Soil Preparation

Robert Bishop 
Frederick County Master Gardener Program

For the most part Frederick County soils are on the heavy side. This is mostly due their high clay content. I get so many phone calls from people complaining about their lousy soil. They say nothing will grow in it, it is like cement; or that the home builder took away all the good topsoil when the house was built. There is a solution, like most projects, it is the preparation that is most important. If you invest time and money into soil preparation it will pay off in the long-term growth and vigor of your plants.

Your existing soil can be amended to provide a rich environment for plant growth. To begin with, you cannot know everything about your soil by simply looking at it. Do a soil test. Call the number below or stop in my office to pick up a soil test kit. The kit will instruct you how to take the sample and provide the address for the University of Maryland soil testing lab.

Take the sample and mail it yourself, the resulting report will be mailed back to you in 2 - 5 weeks depending on the time of year. The basic soil test will cost you between $8.50 - $9.00 dollars including postage. It will tell you the soil pH, the existing level of the nutrients: magnesium, phosphorus and potassium and the soil texture.

Soil texture refers to the physical consistency of the soil. All soils consist of varying amounts of three soil particle sizes. Sand is the largest, silt is smaller, and clay is the smallest. When we say the soil has a silt loam soil texture, it means that it predominantly consists of silt size particles, while a sandy loam has more sand particles. From the test results you will be able to determine what fertilizer (if any) and amendments you will need to add to your existing soil to improve it.

Besides supplying nutrients, soils also provides a place for root growth and development. Most plants roots do better in lighter soils that drain well. Drainage depends on the amount of spaces (pore space) between the soil particles. Pore space also provides a place for air exchange with the roots.

Heavy clay or compacted soils have less pore space and this restricts water movement through them and reduces air exchange. The best soils both drain well and retain moisture. The addition of organic matter (compost, peat moss, composted manure) to a soil will increase water retention while improving drainage. It also provides some nutrients and living soil microbes as well. A

s the microbes decompose the organic matter they produce a glue like byproduct which bonds soil particles together (soil structure) and further increase pore space. Gypsum also has the same effect on soil structure, and I personally feel it is under used by most gardeners with heavy soils.

Gypsum is calcium sulphate. It will not raise or lower pH. It is also useful for removing salt build-up (from de-icing materials) from soils.

My favorite method for amending the heavy clay soils here is to incorporate the following soil amendments:

  • Compost - spread 2" thick over the area to be planted
  • Finely ground pine bark (sold as Pine Fines)- spread 2" thick over the area to be planted
  • Gypsum - 10lbs. Per 100 square feet

I use a roto-tiller to mechanically mix the amendments in, try to till as deeply as possible. Sand is another good choice, but you will need a large amount. Depending on your soil test results you may have to add lime to raise the soil pH or iron sulphate to lower it. Fertilizer should only be added if existing nutrients are reported as low.

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