The rapid growth in the global population has substantially increased the demand for agricultural produce. To meet this agricultural demand, fertilizers are overtly used in agricultural lands. Typically, the formulation of chemical fertilizers is associated with high energy requirements, non-renewable resources, and greater carbon dioxide (CO2) emissions. Therefore, scientists have been working on developing sustainable fertilizers.
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The Need for Sustainable Fertilizers
The World Population Prospects 2022 projected that the global population will grow to 8.5 billion by 2030 and reach 9.7 billion by 2050. This increase in the global population automatically enhances the need for a sustainable agriculture system.
A sustainable agriculture system ensures optimal water, energy, and nutrition usage and decreases environmental impacts. It also encourages a reduction in the dependence on depleting natural resources and maintains global economic strength.
Plants require both macronutrients and micronutrients for their growth. Since most agricultural soils are nutrient deficient, farmers use additional fertilizers to increase crop growth.
Macronutrients, such as nitrogen (N), phosphorus (P), and potassium (K) are required in larger amounts. In contrast, micronutrients, such as calcium (Ca), boron (B), copper (Cu), manganese (Mg), zinc (Zn), and molybdenum (Mo) are required in relatively smaller amounts.
Plant nutrients are available in the form of organic fertilizers or synthetic fertilizers. Organic fertilizers include fish meal, water or sewage treatment waste, alfalfa meal, manure, blood meal, and wood ashes. Both synthetic and organic fertilizers are required to feed the global human population.
Since fertilizers (synthetic and organic) require energy and other resources, it is crucial not to overuse them and minimize their negative consequences. In ammonia, nitrogen is produced using the Haber-Bosch process, reducing atmospheric nitrogen to produce ammonia in the presence of an iron-based catalyst and high temperature. Subsequently, the use of ammonia-based fertilizers induces nitrous oxide formation, which is a known greenhouse gas.
Phosphate is derived from a non-renewable resource that lasts between 40 and 400 years. It is obtained from mining and the chemical reaction of phosphate rock and phosphoric or sulfuric acid. Potassium is also obtained by the mining of underground brines. A large amount of energy and water is required to obtain these fertilizers. Therefore, it is crucial to make fertilizers sustainable in agriculture.
Different Pathways for Sustainable Fertilizers
The U.S. Environmental Protection Agency (US EPA) described sustainability as “[the ability to] create and maintain the conditions under which humans and nature can exist in productive harmony to support present and future generations”. The guiding principles of sustainable fertilizer comprise 4-Rs, i.e., recovery, reduction, recycling, and reuse.
Recovery and reuse are involved with processes that directly generate useful waste products (e.g., sewage treatment) and reuse them as fertilizer.
Recycling is associated with converting wastes to useful products, while reduction involves decreasing the use of natural resources, energy requirement, and the cost of fertilizer production.
The manufacture of nitrogen fertilizer can be made more sustainable by using alternative methods.
Some strategies include replacing natural gas with renewable energy and developing a superior catalyst that decreases the temperature and pressure required for ammonia production. Electrochemical or photochemical methods can also produce ammonia that utilizes renewable and non-CO2-generating energy.
Nanotechnology has been applied for sustainable fertilizer nutrient recovery processes. Scientists use nanotechnology for the slow release of fertilizers in soluble forms. The nano-based fertilizers are developed using green methods, less harmful polar organic acids (e.g., citric acid), reduced water and energy usage, and minimum waste. Applying nano fertilizers has substantially increased crop yield (e.g., corn) compared to conventional fertilizers.
Several soil bacteria convert nitrogen (ammonia) fertilizer to nitrate, which is useless for plants. Therefore, inhibiting the conversion of ammonia to nitrate or urea to ammonia gas would significantly decrease the excess nitrogen fertilizer requirement for plant growth. Adding nitrification inhibitors and urease in fertilizer formulations can prevent nitrogen losses and inefficiencies.
Another form of sustainable fertilizer is bioformulation fertilizers. This type of fertilizer contains encapsulated microorganisms that benefit plants' nutrient fixation and mobilization. The introduction of nitrogen-fixing bacteria or phosphate-solubilizing bacteria in the soil increases the bioavailability of nitrogen or phosphorus, improving crop growth and yield.
The effectiveness of fertilizers depends on the time and mode of application. A nano-based formulation that ensures sustained or controlled release of nutrient components over time is highly economically beneficial with a higher yield.
Reduced synthetic fertilizer use is linked with lower surface water contamination, improving overall water quality, and decreased eutrophication.
Utilization of organic wastes as fertilizers enhances fertilizer sustainability and decreases pollution. For instance, waste from wastewater and sewage treatments can be directly used as fertilizers. However, in some instances, the levels of nitrogen and phosphorus require reduction before their application in soil.
Biorefineries are designed to improve the sustainability of fertilizer production. This also aids in the circularity of the economy via the reuse and recycling of waste for new products.
Future Perspectives on Sustainable Fertilizer Developments
In the future, alternative greener methods will be required for fertilizer development. Using nano-based, energy-efficient fertilizer production and recovery methods promotes sustainable fertilizer management. Microbes and chemical additives are currently used in fertilizer formulations to improve nutrient use efficiency.
Small and large companies have begun incorporating recovered nutrients into their fertilizer products. This technique is expected to continue to grow in the future. A single technology will not work in all settings. Therefore, the integration of many technologies could yield better results.
At present, strategies to produce green ammonia have been regarded as an essential step for sustainable fertilizer production. A successful strategy could modify or replace the Haber-Bosch process for nitrogen fertilizer development.
References and Further Reading
Sustainable use of nutrients. (2023) [Online] Available at: https://agriculture.ec.europa.eu/sustainability/environmental-sustainability/low-input-farming/nutrients_en
Avery, H. (2022). The Role of Organic Fertilizers in Transition to Sustainable Agriculture in the MENA Region. New Generation of Organic Fertilizers. http://dx.doi.org/10.5772/intechopen.101411
Babcock-Jackson, L. et al. (2022) Sustainable Fertilizers: Current Publication Landscape and Challenges. DOI: 10.26434/chemrxiv-2022-fxnlc
Lv, C. et al. (2021) Selective electrocatalytic synthesis of urea with nitrate and carbon dioxide. Nature Sustainability, 4, pp. 868–876. https://doi.org/10.1038/s41893-021-00741-3
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