Take a handful of moist soil, rub it between your finger and thumb and mould it in the palm of your hand. If the soil is sticky, workable like Plasticine, stains your skin, can be smoothed and made shiny, and doesn't crumble when released, it's clay. This is quite likely to be found at a depth, but soil from nearer the surface may crumble when released. This is a clay loam. Dark soils are due to decomposing plant material; a gritty feel means there's sand or gravel. Silty soils feel silky but you can't smooth and polish them. Good garden loam feels neither sticky nor silky.

If you have really clayey soil in the garden, the action of digging may smear the sides of the hole sufficiently to keep water in. It's worth digging a trial hole and filling it with water to see how long it remains full. The depth of topsoil will determine the final water level. Water may leak away through the topsoil, but should be retained by the subsoil.

If the soil isn't clayey enough to hold water without help, then try smearing the sides of the hole with a thick layer of wetted soil from the spoil heap and puddling it to create a watertight liner. However, any stones or organic material present will prevent you from being able to squeeze out every last gasp of air from the clay lining.