Gardening Article
April 17, 2004
Danny Lauderdale
Horticulture Agent
NC Cooperative Extension Service
Pitt County

Topsoil

I often get questions or hear comments made about using topsoil in landscape installation or renovation. A call just last week inspired me to revisit this topic. If selected, analyzed, and used correctly, topsoil can be a great addition to a landscape. However, if selected poorly, not soil tested, and used incorrectly, topsoil can cause several problems in the landscape. Three of these are plant nutritional problems, drainage difficulties, and weed infestation. I hope to address these issues in this article. Some of the information in this article comes from the North Carolina Department of Agriculture and Consumer Services Agronomic Division Soil Fertility Note 14. I have added additional information based on my own experiences and observations.

What is Topsoil? Some people have the notion that topsoil is a marvelous naturally deposited material that contains all the essential chemical, physical and biological components necessary for growing beautiful plants. Unfortunately, this is rarely the case. Topsoil is the top layer of soil which is excavated from one site and moved to another. Topsoil often needs amending with lime, fertilizer or organic matter. Use of topsoil is increasing in rapidly developing areas. Questions rarely asked include: 1. When should topsoil be used? 2. What factors are important in selecting topsoil? 3. How can the quality of a topsoil be evaluated? 4. How does addition of topsoil affect drainage?

To use topsoil effectively, you must know two things: what kind of soil you are amending and what kind of result you hope to achieve. If your goal is to improve drainage, your strategy will be different than if your goal is to increase water-holding capacity. Applying the right amendments can help prevent either problem. In North Carolina, topsoil is often applied to improve existing soils. The topsoil chosen to amend clay soils is likely to be sandy. This scenario sets the stage for increased nutrient deficiencies, acidity problems and moisture stress. The very qualities of sandy soils that improve drainage can trigger these problems. In such cases the soil is better amended with a combination of topsoil and organic matter. Amendments like composted yard waqste, cotton gin waste, aged or composted manure, or ground pine bark can improve drainage, soil porosity, nutrient retention and soil pH. They can also increase biological activity and promote root growth.

Your decision to apply topsoil will be based on the qualities of the topsoil and the qualities of the soil you want to amend. Topsoil is commercially available either bagged or in bulk. Bagged topsoil usually is sold in 40 to 50 lb quantities and has been amended with lime, fertilizer and organic matter. Bulk topsoil generally is a native soil taken from the surface and sold in truckload lots. Unfortunately, bulk lots of North Carolina topsoil rarely meet the standards of a good topsoil. The texter and fertility of bulk topsoils vary across geographic regions. In North Carolina, coastal plain soils are generally sandy wheras piedmont and mountain soils contain more clay. Although the topsoils from these two regions are vastly different, both can be made productive when properly amended with lime and fertilizers. Sandy coastal plain soils have physical properties that make them easy to distribute, but they have low nutrient and water-holding capacity. As a result, plants growing in these soils are more subject to nutrient deficiency and drought stress. Sandy soils generally require more frequent applications of lime and fertilizer, particularly nitrogen, potassium and sulfur. Addition of organic matter increases the water and nutrient retention of these soils. In contrast, clay soils aggregate and are hard to distribute. However, they hold water and nutrients well. As a result, lime and fertilizer applications are needed less frequently than on sandy soils.

A soil test is the most reliable way to determine the quality of topsoil. Most native bulk soils tested have low pH and nutrient content, both of which can be corrected with appropriate amendments. The addition of amendments, such as lime and fertilizer, also improves granulation of soil particles. Keep in mind, however, that clay remains clayey and sands remain sandy even after amendments are applied. You can modify the chemistry of soils, but their basic structure remains the same. Topsoil can be tested by sending samples to the NCDA & CS Agronomic Division Soil Testing Section. Soil sampling kits are available at all NC Cooperative Extension County Centers.

Topsoil is often applied to landscapes to improve drainage. However, the mechanics of water movement through soil is often misunderstood. If a layer of sand is placed on top of a clay-based soil, water moves readily through the sand and is absorbed by the clay. In contrast, if sand is placed below the clay, water moves into the sandy layer only when the clay soil is totally saturated. Therefore, placing a layer of sand on a poorly drained clay-based soil has no effect on drainage of the clay. Drainage of soils can be improved with any treatment that enhances granulation or aggregation of soil particles. Such treatments include lime, gypsum (calcium sulfate), organic matter, sand or a combination of all of the above. When adding topsoil or other treatment to an existing soil, mix the two thoroughly for best results. The existing soil should be tilled, topsoil added, and the area should be tilled again. Treat topsoil as an amendment. Placing it on top of already compacted soil may temporarily solve planting problems but will likely result in poor plant growth in the long run.

Finally, weed infestations often occur when topsoil is brought in. On some occasions the weeds are easy to control annual weeds like crabgrass or annual bluegrass. However, on many sites, tougher to control perennial weeds like nutsedges, Virginia buttonweed, bahiagrass, or bermudagrass are brought in with topsoil. If possible before receiving delivery of topsoil find out where it is coming from or being stored and check out the site for weed problems. If this is not possible, be prepared to keep a close eye out for weeds as they begin to emerge and identify them immediately. Weeds are much easier to control when they are young than after they have time to mature.

If you have further questions about gardening in Pitt County, give the Pitt County Master Gardener Extension Volunteers a call at 757-2801 Extension 40. Volunteer staff answer and return your calls on Monday, Wednesday, and Friday from 10 am to noon. You can also email questions to pittcomgv@hotmail.com. Another opportunity to get gardening, landscaping, and lawn information is coming up on Monday April 19. I will be giving a presentation on Dealing with Fire Ants and other yard pests beginning at 7:00 pm in the Demonstration Kitchen at the Pitt County Agricultural Center. If you are interested in attending call Wendy Garner at 757-2801 Extension 20 to register. There is no cost for attendance.