Many farmers have purchased liquid fertilizer this fall and are storing it in poly tanks for use next spring, however, storing liquid fertilizer on farms can present safety, environmental and economic issues. The cost of liquid fertilizer is about $5 per gallon.

There are no rules or regulations dealing with storing bulk liquid fertilizer in North Dakota. However, the North Dakota Department of Agriculture is developing rules relating to liquid fertilizer storage both on and off the farm.

Not all poly tanks are the same. NDSU Offers Advice on Using Poly Tanks for Agricultural Storage

“The material a poly tank can hold is determined by the amount of plastic resin per square inch,” says Dale Siebert, North Dakota State University Extension Service cropping systems agent in Richland County. “Specific gravity measures tank strength, so the higher the specific gravity, the denser and stronger the tank.”

Water weighs 8.24 pounds per gallon and has a specific gravity of 1. Liquid fertilizers weigh 10 to 12 pounds per gallon and should be stored in poly tanks designed with a specific gravity of 1.5 or greater. Pesticides also should be stored in a high-density tank to reduce the risks to the environment.

Siebert advises producers not to guess the tank’s specific gravity based on appearance, but to contact the tank salesperson or look at the specification sheet to get the information.

“Don’t assume that a poly tank is safe for storage after several years of use, regardless of the density or specific gravity,” Siebert says. “Tanks stored outdoors are subject to ultraviolet deterioration. Ultraviolet (UV) deterioration causes the tanks to become brittle, rigid and inflexible as they age. Generally, tanks begin to show signs of UV damage between 5 and 8 years of age.”

Keeping tanks indoors will help protect against UV damage. The color of the tank or painting the tank will not prevent UV deterioration because the tank contracts and expands, which causes the paint to crack and chip.

Tanks can be inspected using a water-soluble marker. Rub the marker on the tank and wipe off the excess ink. If you see clear plastic or just a few scratches, the tank likely is good for another season of use. If you see crazing (a patchwork of interconnecting markings that has a dry-rot appearance), the tank likely has lost its strength and should be replaced.

If your tank shows signs of crazing, you can further evaluate it by hitting the empty tank with a baseball bat. A good tank has the flexibility to bend and will not be harmed, but poly tanks that are brittle will crack or show signs of cracking. Tanks that exhibit signs of crazing or cracking should not be used to store liquid materials and should be replaced to prevent tank failure.

“Even though North Dakota does not require containment structures to collect and hold fertilizer in the event of tank failure, it is a wise management practice to use them,” Siebert says. “A ring dike installed around the tank that will contain a volume equal to the tank volume will prevent loss of material and costly cleanup in the event of a failure.”

NDSU Agriculture Communication