When you are improving and enriching your soil, be sure not to mix a different type of soil with your existing soil. For example, you wouldn't want to mix a sandy soil with a clay soil this would produce disastrous results. Also, when adding organic material, make sure you don't use plant residue that consists of diseased plants or organic material that has meat scraps. Finally, sewage sludge can be used but it may contain high levels of lead and cadmium so it is best to use this material only on plants that you are not intending to ingest.