Answer: Purchased topsoil varies dramatically in quality and in type, and in many cases is a poor match for the existing soil. This can cause problems in the long run, so usually it is best to use some of the existing native soil and mix it with organic matter when filling the beds. If you loosen the soil beneath the beds and further "fluff" it up by mixing it with organic matter such compost, rotted leaves, aged manure and bedding, you will begin to approach the needed volume. Over time, as the organic matter rots down you will have manufactured your own "top soil" in the beds. In my experience there are no exact proportions to follow as it depends on which type(s) of organic matter you are using, but half existing soil and half organic material would be a good start. Your end result is ideally one that will hold both water and air and can be dug with your bare hand; this can take a number of years to achieve and in any case more organic matter should be added on a regular basis. You will also need to test your soil periodically to determine what amendments if any you need to add. Peat moss is slightly acid to neutral in pH and has very little nutritive value but it is a source of organic matter. It should be moistened before you add it because it can be difficult to get totally dry peat moss to reabsorb water. You may find that there is a locally available material that is less expensive than peat moss as well.
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