Place a few handfuls of various soil samples in a clear jar with a lid. Fill the jars about 2/3 full of cold water, then close them tightly. Shake the jars, then set them on a flat surface. After a few minutes, the dirty water will seperate into layers. Lighter materials (such as dead plants and other organic matter - i.e. humus) wil float to the top, whil heavier stuff (such as small rock fragments) will sink to the bottom.