Using Fishy Water and Fly Frass to Grow Sweetpotatoes
Sweetpotatoes shown with their ‘slips,’ or cuttings, in the background. ARS researchers are developing a method to grow Sweetpotato slips in an aquaponic system. (Getty Images)
A collaborative project between the Agricultural Research Service (ARS) Aquatic Animal Health Research unit in Auburn, AL and the University of Arkansas at Pine Bluff, may provide the keys to unlocking several nutritional and economic opportunities. The passwords are “aquaponics” and “sweetpotatoes.”
Aquaponics is a blend of aquaculture and hydroponics. Aquaculture is the farming of aquatic organisms (fish, shellfish, and algae), while hydroponics is the production of plants using soilless growing systems in indoor growing facilities.
Aquaponic systems use water from fish production to irrigate and feed the hydroponically grown plants. The water is then recirculated back to the fish tank for reuse.
“Fish excrete waste, bacteria convert the waste into nutrients, and plants remove the nutrients, and improve water quality for the fish,” said Carl Webster, project leader and fish biologist at ARS’ Aquatic Animal Health Research unit.
Sweetpotatoes are a staple agricultural product in the United States and around the world for human nutrition, livestock feed, biofuels, and many industrial products. Despite the word “potato” in the name, sweetpotatoes are not potatoes; they are from different botanical families. And, because sweetpotatoes are grown from cuttings rather than seeds, they are the perfect match to be the plants in Webster’s project.
“One of the criticisms of aquaponics is that plant production is often limited in scale compared to terrestrial farming, where many thousands of acres of land can be planted,” Webster said. “However, for sweetpotatoes, which rely on cuttings (or ‘slips’) from a mature plant, commercial production of slips must be done indoors, so they can be available for farmers to plant outdoors.”
This process is good for sweetpotato growers because obtaining slips is a traditional bottleneck in the production process, Webster said. Optimized slip growth and production with aquaponics would help the sweetpotato industry produce more slips to plant outdoors.
One unique aspect of the project is Webster’s use of black soldier fly frass as an organic fertilizer. Black soldier flies are grown for their ability to recycle organic waste; frass is the waste material left behind by fly larvae. The fly frass can be used as fertilizer.
“This study will provide data for the domestic and global aquaponics industry,” Webster said. “As aquaponics is a fairly new commercial industry, [we] are conducting research on numerous aspects to provide techniques and information to people who want to grow plants and fish aquaponically.”
According to Webster, the global aquaponics market size was about $1.21 billion in 2024 and is expected to reach $1.92 billion by 2029.
“Aquaponics is a small, but rapidly growing, industry,” he said. “Mass-scale production of aquaponic crops has not been realized yet, but with more research to provide information to producers, aquaponics could be a very viable form of agriculture.”
With fish waste serving as the primary nutrient for plants in some aquaponic systems, organically grown crops are a largely untapped market for emerging aquaponic farms and aquaponic system providers, Webster said. Sales of organic fruits and vegetables rose by 5.6% to $19.2 billion in 2021, making the United States one of the leading markets for organically grown fruits and vegetables.
Aquaponic systems are also environmentally sound and hold great nutritional promise. Being indoor facilities, aquaponic systems may be housed in urban areas or warehouses where they leave a smaller and more sustainable environmental footprint. These facilities can feed people who live in “food deserts,” where there is little to no access to fresh fruits and vegetables.
“Indoor aquaponics systems enable you to grow food year-round,” Webster said. “Further, fresh produce can be locally grown, no matter where the aquaponics facility is located, for instance, downtown in a large metropolitan area. People can have access to fresh food any time of year, and consumers would know where and how the produce was grown. As more than half the world’s population now live in cities, aquaponics could supply those large populations with locally grown fish, fruits, and vegetables.” – by Scott Elliott, ARS Office of Communications