The biggest problem I had using the clay subsoil was that our red clay is very dense and fine-particled. This means that when you start to mix it with water, the clay will absorb a certain amount of water and then proceed to clump together. I experimented to see if this could be avoided, and it made no difference whether I first ground the clay into a fine powder or just threw in large lumps. So it took quite a bit of effort to get it to mix evenly without too many lumps. In the future I need to use a method other than hand-kneading as this take a very long time.