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Topsoil, which is the uppermost layer of the earth's surface, is a very valuable resource. Because of the need for more and more space it is becoming a scarce and expensive commodity. Top soil has been created over the millennia by the physical and chemical action of climate and weather on the earth's parent rock materials and the accumulation of decaying organic matter derived mostly from the evolving world of plants. Depending on where a garden is situated, its top soil may be anywhere from a few inches to many feet in depth.
Frequently because of gardening and landscaping activities, additional topsoil is needed. Raising up mounds, berms or beds to grow plants above the level of the native soil has become increasingly used in our gardening efforts. These are especially appropriate methods when the native soil is poorly drained or when there is no topsoil, such as in areas where rock in all stages of evolution predominates. Unfortunately, genuine topsoil created by natural forces is often unavailable because it is so scarce, and when it can be obtained it is often very expensive.
To make up for this scarcity a combination of materials is often sold as topsoil. They may be called 3-way or 5-way mixes, but generally no matter what they're called they usually consist of mixtures of sub-soils, sand, sawdusts, manures and biosolids. Because there are not legal standards for these materials, their makeup does vary depending on the preference of the maker or the availability of materials throughout the year.
A good "topsoil" should have a loose and open structure so that it is fast draining, but at the same time it must be able to retain enough moisture in order that plants growing in it are not constantly subjected to drought stress. The above materials, when combined in proper proportions, should provide these physical qualities.
A lack of success when using these mixes may be related to several causes, Often the mix is simply thrown down on top of the ground and no attempt is made to incorporate some or all of it with the native soil. When this is done, a perched watertable is created, since water will not move freely across interfaces created by two radically different soils butting up against one another. This area will tend to remain soggy to the point that plant roots are suffocated.
When any mix is added to a garden it should be incorporated into the native soil, or at the very least a shallow transition zone of the two soils mixed together should be created. Accomplishing this will eliminate the problem of water being hampered in moving through them.
Vegetable gardens and areas intended for new lawns should be treated this way. The same precautions should be taken when digging planting holes for shrubs and trees. Never discard the native soil completely. If a manufactured mix, or even a natural topsoil for that matter, is going to be used as backfill, it should always be incorporated in the native soil. In most cases though, for the majority of woody plants, it is not necessary to amend native soils unless they are very sandy or predominantly clay.
An exception to the above might be made in the case of plants in the Ericaceae, or heather, family. This includes rhododendrons, azaleas, heather, blueberries and other similar fibrous rooted plants. They do best in soils with a great deal of organic matter and the incorporation of peatmoss, bark, sawdusts and composts is usually advised. On occasion, manufactured mixed have been found to contain substantial amounts of soluble salts which have prevented seeds from germinating or caused young plants to be stunted or to die. It's never easy to determine what the sources of these salts are, but manures and synthetic fertilizers added to the mixes are some possible culprits. Before planting in such mixes it might be prudent to allow rainfall or several irrigations to carry away any possible excess soluble salts. Incorporating commercial mixes with the already existing soil will also help to eliminate the problem.
Many soils available contain biosolids or 'humanure' composted with vegetation or wood. Products produced using currently approved practices do not contain hazardous levels of pathogens or heavy metals.
For further information contact your local WSU Extension Office.
