Bentonite in Ponds for Killing Weeds

Making a Seal

• When a pond is built, it's necessary to make the pond bed compact and water-resistant so leaks don't develop. Clay is a medium frequently used to this end, and it's usually brought, or bought, from an outside source. Bentonite is a type of clay especially useful for making seals because of its expansion properties. Usually bentonite comes in a powder or pellet form and is applied to the pond floor before the pond is filled with water. Bentonite can also be used on a pond that's already filled with water; it's sprinkled on the surface and eventually falls to the floor, where a seal is made.

Sealing out Weeds

• Bentonite has been used as a means for sealing out weeds and weed seeds. This can be done only as a preventive measure, however; it has no effect on weeds that are already in a pond. This method is also used on dry ground for the same reasons.

Aluminum Content

• All varieties of bentonite have at least a low amount of aluminum, and some types have a moderate quantity of the element. Those with more aluminum content are effective at killing weeds in a pond without endangering fish or other wildlife around the pond. The aluminum content in bentonite deactivates phosphorous in the water, and phosphorous is an essential nutrient to most weeds, so if there's no phosphorous in the water, the weeds can't survive.

Practicality

• Bentonite is an effective tool for making seals for pond beds, but the seal will only temporarily mitigate weed issues, so bentonite is therefore inefficient as a weed killer. It can be hard to find bentonite clays that have an aluminum content high enough to effectively deactivate phosphorous, and though at the outset the aluminum content is less harmful to pond wildlife than many other solutions are, high levels of aluminum are toxic to humans and other organisms, so bentonite application should be done in very conservative doses.