Another potential strategy is the development and use of nano-fertilizers that can improve nutrient use efficiency. Nanomaterials are applied as biofertilizers, major element fertilizer and potential nutrient delivery systems that can encapsulate fertilizers protecting them from leaching. A combination of biotechnology and nanotechnology develops smart fertilizers with controlled nutrient release through degradable delivery systems and less negative impact on environment. 37 Bioformulation fertilizers contain encapsulated microorganisms beneficial for plant nutrients fixation and mobilization. Use of smart fertilizers is a promising step towards sustainable agriculture without harming the environment. The effectiveness of fertilizer use is strongly dependent on when and how it is applied. 6 Improved monitoring of field conditions makes fertilizer application more effective. Formulations of fertilizer that would reduce the time sensitivity of fertilizer application by sustained or controlled release of components would be attractive to farmers if sufficiently economical, either by increasing yields or by reducing labor costs. If fertilizer were formulated to require less nitrogen/nutrients, its costs in energy and resources could be reduced. Nitrogen in fertilizer is metabolized by soil bacteria to nitrate, which is not useful for plant nutrition, while urea (an alternative to ammonia and ammonium nitrate) is processed by urease enzymes to CO2 and ammonia, which is lost as gaseous ammonia and thus unable to function as a nutrient. Inhibiting the conversion of ammonia to nitrate and the conversion of urea to ammonia gas would thus reduce the amount of excess nitrogen needed in fertilizer. Urease and nitrification inhibitor additives to fertilizer formulations are helping reduce these losses and inefficiencies. Another refinement that could help reduce the nutrient content of fertilizer is the use of biofertilizers containing living microorganisms which colonize the soil or plants, and which increase the supply of nutrients to plants. The use of bioinoculants such as nitrogen-fixing bacteria or phosphate-solubilizing bacteria , for example, can be used to improve the bioavailability of nitrogen or phosphorus in soils and applied fertilizers, increasing plant use efficiency and thus reducing the amount of nutrients required. Reducing the amount of nutrients needed would also reduce the amount of fertilizer contamination in surface water, improving water quality and decreasing eutrophication. Organic fertilizers may be able to help as well. The use of wastes as fertilizer or fertilizer components can both improve fertilizer sustainability and decrease pollution. The wastes from sewage and wastewater treatment can be used as fertilizer directly, but wastewater processing to reduce the levels of nitrogen and phosphorus is often necessary. If the phosphorus and nitrogen from wastewater processing could be effectively converted to phosphates and nitrogen useful for fertilizer, then eutrophication from wastewater pollution would be reduced while fertilizers would be made more sustainable. For example, struvite is a product of water treatment that may be of use as a fertilizer. Improvements in processing methods for sewage treatment could also make those wastes more useful fertilizers by reducing the content of heavy metals, pathogens, and undesired contaminants and thus reducing the amount of synthetic fertilizer needed. The development of biorefineries may also be able to improve agricultural sustainability in multiple ways. Biorefineries are designed to use biomass, including crops raised for such purposes, as feedstock to replace nonrenewable sources for plastics, fuels, or other important materials. To sustain the crop yields necessary to replace other feedstocks, large amounts of fertilizer are needed. Growing crops and raising livestock on one farm allows livestock manure to be used as fertilizer. The need to transport manure (which is bulky and expensive to transport significant distances) is elided and applications of synthetic fertilizer can be reduced. The waste from the biorefined crop can be used as food for livestock or as additional fertilizer; in some cases, it may also be used for biogas or as fuel to provide energy for the farm. Biorefineries thus may improve the sustainability of fertilizer production and the circularity of the economy through reuse and recycling of wastes for new products. This article is intended to discuss the development of sustainable fertilizers. Methods for nitrogen fixation which may provide nitrogen fertilizers with reduced fossil fuel energy consumption and carbon dioxide emissions, methods for the use of wastes (particularly those from wastewater treatment) and biochar in fertilizers, and the design and development of biorefineries for more efficient and sustainable resource use are discussed.