Exposed Subsoil

Subsoil on the soil surface is an unfortunately all-too-common result of home construction, one that many new homeowners must face along with a season or two of mud or dust. Not only does the subsoil create a mess, it’s also next to impossible to grow much in it unless it is improved.

When new subdivisions are developed, it is not uncommon for contractors to bulldoze the entire area, sometimes removing the existing topsoil as well as trees and other plants. In some cases, the topsoil ends up under the subsoil as the construction equipment backfills around buildings. Even fill dirt brought from another area may be subsoil.

Changing the grade and general configuration of the land is often the excuse for removing the topsoil. Fortunately, this happens less often with an individual home built to an owner’s specification than it does with subdivisions. Anyone planning to build a home should make every effort to have the topsoil saved and then replaced after construction is completed.

When the subsoil ends up on the surface as the result of grading and excavating, even weeds don’t grow, and the resulting erosion on the bare soil surface can cause problems. A heavy rain can create gullies in no time. In addition, subsoil is very low in organic matter, and may include mineral salts leached from the soil layer originally above it.

Regardless of which of the following solutions are adapted, the first thing to do in this situation is to test the soil, even though the subsoil may look more like clay or, at the other extreme, more like a sandy beach than garden soil. After testing, you can proceed with one or more of the alternatives below, depending on the size of the project, budget, and availability of materials and labor.

  • Find a supplier of topsoil and order enough to cover the area to a depth of at least 4 inches. It may be necessary to remove some subsoil to keep from raising the grade and causing drainage problems. If the purchased soil is a different texture than the native soil, till some in to form a transition zone, then spread the rest on top. Some contractors can supply good topsoil and do the entire job of spreading, tilling, grading, and fertilizing. This may be an expensive solution, but there is comfort in the fact that once it’s done, planting can begin. Cover the soil with mulch and plants as soon as possible to prevent erosion.
  • Plant a cover crop—a green manure—and then cultivate it into the soil to a depth of at least 6 to 8 inches. It’s important to get the soil covered with plant growth as quickly as possible. First, till the soil and level the surface. Plant grasses or legumes, then till the crop into the soil when it grows to 6 to 8 inches tall. Ryegrasses, fescue grasses, sweet clovers, and crown vetch are among the best plants to use as green manures or cover crops to prevent erosion. Repeat as often as you have time for.
  • If the area with surface subsoil is fairly small, purchase potting mix from a nursery and add 4 to 6 inches on top of the subsoil to make a raised bed. Raised beds can be set off from the rest of the yard or garden by timbers, railroad ties, bricks, stones, or any other appropriate, often ornamental, material. They also can be merely raised mounds above the original soil surface level.
  • Spread a 2- to 6-inch layer of leaf mold, manure, shredded leaves, compost, or other organic material on the subsoil and till it into the soil to a depth of 6 to 8 inches. Add some nitrogen fertilizer to pine needles, leaves, or other low-nitrogen mulches to speed decomposition. Repeat this procedure a few times, letting the soil rest a few weeks between applications, then mulch the surface and commence planting. In some regions, organic materials may be regularly available as industrial waste products or agricultural crops.
  • If it isn’t possible to till the organic material into the soil, just spread it over the surface and wait. If you spread an inch or two of planting mix on top of it, you can plant a cover crop to beautify the yard while you wait for earthworms and other soil life to dig the mulch into the soil for you. In a year, the soil will be much improved.