The Dirt on Topsoil

Topsoil is a nebulous word. It means the top layer of soil. When ordering topsoil, you need to be more specific. If you need to import soil to change the grade of the landscape, the imported soil needs to be similar in texture to the existing soil. Never cover a sandy soil with a clay or silt soil or cover a clay or silt soil with a sandy one. Importing the wrong soil will cause drainage problems.

            Before importing soil, have your current soil tested to determine the amount of sand and silt and classify your soil as loamy sand, sandy loam, loam, silt loam, clay loam, etc. When you know what soil you have, you’ll be set to order a similar soil.

            You also need to specify other factors: percent organic matter, pH and absence of such objectionables as stones, wood, weed killers and man-made inerts. It should be free of stones larger than two inches in diameter, tree roots, wood waste and metal, glass and plastics. If the soil has been taken from farms that have cultivated corn, soybeans or sunflowers, it should be guaranteed free of pre-emergent weed killers. Imported soils contaminated by weed killers can kill plants.

            I specify that topsoils from active agricultural fields not be used for at least three years after the last harvest. A good indicator the soil is free of weed killers is an abundance of healthy weeds.

            In addition to proper texture, a good imported topsoil should have a minimum of three percent organic matter.

            When you’re not needing truckloads of soil, it is often better to manufacture your own topsoil by adding compost to your existing soil. By doing so, you avoid all of the problems mentioned above. I have recommended this procedure many times to landscape contractors and landscape architects.

            Start by ordering mechanical analysis and nutrient tests of your soil. If it is rich in phosphorus and low in potassium, order yard debris compost such as LeafGro, restaurant-waste compost or horse-manure compost. If the soil is rich in potassium but low in phosphorus, order biosolid composts such as Orgro or Bloom.

            If your soil is loamy sand or sandy loam, apply six to seven cubic yards of compost per 1,000 square feet. If your soil is loam, silt loam or clay loam, limit the rate of application to four cubic yards per 1,000 square feet. The soil test results should contain recommendations as to the amount of limestone needed if the soil is acidic.

            Spread the compost and liming material uniformly and plow or rototill them deep into the existing soil. After the compost and lime have been thoroughly mixed with the existing soil, collect a random cup of mixed soil. Place in a plastic bag and moisten if the sample is dry. Store the soil samples in a warm place for at least a week before submitting for retesting. A final soil test is needed before planting to make certain that no additional amendments need to be added.

            Mother Nature has taken centuries to convert subsoil to topsoil. With quality compost readily available, you can make high-quality topsoil in days. Many suppliers of sand, gravel and topsoil also sell manufactured topsoil. They use the same method I’ve described on a larger scale, blending subsoil with compost using large cement mixers or continuous belt mixers.

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