Potassium and potassium fertilizer terms show up very infrequently in these associations found in our cluster diagrams and charts. However, a few instances of publications on more sustainable sources were found for potassium, which could perhaps have greater utility in a more sustainably focused future. Historically potassium has been derived from potash mining, brines, wood ash and other ashes, potassium-silicate minerals, and even from kelp, though generally mining of potash has been economically more competitive as an industrial enterprise. Availability and affordability of mined potash globally has become an issue, particularly for poorer nations in the Southern hemisphere. The earlier use of ashes and kelp as potassium sources indicates that improved technologies and methods for nutrient recovery may make these sources useful again. Kelp is farmed for other products such as alginic acid and carrageenan, used in foods, and is recently being proposed as a low-cost means for carbon sequestration and mitigation of ocean acidification. Could the promise of another profitable potassium product from kelp cause further regional interest in kelp farming? Time will tell. A variety of methods have been used to recover potassium struvite (MgKPO4 6H2O) from bio-wastes, such pig slurry after nitrification-denitrification 66 , by combined partial nitration-anammox from municipal wastewater , by selective recovery of two struvite forms, magnesium ammonium phosphate (MgNH4PO4 6H2O) and potassium struvite, by CO2-assisted extraction from poultry litter , extraction of potassium struvite from pumpkin wastes 69 , from sugarcane vinasse by an integrated electrodialysis and precipitation process 70 , and in multiple potassium mineral forms found in biochars produced from sugar palm fiber, coconut fiber, durian shell, and palm oil fruit. So once again we find there are multiple sources for recovery of fertilizer nutrients, including potassium, from waste and wastewater sources. Sewage sludge generated by wastewater treatment plants is a source of both biogas energy and nutrients such as phosphorus, nitrogen and potassium. The nutrient content depends on sludge type (biochemically treated activated sludge, anaerobically digested sludge, lime treated sludge) and processing methods. Sewage sludge can also be a primary feedstock for producing biochar. Biochar produced via sewage sludge pyrolysis is rich in P, N, and K-based nutrients which makes it a potential fertilizer if other contaminants can be removed. Charcoal and ashes derived from municipal sewage sludge contain a lot of phosphorus-based nutrients. Hydrochars prepared by hydrothermal carbonization of wastewater sludge are not only important adsorbents of nutrients, but also have potential as controlled-release fertilizers and methane production enhancers. Anaerobic digestion (AD) is the natural process in which microorganisms break down organic materials in closed spaces where there is no air (or oxygen). Anaerobic digestion is the preferred treatment method for organic fractions of agricultural, industrial, and municipal solid waste. AD of organic matter proceeds via hydrolysis (polymer decomposition), acidogenesis (volatile fatty acid production), acetogenesis (acetic acid production), and methanogenesis (methane production). The process produces two main products: digestate and biogas. The sludge (containing 90-95% of the original organic matter) which remains after anaerobic digestion is called digestate. Nutrient recovery from anaerobically digested swine wastewater in the form of crystallized struvite was achieved by employing a sequencing batch reactor and a continuous-flow reactor. Calcium phosphate and struvite were accumulated during anaerobic digestion of sludge from biological phosphate removal treatment. Compost, including vermicompost, appears among the products found in our chart associated with sustainable fertilizer patent products. Composting is the aerobic, thermophilic, microorganism-mediated bioconversion of organic matter into humic substances called compost. The process of composting: