## 6 Nanofertilizers
### Noble Metal Nanoparticles  
Noble metal nanomaterials have been used in agriculture and, among them, silver is the most widely studied. Silver has a long history of antimicrobial effect and that property has been utilized to remove unwanted microorganisms in the soils, plants and hydroponic systems with improved effect from nanosilver [94]. Apart from promoting plant growth and seed germination, silver has also been applied in the control of fungi, rot and various plant diseases [95]. On the other hand, there could be some negative influence. [PERSON] et al. reported that oxidative dissolution of Ag and polyvinyl pyrollidone (PVP) coated silver nanoparticles in the soil system negatively affect the nitrification processes [96]. Other noble metals such as gold nanoparticles have not been well utilized in plant production, as they are not among the micronutrient required in crops. However, they have been limitedly applied as pesticides and also support rapid plant growth. Gold nanoparticles at lower concentration in the soil have been reported to enhance the shoot to length ratio in _Lactuca sativa_ seeds, without affecting the microbial concentration or causing any toxicity [87]. In other studies, they improved the seed germination, growth rate and yield with early flowering, high pod length, excellent chlorophyll and sugar content and enhanced free radical scavenging due to increased flavonoids content [87; 97]. The mechanism of free radical scavenging follows that plants contain varied biomolecules such as flavonoids, phenolic substances, etc., and these substances by virtue of their functional groups and antioxidant properties can complex with the metallic ions in the plant and scavenge the free radicals that causes oxidative stress [60; 98].  
[PERSON] et al. reported that exposure of 5-15 mg/L of phytosynthesized gold nanoparticles to maize aged seeds significantly improved their germination and physiology without any toxicity [99]. The results obtained reveal active presence of the nanoparticles in the seed tissues, but were not translocated to the root or shoots, possibly due to the low concentration used. On the contrary, as applied in tobacco plants, gold nanoparticles have been observed to be absorbed in both the root and the shoot of the plant, suggesting more bioavailability of the nanomaterials to some plant species than the others [100].  
Platinum nanoparticles are another group of noble metal nanomaterials with interesting applications in plant growth. They have been reported to practically affect the growth mechanism of plants by increasing the length and weight of the plant root system [101]. The reactions of thenanoparticles are such that, if there is agglomeration, it becomes difficult for the nanoparticles to be absorbed and bio-accessibility is hampered. [PERSON] et al. [101] reported enhanced plant protective mechanisms due to increased flavonoids concentration resulting from nanoplatinum application. In another study of platinum nanoparticles uptake using _Sinapis alba_ and _Lepidium sativum_ plants, both plant species showed uptake of considerable amounts in the roots and shoots at different concentrations [102]. In this study, both the metal salts and nanoparticles were bioavailable with no recorded phytotoxicity and the _Sinapis alba_ plant recorded higher nanoparticles translocation with increasing concentration than _Lepidium sativum_.  
[PERSON] et al. [103] used some nanocrystalline metals (Fe, Co, and Cu) to treat soybean seeds, and recorded improvements in the chlorophyll index, number of nodules and amount of crops. Nanocobalt powder exhibited the most interesting crop growth and development among the three nanometals investigated.