Is freshwater fish compost as effective as saltwater fish compost on vegetable production?

Final Report for FS14-284

Project Type: Farmer/Rancher
Funds awarded in 2014: $10,000.00
Projected End Date: 12/31/2015
Region: Southern
State: South Carolina
Principal Investigator:
Dale Snyder
Sweetgrass Garden Co-op
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Project Information


The comparison of compost made with fresh fish carcasses and salt water fish carcasses found that fresh water fish compost was not inferior.


A previous study from The Sweetgrass Garden demonstrated the efficacy of compost made with fish carcasses. The source of fish for the study was a commercial, off-shore fisherman and included a variety of wild-caught (not farmed), native coastal species.

A subsequent grant proposal, which was not funded, involved a comparison of this fish compost with pluff mud, a rich, and abundant organic material. One criticism of this project was that fish carcasses are readily available on the coast, but not inland. Thus, results would not be universally applicable.

However, on reflection, we realized that fresh water fish are widely available inland. But would compost made with fresh water fish carcasses be as effective as compost from salt water fish? It seems that the two would be equivalent, since the composition of all animal tissue is identical, including fish species. For example, intracellular and extra-cellular electrolyte levels are identical for humans, moles, birds, insects, and all fish species. Because a fish lives in salt water, its flesh contains no more sodium than other animals. On the other hand, it is possible that trace element concentrations are different with ocean fish.

This study is a field comparison of two compost materials, one made with fresh water fish carcasses and the other, salt water fish carcasses. Essentially, it was a non-inferiority trial testing whether fresh water fish compost is non-inferior to that made with salt water fish , or compost that is plant based.

Project Objectives:

We planted three 100 foot rows of eggplant with 50 plants in each row and three 100 foot rows of green peppers with 50 plants in each row. 

We applied saltwater fish compost on one row of eggplant and one row of peppers.

We applied freshwater fish compost on one row of eggplant and one row of peppers.

We applied no compost on one row of eggplant and one row of peppers. 

Our objective was to measure the yields of each row for a comparison of saltwater fish compost versus freshwater compost versus the control rows with no compost.

The compost preparation was the same for both the saltwater fish compost and the freshwater fish compost.


Materials and methods:

Fish compost was made using the method described in our previous report, using 4x4x4 foot bins. A 4-inch wall of finished compost, a bio-filter to control odor, lines the compost bins. At the bottom of the bin four 5-inch, perforated, poly sewer pipes are laid, with their ends extending beyond the bin, thus allowing free flow of air into the base of the bin. As the bin heats, air is pulled from the pipes up through the compost pile, much as a hot chimney draws. For this reason the compost does not need to be turned as it cooks. The compost pile is watered as needed to remain moist.

The compost was constructed in layers: 2 inches of fish carcass including head, tails and entrails, 4 inches of straw or leaves, and a half-inch of garden soil. Within a couple days the bins heat to more than 130 degrees F.

Ocean fish carcasses were provided by Mark Marhefka, a commercial fisherman.
Fresh water fish compost was produced using the above method by Don Graham, a fishing guide on Lake Moultrie, South Carolina.

Compost produced at the Charleston County Landfill, using waste from road-side mowing, forest debris and other cleanup operations, was the third compost tested. There was no control row that used no compost.

The test plot on the Sweetgrass Garden included six 100ft test rows, separated by 4 feet. Two of the six rows acted as control rows with no fish compost inputs. They were planted in the spring with equal numbers of eggplant and peppers. The original proposal indicated the use of kale starts, however a generous donation of eggplant caused us to modify the plant selection. Each row was prepared with a two inch layer of test compost per foot that was worked in by hand, and planted the next week. After 4 weeks, plants were side dressed with one quart of compost. Drip irrigation from a well was used; since the three rows were served by drip tape form a single conduit, the amount of irrigation was identical for the six test rows. An 8 foot deer fence was placed around the test plot.

Research results and discussion:

The three, 100 ft test rows had from 50 plants per row. Eggplant fruit was harvested over an 8 week period, late July –late August.

Total yield: Plants/row Yield/plant

Fresh water fish compost = 144 lbs

Salt water fish compost = 101 lbs

Control = 123 lbs

Based on this field trial on Johns Island, SC, compost made with fresh water fish carcasses was not inferior to that using salt water fish carcasses.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

On October 31, 2015, upon completion of our data collection, we hosted more than 150 members of the Johns Island and Wadmalaw Island community to the farm to share the results of our research. We explained the experiment, we showed farmers the test plot, we outlined the process and we took questions regarding the implications of using fish compost, saltwater, freshwater, and otherwise. 

In addition, we published the final report on our FaceBook page, which currently is viewed by more than 750 people, including many farmers from Charleston and around the country.

Project Outcomes

Project outcomes:

In a previous study comparing fish compost with compost made with vegetable matter, we found that plant vigor appeared better with fish carcasses as a component. The previous study also found that the fish compost was superior to gardening without any soil amendment.

In this study we demonstrate that compost made with inland lake fish is at least as efficacious as wild-caught salt water fish.


Potential Contributions

There are commercial fisherman working on mid to large sized rivers, and on large lakes. They typically filet the fish and discard the carcasses. Our studies indicate that these organic remains are effective as a component of garden compost. Composting is inexpensive, and with the method described, not labor intensive (e.g. the compost does not require turning)
The benefit to the small farmer who uses fish compost is obvious (improved yield). Furthermore, like our supplier on Lake Moultrie, composting could be a profit center for the commercial fisherman, usually a small operator.

The present study did not address the use of carcasses from commercial fish farms. Typically these use ‘the whole fish’, grinding carcasses for use in fish meal, and as animal food. Our study suggests that these fresh water fish carcasses could also be used in traditional composting.

Future Recommendations

There is business potential: consider a garden center selling 5 or 10 pound bags of “Sweetgrass Garden Organic, Wild-Caught, Fish Compost”, with efficacy demonstrated by our field trials. While we have no plans to pursue this, such a composting program could add profit for a small operator, either farmer or fisherman.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.