Garden Tasks

The Dirt on NC Soil

When I moved to North Carolina from Massachusetts in 1993, I was eager to start planting. I had heard how many more varieties of plants could be grown here than in Zone 4, including warm season grasses, and I could not wait to get my hands in the soil. So you can imagine my surprise when, after great effort, I came up with a shovel full of red clay and rock. I wondered how anything grows here! I sought out the advice of local experts, and began the hard work of amending my soil. After 24 years of soil building, resulting in beautiful, rich, black dirt … we decided to move. We chose to build in Durham and I took a soil sample from the new lot. The test results are the 2nd worst I have seen in 20 years in NC horticulture.

To test your own soil, take core plugs from several spots in the yard and mix them together to get an overall sample. Or you can take several plugs per area (lawn, vegetable garden, etc.). A soil probe is a helpful tool for taking core samples. For each sample you will need about 3/4 of a sandwich baggie. The NC Department of Agriculture  offers free soil testing most of the year, charging a small fee December through March. Bring your sample to the lab in Raleigh and they will give you a form and a box to submit your sample. Results are emailed to you in about 2 weeks. Outside NC, check with your state agriculture department.

The NC report will look like this: Soil Test Report

Over the years I have performed hundreds of soil tests on my clients’ yards. The most telling number on those tests is the HM% – the percentage of Humus Matter (organic matter) in the soil. Ideal is 5%, but in my 20 years in the horticulture industry in NC, I have yet to see one above 1%. The average is 0.46%. Mine is 0.04%. Pitiful.

The pH is another important number. The form you fill out will ask what is being grown in the area sampled. I wrote  TifTuf Bermuda because that is what I intend to use for my lawn. I will (of course) also be planting perennials, trees, shrubs, vegetables, herbs, hops… but I digress. Ideal pH for most lawns (not Centipede) is 5.8-6.5 and, as you can see above, mine is 5.3. The lab recommends I add 80 pounds of lime per 1000 square feet. That is a lot of lime. I have work to do.

The solution to poor soil quality is soil building. In the Triangle, our native soil has the following layers: gray clay, red clay, and if you are lucky, a bit of organic matter on top. Most new construction is missing that important top layer. Nature builds soil in these layers, so for best results we should imitate Nature.

To remedy a lack of organic matter,  work compost into new beds and continue to amend your soil by adding a layer of compost yearly. I highly recommend Super-Sod’s Soil3 compost. One cubic yard comes in the signature BigYellowBag and can be delivered to your home! Soil3 is made from grass clippings, wheat straw and cow manure. It is inoculated with mycorrhizae and other beneficial organisms, and turned regularly to maintain a temperature of 160 degrees. The result is rich, humus compost that is OMRI  listed for use in organic vegetable gardens.

There are other important numbers to note on the soil report. The lab tests available Phosphorous and Potassium in the soil, but not Nitrogen because it is too unstable and the results will not be reliable. Under “N-P-K Fertilizer Recommendations” they will tell you how much fertilizer to add. Be sure to do this at the correct time of year for the plants you are growing. CEC measures the soil’s ability to absorb and release nutrients to the plant roots. Ideal CEC is 15+. High levels  of zinc, copper and manganese can be problematic as well. Be sure to read the Agronomist’s comments to see if further action is needed.

We move in to our new home at the end of December, and I will begin shortly thereafter improving my soil. I will start with a dump truck load of 50/50 topsoil/compost, and then top-dress annually with Soil3.  It will be a long process, but the results are well worth the effort. I will keep my readers updated as the new landscape develops. Happy gardening!

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