Step 5 - Pond Bottom Medium

What’s the best bottom medium substrate for a pond? When choosing a bottom medium, I prefer sand but also find river pebbles useful too. I use river pebbles sparingly in specific areas. For example, rivers pebbles work well for the front of an overflow. Also, mix pebbles with sand for a nice effect.

 River pebbles are also good for beneficial bacteria growth.


Bottom Medium for ponds | Natural Pond Lover

River pebbles are an expensive bottom medium

You’ll find that lining an entire pond with river pebbles can become very expensive. Pebbles are also more likely to slide down the slopes of the pond walls. You may find yourself constantly adding more pebbles to bare areas. I’ve been there and done that.

A female bullfrog living in one of my ponds insists on hiding under the flagstone of the overflow. Being as large as she is, I honestly don’t know how she fits between the slabs of flagstone. Occasionally, I find myself having to add river pebbles to the front of the overflow. River pebble edges are usually rounded making them a relatively safe choice for a bottom medium.

Sand as a bottom medium

Sand sticks to the slopes of the pond walls the best. Remember to keep slopes less than a 45-degree angle during construction. You may find yourself resealing the slopes with sand a few times if they are too steep. Aiming for 30 degrees, or less is all the better. Just remember that slowly descending slopes need a wider pond. It’s best to think big when laying out a new pond design.

The general rule of thumb is, the bigger, the better.

Bottom medium | Natural Pond Lover

Certain fish, frogs, turtles, and tadpoles burrow down in the sand. The only problem I foresee with any of these animals is turtles because they have sharp claws. Maybe not as sharp as a raccoon or bobcat, but a determined turtle can dig. If you’re going to keep turtles, make sure the liner has several extra inches of bottom medium. Avoid large and powerful turtles like snapping turtles or softshell turtles when utilizing a plastic or rubber liner.

Alligators are pretty much out of the question too. Alligators are the only exception I can think of when it comes to preferring a concrete basin.


Acquiring a bottom medium

I reuse the sand dug up from the hole when excavating a pond. It’s important to avoid any bagged sand intended for concrete mixing because it’s full of contaminants. Play sand is safer but a bit too fine. Think more of a sandy type of soil.

Sand as a pond medium

Keep in mind that pond plants need some fertilizer when sand is first used as a bottom medium. It takes some time for fish and other pond inhabitant waste to start acting as a fertilizer. In the meantime, you might have to add a fertilizer supplement for the plants. Follow the label’s instructions carefully. Adding too much fertilizer leads to cloudy water or a possible fish kill.

Some bottom mediums you should avoid 

Some organic material you shouldn’t use as a bottom medium for your pond includes store-bought topsoil, potting soil or crushed seashells. Also, avoid certain darker soils found up north that contain high levels of nitrogen. Such soil carries too many nutrients for your pond to start with. This kind of soil also gives off a pungent odor. Out of my six ponds, none of them emit any kind of offensive smell. Some muddy ponds really stink. You don’t want your pond to smell like a swamp. The goal is to replicate a natural spring when building a new pond.

These darker soil types are harder to settle and chronic muddy water often results. Although many pond inhabitants survive muddy water conditions, you’ll be the one missing out because you won’t be able to see what’s swimming around. For this reason, a sandy based soil is the best way to go. Once sand settles, it’s not going anywhere. Make sure the sand isn’t contaminated with gasoline, oil, pesticides or herbicides.

Bentonite clay 

I’ve talked about bentonite clay extensively in my ‘choosing a pond liner’ article. I tried this clay in lining my first pond but failed. Still, it seems that some people have been more successful utilizing this clay as a membrane. I have yet to see a successful natural pond lined with bentonite with my own eyes. If you have any pictures or video of such a pond, please email me.

Bentonite clay continued to haunt me with my second attempt at building this forsaken project. I called in a pond contractor who I made aware of the earlier situation. He used soil contaminated with bentonite to line my new pond. Chronic cloudy water resulted from every time it rained, or if a raccoon stirred it up. These conditions lasted for up to a week or more. In the end, this second attempt only lasted three months before raccoons destroyed the liner. This was because the pond contractor failed to properly bury it. I ended up scrapping the entire project and started over from the beginning.

The future of pond membranes?

Is bentonite clay the pond membrane of the future? I don’t think so, but polyethylene has such potential. I wouldn’t be surprised if polyethylene became the next big thing. It’d be great to see more people discover the advantages of using this liner. EPDM is still a good liner to go with but it’s the heaviest and most expensive. It’s also not as strong as polyethylene. PVC is for less serious pond construction. It’s come a long way over the years, but I wouldn’t use it. 

Choosing a Bottom Medium