticles was relatively concentration‐dependent which implies the need for future studies to adjust the suitable concentrations of Fe nanoparticles and further application to a variety of crops (Zulfiqar et al. 2019).
Foliar application of Mn nanoparticles (0.05 mg/L) to mung bean (Vigna radiata) resulted in improvement of root growth, shoot growth, and biomass as compared to plants treated with MnSO4 (Pradhan et al. 2013). In another study, Dimkpa et al. (2018) showed that application of Mn nanoparticles (6 mg/Kg) did not affect wheat grain yield but enhanced nutrient uptake by the plant. Adhikari et al. (2016) showed that exposure of maize plants to CuO nanoparticles (10 mg/L) resulted in significant increase in growth as compared with untreated plants. Nanocomposites of CuO, ZnO, and B2O3 caused significant increase in the uptake of N, K, Zn, and B in soybean as compared with untreated plants under drought stress (Dimkpa et al. 2017a). Dimkpa et al. (2017a,b, 2019) revealed that root or foliar exposure of soybean or sorghum to the nanocomposites of Cu, Zn, and B or to Zn nanoparticles increase N and K accumulation. These findings indicate the possible role of using nanomicronutrients in fortifying accumulation of macronutrients in plants to achieve better nutrient use efficiency (Adisa et al. 2019).
1.2.5 Non‐nutrient Nanofertilizers
There are other nanoparticles which are not classified as plant nutrients but potentially have positive impact on plants. These nanoparticles mainly include CNTs, chitosan (Cs), cerium(IV) oxide (CeO2), silicon dioxide (SiO2), and titanium dioxide (TiO2). Although they are not of nutritional need to the plant, but they can improve growth and yield (Adisa et al. 2019). CNTs were found to increase shoot length of date palm (Phoenix dactylifera) at 0.05–0.1 mg/L (Taha et al. 2016) and promote growth of tobacco plants at 5–500 mg/L (Khodakovskaya et al. 2012). Studies have showed that Cs nanoparticles improve seed germination, enhance plant growth, increase photosynthesis, and improve crop yield (Adisa et al. 2019). Van et al. (2013) treated Robusta coffee plants with nanochitosan under greenhouse conditions which resulted in increased chlorophyll content and photosynthetic rate as well as increased nutrient accumulation as compared with untreated plants. Cu‐chitosan nanoparticles increased seedling growth, fresh, and dry mass of tomato plants as compared with untreated plants (Saharan et al. 2015). Pretreatment of maize seeds with Cu‐chitosan nanoparticles improved seed germination and growth parameters as compared with untreated control (Saharan et al. 2016).
In a similar manner, CeO2 nanoparticles reported to increase shoot biomass, growth, and grain yield of wheat plants relative to untreated plants (Rico et al. 2014).