Soil Test

If you call the Master Gardener office for advice, chances are good that someone will recommend sending a soil sample for analysis. Soil samples are easy to obtain, and the service is free for NC residents. The results of the soil test will tell you what amendments your soil needs. A soil test is recommended every two years.

Steps for providing soil samples: (from Urban Horticulture Note #5)

 STEP 1: Prepare the Sample Boxes

Use a separate box for each planting area you wish to have tested. Sample areas separately if:

  • Different plants are grown, i.e. lawn, vegetables, shrubs, and azaleas.
  • Plants seem to grow differently, i.e. shrubs thrive in one area but not in another.
  • Specific areas have been treated differently, i.e. front yard has been fertilized but backyard hasn’t.

For each area to be tested, make up an identification code. You may use up to five letters or numbers. These codes are for your own use; they will be shown on the report you get from the lab. The illustration identifies the samples as FRONT, BACK, VEG, and SHRUB.

soil sampling diagram

Keep notes on which code refers to which area! What seems obvious now may not be so obvious in a few weeks when you get your report back from the lab. Write your name, address, and sample identification code on each box. Use a pencil or a permanent waterproof marker to write on the boxes.  Felt-tip markers may bleed.

NOTE: For fastest service, sample soil in summer or early fall. October through April is the busy season for the soil-testing lab. Avoid sending samples during the peak time, which is February through March.

STEP 2: Collect Samples

You can collect soil samples any time of year when the soil is reasonably dry. Do not sample when soil is wet. See the note above about the busy season for the testing lab.

Clear away grass, twigs, and leaves from the surface of the ground. Use a clean shovel to dig a V-shaped hole (8″ deep for a garden, 4″ deep for lawn areas). Take a 1 ” slice from one side of the hole and place it in the bucket.

For each area to be sampled collect a slice of soil from six or eight different spots and mix thoroughly. This is VERY important to get accurate results. Use a clean tool, not your hands, to mix the soil, as salts from your skin can affect the test results. Remove any large pieces of organic material. From this mixture, take enough soil to fill the box up to the line on the box (no more, no less). Discard any soil remaining in the bucket.

Do not line the box with a plastic bag. The first step in testing is to dry the soil in an oven, and plastic bags will prevent the soil from drying properly. It’s okay if the box gets damp from the soil. REPEAT for each area to be sampled, collect a slice of soil from six or eight places per area.

STEP 3: Fill Out the Form

Print your name, address, phone number, and county in the upper left-hand box under “Grower’s Name.” Be sure to-write clearly and provide an email address

Sample Identification Identify each sample with your five-letter/number code. Make sure the information on the boxes matches the information on the form!

Last Crop & Next Crop The lab needs information about what plants will grow in the sampled area, to give good recommendations about what to add to the soil. Different plants have different needs. Last Crop is what has been growing in the area. Next Crop is what you plan to grow there next. If you are converting a patch of your lawn to a rose bed, “Last Crop” would be turf grass and “Next Crop” would be Roses. If your lawn will remain lawn, then Last Crop and Next Crop will be the same. Home gardeners can usually ignore the “Second Crop” space.

Crop Codes Crop Codes are found on the back of the form. Look under “Lawn, Garden, Ornamentals” (lower left corner of the sheet) for appropriate crop codes. Some of the most commonly used crop codes for homeowners are 024 (VegetableGarden), 023 (Flower Garden), 026 (Lawn), and 028 (Roses), and 020 (Azaleas). Note that Centipede grass has its own crop code.

Lime Applied Within the Last Year Lime takes months to fully react in the soil. If you applied lime within the last year, it’s still working on bringing the soil pH up. The lab will adjust their recommendations accordingly. The better your information is, the better their recommendations will be. Write the amount applied in the T/A column. Commercial growers will use tons per acre, but you can use pounds of lime per 1000 ft’ – (abbreviated “M”, e.g. 5OM = 50 Ibs/1000 ft2). YR and MO are the Year and Month you applied the lime. If you’re not sure how much you applied or when you did it, either make your best guess or leave the space blank.

STEP 4: Send In the Samples

Send the form and the soil samples to the NCDA&CS lab. There are many ways to do this:

  • Pack the sample boxes in a sturdy cardboard carton and mail them to the lab.
  • Bring them to the Durham County Extension Center  at 721 Foster St. (They will be delivered to the lab within two weeks.)
  • Deliver them to the lab on Reedy Creek Road in Raleigh.

Results are provided online. You will receive an email when yours are available. Soil test results are no longer provided by mail. If you do not have access to a computer, please contact your Extension agent or a Master Gardener.

A later post will discuss the results of a soil test. -AB

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