Importing Topsoil - March 21, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
Many gardeners bring in a load of topsoil for various reasons. You may be putting in some raised beds, backfilling a retaining wall, or simply need something to cover the rocks and caliche that pass for native soil in our area. Before bring in new soil to solve existing problems, consider some of the pitfalls that your new topsoil may create.
To a soil scientist or a farmer, topsoil is the darker layer of soil that is found at the very surface of the soil profile. Topsoil is dark and fertile because of organic matter decomposition over time: plant litter falls and decomposes, roots die, and other micro and macro organisms make their living above and below ground. These organic matter additions increase water infiltration rate, water holding capacity, add necessary plant nutrients, and create a bank account of slowly released nutrients for the future.
Now consider the range of topsoil products that you may encounter when shopping around. Nurseries sell bagged topsoil products that I would categorize as organic amendments. Bagged topsoil is usually a very good plant-growing medium, but can get quite expensive when used for large jobs. Some nurseries also mix and market their own topsoil by the truckload. These products may include mineral soil, compost, and possibly some fertilizer. In this case, the nursery should be able to tell you what the materials are and where they came from. In general, these are reliable products and you get what you pay for.
Inexpensive topsoil may actually be better described as "fill". These materials may have been excavated from deep within the soil profile. These are actually subsoil rather than topsoil. If so, they contain some small soil-sized particles mixed and partially decomposed rock with little or no organic matter. When considering this type of material for a plant growth media, compare it to your existing soil before making a final decision.
Worse yet, the material may be labeled as "free fill". Before bring it to your yard consider what it could contain and inspect it for construction debris, caliche, or other unwanted materials. Another consideration is whether the material contains weed seeds or rhizomes. By looking around at where the soil came from and learning about the weeds found in those areas, you can get a pretty good idea as to whether it contains weeds or not. If the seller or giver cannot provide you with the source location, then don't take it for landscape purposes.
Following are some weedy species you should be aware of in the Verde Valley area. The following examples are some of the worst offenders and prior vigilance could prevent a serious weed control effort later on.
Johnsongrass (Sorghum halepense) soils could contain seed and underground rhizomes. It often grows on roadsides and ditchbanks and can be between 2 to 8 feet tall. The rhizomes can be almost as big around as your finger.
Bermudagrass (Cynodon dactylon) soils could contain seed, aboveground stolons, and underground rhizomes. Stolons and rhizomes are small1/16 to 1/8 inch in diameter, jointed like bamboo, and may or may not have roots and/or new shoots emerging from the joints.
Yellow Starthistle (Centaurea solstitialis) soils could contain seed. This plant can be found on roadsides on the south end of Camp Verde (Hwy 260, Quarterhorse Ln, Verde Lakes area). Fill from this area could contain seed.
Curly Dock (Rumex crispus) spreads by seeds and possibly by transplanted fleshy taproots. The plant grows in a rosette of wide, fleshy leaves. Flowers are on long spikes that start out green and turn reddish with age.
If you have read this far and are still not afraid, then here are some suggestions to make your topsoil transition successful. Don't skimp - if you are going to add soil, put down at least 4 inches and incorporate it into the existing soil. Putting good soil on top of bad will create a boundary that will not allow good water infiltration and root penetration.
Here is a simple recipe for adding topsoil successfully. Before adding any materials to your site, clear as much of the surface rock and debris as possible. Water and fertilize the area to encourage weed growth, and then apply an herbicide to kill any weeds that grow. Wait 10 to 14 days and reapply if necessary. Apply one or two inches of new soil and incorporate it with a tiller or by hand to create a transition layer. Next, put down enough new soil to bring the soil surface to the finished grade and water thoroughly. Fertilizer may be incorporated at this point or applied to each individual plant at the time of planting. If necessary, install or modify the irrigation system then re-level the area, and check the area/slope for drainage. Plant the area, irrigate appropriately to promote establishment, and monitor and treat for weeds if they appear. I know it seems like a lot of work, but these steps will pay off if followed.
Watch for the debut of the Backyard Gardener website. It will contain the current column and a searchable archive of past columns. The University of Arizona Cooperative Extension has publications and information on soil, irrigation, fertilizers, and plant selection. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at email@example.com and be sure to include your address and phone number. The Yavapai County Cooperative Extension web site is http://ag.arizona.edu/yavapai/.
| Arizona Cooperative Extension
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Last Updated: March 15, 2001
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