How to Test your Soil for a Pond Site
If you are planning to build a compacted pond without a liner, it is a good idea to test the soil to determine the clay content. The clay content will tell you whether or not the existing soil can be compacted. If you have clay content of 30% or higher, you can be pretty confident that with proper compaction, your pond will seal.
The first thing you need to do is decide where you would like to put your pond, and the depth. It is a good idea to have an alternate site. Then simply dig a hole down to the depth of your proposed hole, and excavate some subsoil from that depth. You can do a quick soil test by trying to make a ball out of a handful of subsoil. If you can pack it tight without it crumbling it has some clay in it. If you can roll it into a skinny snake than it has even more clay, and if you can bend the snake into a doughnut, you have even more clay. If your soil falls apart when you are trying to make it into a snake, you may not have enough clay to seal a pond.
A more accurate test would be to excavate some subsoil to put into a mason jar filled with water. Fill up the mason jar with water and put the cap back on tightly. After that, shake up the soil vigorously for 1 minute, then set the jar down on a level surface to settle. Within a few minutes, you will start to see the sand layer develop. After a few hours, you will see the silt layer start to develop, and after 12-24 hours, you will see the clay layer that will be the top layer. The clay layer can be a bit hard to see, so it is a good idea to put your jar up to the light to see the layer. Be careful not to shake it up as you do this.
Unfortunately for me, I ended up with only a 10% clay layer on both my test sites. You can still have a pond in this situation, but you will have to do some extra work, and spend a little more money. Below are some options if your soil is less than ideal. I am going to use sodium bentonite to seal my pond.
If you end up with less than 30% clay, then there are other options.
1. You can choose a different site.
2. You can mix in sodium bentonite.
3. You can bring in clay and mix it in.
4. You can use a liner.