Subirrigation systems, also know as zero runoff, are an environmentally responsibly alternative that conserve water and fertilizers. They are being installed by greenhouse growers to improve product quality, achieve more uniform growth and increase production efficiency.
In subirrigation systems, water and nutrient solution provided at the base of the container rises by capillary action through holes in the bottom and is absorbed by the growing media. These systems are adaptable to crops grown in pots or flats.
Advantages of subirrigation systems
In a capillary mat system the pots are set on a mat that is kept constantly wet with a nutrient solution. Several styles of fabric mats are available from 1/4 inch to 1/2 inch thick. The pots take up the solution through holes in the bottom. The mat is places on a level bench over a layer of plastic. Water is supplied from drip tubes laid on top of the fabric.
To keep algae under control, a layer of perforated film plastic is sometimes placed over the top of the mat. Algicides can also be used. Some growers turn the mat over when a new crop is started. Containers holding nutrient solution and piping should be enclosed in black plastic or painted black to eliminate light and algae formation.
In this system, plastic or metal troughs are placed on existing benches or supported overhead from the greenhouse structure. The troughs are installed at a slight slope (3 inches to 6 inches per 100 ft) from one end to the other. Pots are spaced along the trough. Nutrient solution, supplied from spaghetti tubes, is pumped to the high end, flows past the base of the pots and is collected in a cross gutter at the low end. The solution returns to a storage tank under the benches or below ground to be recycled.
One advantage to this system over other ebb and flow systems is the air circulation that occurs between the troughs. Another is the ability to space the troughs for different size pots. Trough systems tend to be less expensive than bench systems and can be easily installed in existing greenhouses.
This system uses 4' to 6' wide watertight benches or water-tight movable trays to contain the nutrient solution. The benches, usually of plastic or fiberglass construction are installed perfectly level to maintain a uniform depth of liquid. They can be installed as either fixed or movable depending on the crops to be grown. Channels in the bottom of the bench allow the water to distribute evenly and to drain rapidly when the water supply is shut off. This allows the bench top to dry reducing algae growth and disease potential.
In operation nutrient solution is pumped from a holding tank to a level of 3/4 inch to 1inch depth in the bench and held there for 10 minutes or long enough for the media in the container to absorb the solution. A valve is then opened and the liquid is quickly drained by gravity back into the tank. Low cost PVC pipe is used as it is not affected by the fertilizer in the water. A filter removes any solid matter. The holding tank, usually located in the floor below the benches should have a capacity for about 1/2 gallon/sq ft of bench area.
The nutrient solution is used over again but adjustments in pH and soluble salts may have to be made as water is added. Water treatment with chlorine, uv light or ozone is used by some growers. Control of the nutrients and flow can be manual or with a controller. Watering may be once or twice a week to several times a day depending on the weather and the size of the crop.
Flooded floors work on the same principle and with the same equipment as ebb and flow benches. A watertight concrete is necessary for the floor surface and it must be installed as smooth as possible to avoid pockets. A laser transit is used to get a perfect slope, usually 1/4 inch in 10 feet. A concrete contractor having experience with flood floor system should be hired. Berms may be installed at the post line in gutter-connected houses to create zones. PVC pipe with slots or holes is usually installed in the floor in the center of the bay to supply and remove the nutrient solution as quickly as possible.
Large holding tanks are necessary, usually made of concrete and lined with plastic or coated with an epoxy paint. Typically a 21' x 200' bay will require 2000 to 3000 gallons of solution. In larger greenhouses, the tank has to be large enough to hold the liquid from several bays that are operated as a single zone.
Plastic piping is used to transport the nutrient solution as it is inert to fertilizers. Monitoring of the nutrient solution is done by a computer. Fertilizer is added, usually as individual elements, to maintain the desired nutrient level.
Best results are obtained if a floor heating system is installed. This provides uniform heat in the root zone area and quickly dries the floor after the solution is drained to reduce algae formation and lower disease potential. A horizontal air flow (HAF) circulation system will reduce moisture in the plant foliage. To save handling labor, a fork lift transport and spacing machine could be used.
John W. Bartok, Jr.2009