Did you know that it can take several hundred years for a layer of soil just one inch thick to form? That is one of the many reasons why topsoil should be preserved and not be allowed to 'run off' or be washed away. But what exactly is soil, or earth, made from? Well, soil is a mixture of either sand or clay with water and decayed, or decaying, material called humus. Humus is formed from dead organisms and their nutrients become part of the soil. They can then be taken up by plants and enter the food chain once more.