Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
Subsoil materials are frequently encountered at the surface of the ground as a result of erosion of the native topsoil or severe soil disturbances associated with earth moving and construction activities. Unlike topsoil, this material is often quite clayey and dense, devoid of organic matter, and resistant to plant growth. Subsoils in humid regions are usually highly leached, acidic, and infertile, while subsoils from drier areas tend to be much less acidic, but may be high in shrink-swell clays and/or salts. Subsoils in any region may be rocky.
Usually, the most important factor to correct immediately is the low organic matter content. Large amounts of compost or other organic materials must be repeatedly mixed in deeply (one foot or more, if possible). Over time, the organic matter decomposes and stabilizes the new surface soil, aiding in essential soil particle aggregation and building nutrient supplies.
Next, problems of acidity and infertility must be solved through liming and use of appropriate fertilization strategies. Have pH and fertility tests done and follow recommendations. Remember that the maintenance of organic matter in the soil does much to aid fertility.
Most subsoils are dense and/or clayey, so particular attention must be paid to the problems of poor drainage and water saturation. Even the addition of trucked-in topsoil usually will not solve poor drainage problems caused by heavy or compacted subsoils. Before new topsoil is added, poorly drained, exposed subsoils must be deeply, but coarsely, tilled. In some cases laying tiles below grade to facilitate drainage is advised. In many situations the use of raised beds greatly eases the modification of surface soil properties.
Subsoil materials left by construction and earth moving are often unnaturally mixed and are variable from point to point. Whenever possible when undertaking soil testing, analyze samples from numerous points, rather than putting all samples together; this allows you to isolate the problem areas that need special attention.
(Originally published as "Managing Subsoil Material at the Surface," by W. Lee Daniels, Associate Professor, CSES, in The Virginia Gardener Newsletter, Volume 8, Number 7.)
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