The ideal minimum clay content of a soil for a leakproof pond is 60%.

To test your soil, take a sample, let it dry out, then break it up as well as you can before adding it to a container of water. A tall straight-side container (e.g. a spaghetti jar), is best because it's easy to see the relative amounts of each consituent. Half-fill the container with water, add the soil, then top up with water. Turn it upside down as many times as necessary to get the soil in suspension and then watch the soil settle out.

Most of the larger particles of sand will settle in about 20 minutes, with the fine clay staying in suspension for a long while and any organic matter floating to the surface. Once all the soil has settled, measure the total depth of the soil in the container, plus the amount of organic matter. Then measure the depth of clay particles (ie those that are obviously not sand).

To calculate the amount of clay:

1. Divide the depth of clay by the total depth of all the soil and the organic matter.

2. Multiply the answer by 100 to obtain the percentage of clay.

eg. if there's a total of 10cm of soil and organic matter, of which 2cm is clay, divide 2 by 10 (=0.2), then multiply by 100 (=20) to obtain the percentage (20%).