ALTERNATIVE GREENHOUSES: New Ideas for Design and Operation (1990)

Note: This is a reprint of an article originally published in 1990.

(1990, March/April). ALTERNATIVE GREENHOUSES: New Ideas for Design and Operation. Missouri Farm Magazine, pp. 35-XX

Mark McMurtry’s greenhouse integrates the production of animals and plants in a system that recirculates water, repeatedly between fish tanks and vegetable growing beds. The relationship is mutually beneficial: The fish produce high-quality protein while their liquid and uneaten feed fertilize the vegetables. The plants in turn take up nutrients that would accumulate in the water to levels that are toxic for fish. The fish require only 1/100 the amount of water they would in fishponds, and the vegetables are fertile with heated water and rich fertilizer needs. The controlled environment and efficient use of water in this polyculture system enable it to operate in arid regions where producing food would not be feasible otherwise.

With examples like these, it’s no wonder that the 90 participants in a recent workshop on alternative greenhouses were attractive during the proceedings at Meeting Osage Project at Fox, Arkansas. (Additional sponsors were the Kerr Center for Sustainable Agriculture, Poteau, Okla.; hosted by the Ozark Small Farm Viability Project, Parthenon, Arkansas.) These participants realized that the innovative ideas presented were independent of scale and could be adapted to a wide range of applications. Also clear was the intriguing possibility of combining Edey’s and McMurtry’s approaches into a single culture system with the best features of each. Here are the details as presented by the workshop participants received.

He fed vegetables and four inches for lettuce. Many of the greens will produce until May without bolting; others require a new planting every two or three months. Fish farming is also easily to be successful in solving, but Edey fertilizes with liquid seaweed when the seedlings emerge, again after transplanting, and every other week thereafter.

Edey set out to show that it was possible to produce high yields of food without using fossil fuels and chemicals, or creating pollution — and make a living at it. She was successful beyond even her expectations, and she has developed detailed plans for her greenhouse. She also does consulting and design work for a fee by telephone or mail. In addition to lecturing and working on a book to let others know about her methods and to encourage more producers to use them.

The economic potential of fish-vegetable production

Mark McMurtry’s greenhouse is very different from Edey’s, but is equally successful in its own right. According to two economists at North Carolina State University, where he developed the system, the economic potential of McMurtry’s polyculture is very attractive. These economists used current industry figures for costs and returns to compare tomato production by conventional methods with fish-vegetable polyculture. While conventional tomato producers realized a net profit of 43 cents per square yard per year, McMurtry’s operation showed a potential net profit of $21 per square yard per year. McMurtry is confident that his system can be scaled up for commercial production. In fact, several commercial operations are under consideration, and some giant food corporations are interested in using his technology.

Advantages of recirculating systems

McMurtry’s system utilizes water more efficiently than conventional irrigation. Furrow irrigation of tomatoes typically requires 140 gallons of water to produce one pound of food (dry weight). Trickle irrigation is more efficient and can give the same production from 43 gallons of water. In McMurtry’s recirculating system, a pound of tomatoes uses only 23 gallons of water, and the fish are a “free” bonus. That is not the whole story either. Under conventional irrigation practices, plants have only one opportunity to use the water before it moves out of their root zone. In the recirculating system, plants use each volume of water that is applied 100 times or more as it cycles between fish tank and growing beds.

Another advantage to recirculating water between fish and vegetables is that the plants require no fertilization other than fish wastes. McMurtry’s research has shown that vegetables fertilized and watered eight times daily thrive on nutrient concentrations of 1/10 to 1/100 those applied in conventional practice. The reason for the difference is that nutrients are replaced by the frequent application of “fertilizer” as they are used up.

Flushing the growing beds regularly also facilitates gas exchange within the sand medium and this stimulates conversion of fish wastes into plant nutrients by aerobic microorganisms in the filter. Frequent gas exchange also creates conditions in the root zone of the vegetables that promote mineral uptake.

Usually, the water in recirculating aquaculture systems becomes acidic because of a chemical reaction between the water and the ammonia from fish wastes. Even clarified and mechanically filtered effluent cannot prevent this acidification, and aquaculturists usually resort to carbonates to neutralize it. In McMurtry’s polyculture, filtration and microbial action in the sand, combined with nutrient uptake by the plants, prevent the water from becoming acidic. Seven consecutive trials using the same water proved that it remained suitable for fish and plants.

The “how-tos” of fish-vegetable polyculture

McMurtry raised tilapia, a fish native to tropical Africa, in his polyculture. Because tilapia are extremely tolerant of poor water quality, reproduce readily in captivity, and grow rapidly to highly prized food fish even in crowded tanks, they are a favorite of aquaculturists all over the world. Depending on the biomass of plants in the system, McMurtry stocked as many as 100 fingerlings weighing one-third of an ounce to a 132 gallon tank. (Throughout this section, figures have been converted from metric.) The bottom of the tanks slope at 45 degrees to cause feces and uneaten feed to accumulate at the lowest point. Eight times daily, water and sediment are pumped from the bottom of the tanks and delivered into a furrow along the surface of the sand-filled growing beds to water and fertilize the vegetables. In addition to a complete turnover of water daily, the tanks receive continuous aeration to keep the water oxygenated adequately for good fish growth.

Sand is essentially the medium for the growing beds because of its incredible surface area. The grains in a tablespoonful have an aggregate surface area as large as a football field. The bacteria that break down the fish wastes that are filtered out by the sand thus have a tremendous surface area for their substrate. Sand also allows water to drain rapidly after each application. The clean water returns by gravity to cycle again through the fish tanks.

Tomatoes and cucumbers were the principal vegetable crops that McMurtry tested in his trials. A variety of other plants also grew well in the polyculture, including legumes, root crops, peppers, eggplant, melons and herbs. He planted four vegetables per square yard in beds of sand one foot deep. The experiment tested ratios of water volume (in cubic yards) to plant area (in square yards) ranged from 1:1 to 1:6.75. These ratios were tested to determine the most critical relationship on the growth of the fish and vegetables.

Yields from the fish-vegetable system

Average yield of tilapia was 173 pounds per cubic yard of water over a nine month period. Fish survival was 100 percent, and growth was rapid; some individual fish reached 2/3 pound in 12 weeks, and 1 pound fish were common at the end of the period. McMurtry harvested the larger individuals periodically to balance the biomass of fish, hence reducing their waste production, with the nutrient requirements of the vegetables. Tilapia were fed as much commercial fish feed as they would eat in a 15-minute period twice daily.

Tomatoes and cucumbers produced well under the conditions of McMurtry’s recirculating system. He began harvesting tomatoes seven weeks after he transplanted them, and cucumbers four weeks from the time he seeded them. The average yield from a tomato plant was 13 pounds. Cucumber yields were also good, but McMurtry was unable to obtain production figures because invading voles took a nearly greenhouse swipe at themselves to over half of the harvest.

Guidelines for the design of a polyculture system

Operating a fish-vegetable polyculture successfully is an art as well as science. McMurtry advises against anyone jumping in “with both feet unless he or she has experience with closed-system aquaculture and greenhouse horticulture. Having success with fish-tips in ponds and vegetables in a garden is not adequate preparation.”

McMurtry recommends starting small. Even an aquarium connected to a washtub of sand planted with vegetables can be useful in learning to balance the animal and plant components of the system. Ideally, fish wastes should provide exactly enough nutrients for optimal vegetable growth, and the vegetables by taking up all of the fish wastes should provide optimal water quality for the fish. Achieving and maintaining ever an approximation of this ideal is difficult because the mass and metabolism of the fish and plants changes continually.

McMurtry offers some guidelines to start the uninitiated off in the right direction. The first step is deciding which water: sand ratio gives the best prospect of achieving one’s objectives. For example, is the objective to maximize caloric output per volume of water used, or to maximize profit per dollar invested? Deciding which objective is foremost will tell you which water: sand ratio to use. (The latter objective is probably the most attractive goal to prospective practitioners of fish-vegetable polyculture in the United States.)

To have a system with maximum potential for economic return and biological sustainability, McMurtry recommends a 1:1 ratio of water volume to sand volume. (With beds one foot deep, this volume amounts to three square yards of growing area.) McMurtry has a rule of thumb to relate fish and vegetables in his system appropriately. Each pound of gain in fish weight over a three to four month vegetable crop can support one point. On that basis, stocking 200 tilapia weighing one-third ounce each per cubic yard of water should provide an appropriate level of nutrients for the 12 tomatoes, or other vegetables, that would be planted in the associated sand beds during one growing period. A mutually beneficial relationship between fish and vegetables could be maintained by harvesting the largest fish periodically, as appropriate to the changing situation.

McMurtry developed his polyculture because he knew first hand that people in arid parts of the world desperately need a way to grow food. His system is a success. It produces food intensively, uses a minimal amount of space and water, and requires no equipment except a means of moving water. The method can do elsewhere “in any culture and accommodate to any level of technology. Moreover, it can be applied to a wide range of horticultural activities, for instance, mass producing seedlings for reforestation. McMurtry will soon be in Africa helping to bring his technology to bear where it is needed most.

The pioneering ideas demonstrated by both McMurtry and Edey not only “work” in themselves, they also provide a testing-board for further advances in the development of sustainable methods for producing food. The opportunities are exciting, and they are there for anyone to take.

For further information contact: Anna Edey, Box 682 RFD, Vineyard Haven, MA (508) 693-3341; and Mark McMurtry, iAVs Research Group, Box 7609, NCSU, Raleigh, NC 27695-7609 (919) 851-3604.

A checklist for successful aquaculture in tanks

Ken Williams, aquaculturist at the Kerr Center for Sustainable Agriculture in Poteau, Oklahoma, gave participants at the conference a crash course in growing fish in tanks. He summarized the practice in nine key points:

1. Grow tilapia. Willard and Mark McMurtry concurred that this fish is hard to beat, especially for a beginner, because it is forgiving of mistakes and because it is as delicious as it is hardy.
2. Maintain good water quality. Without it, even if the fish are alive, they will not grow or reproduce. Good quality means dissolved oxygen, a pH of approximately 6 to 9 pH, and a total ammonia below 1 ppm. Temperatures between 85 degrees Fahrenheit are best for rapid growth. Inexpensive test kits are available to test for these variables, and quality should be monitored daily, or even more often.
3. Feed on demand. Feed daily, and only feed as much as the fish will eat in 15 minutes. Uneaten feed can cause oxygen depletion in fish tanks.
4. Use a complete feed. Fish in tanks must have a feed that meets all of their nutritional needs. They don’t have access to supplemental sources of food like fish in ponds do. Standard fisheeds are not adequate because they trace elements that will accumulate to toxic levels.
5. Aerate continuously. Some mechanism for pumping air into the water is essential in fish tanks to maintain dissolved oxygen levels suitable for good fish growth.
6. Stocking rate. A total biomass of approximately one pound of fish per gallon of water is a good target. Stock at harvest as necessary to keep within the range of 1/10 pound to one pound per gallon.
7. Plan for emergencies. Power failures and similar emergencies are fatal very quickly to fish in tanks. Have a back-up system in place for such eventualities.
8. Have a marketing strategy. Know how and where you will sell your fish beforehand. Harvested fish must be moved quickly, and holding harvestable ones quickly eats into profits.
9. Like what you are doing. Aquaculture is a scientific art. You will never develop the necessary “blue thumb” unless you really want to.

Publication Details:

• Magazine Title: Missouri Farm Magazine (currently Small Farm Today Magazine)
• Location: Clark, Missouri
• Editor/Publisher (likely): Ron Macher
• Date: March/April 1990
• Article Title: ALTERNATIVE GREENHOUSES: New Ideas for Design and Operation
• Starting Page: Approximately 35 or 36

Small Farm Today is the original how-to magazine of alternative and traditional crops and livestock, direct marketing and rural living. With circulation concentrated mainly in the Midwestern states, Small Farm Today provides small farmers and rural Americans with information they can use in their lives and on their farms. 

The Original How-to Magazine of Alternative and Traditional Crops, Livestock, and Direct Marketing—Established 1984