We live in a great state that provides free soil testing most of the year! In North Carolina, soil samples are free for eight months (April-November) of the year. The other four months (December-March) of the year they cost $4.00/sample. This is a great reason to soil sample your home lawn, garden, or ornamental bed. The NCDA&CS Agronomy Division has a testing facility in Raleigh that processes in excess of 450,000 samples annually. They receive most of these samples within the four-month period in which they charge.

How do you take a soil sample? Sample kits are available at your local extension office. The kit will include a soil sample box and a form. To fill out the form, enter your information and then look for your plant code on the backside of the sheet. For example, centipedegrass would be code 022, or a vegetable garden would be code 024. Enter this code on the front of the form in the correct space. This is very important so that you can get a good report back. Some of the parameters contained within the report are dependent upon what plant you are growing. Next, take several plugs of soil using a garden spade or soil probe. For lawns, you want four inches of soil, gardens 6-8 inches, and for trees we would like 16 inches of soil, but that’s not really practical, so go as deep as you can. Take at least 10 sample plugs from around the area, mixing them up in a plastic bucket. Do not use a metal or galvanized bucket because these may contaminate your sample. Now, pour the mixed soil into the provided box, filling it to the red line. Place these soil sample boxes into a shipping box and send them to the NCDA&CS Agronomic Testing Facility.

The lab will generate a report that contains several parameters that we need to know so we can grow our plants. For our purposes, we need to look at two of these parameters. The first of these, and perhaps the most important of the two, is the pH of the soil. This is a measure of how acidic or basic the soil is. Values on this scale, less than seven, are acidic, above seven are basic, with seven being neutral. Most plants like the pH to be slightly acidic, around 6.0 – 6.5 on the scale. Depending on what we are trying to grow, pH will determine if nutrients are in a form in which they can be taken up and used by the plants. If the pH is more acidic than we need, which is the tendency in most of Eastern NC, there will be a lime recommendation. Lime is used to raise the soil pH and sulfur is used to lower the pH of the soil. Both of these products take a while to change the pH of the soil, so they need to be applied well in advance of when we are trying to grow our plants. If we are growing a spring garden, then a good time to get a sample turned in would be October through November. This way, the results are back and we can apply lime well before the growing season if recommended in the test report.

The next parameter to look at is the fertilizer recommendation. This is going to give us the amount we need to use in pounds (lbs.) with the analysis of the product beside it. The analysis is usually shown in a form like 10-10-10 or 15-0-14. When we see these numbers, we need to decipher them to know what we need to apply. For instance, if we need 10 lbs. of 10-10-10 that means we are using 10 lbs. of a product that contains 10% Nitrogen, 10% Phosphorus, and 10% Potassium (this is our 10-10-10). This is a unit per area measurement that is always in lbs. per 1,000 square feet.

How do we measure area? We take the length multiplied by the width of our garden. If my garden is 20 feet wide by 50 feet long, I have 1,000 square feet, so I would apply 10 lbs. of this product. How much actual fertilizer am I getting per 1,000 sq. ft.? If I look at the analysis 10-10-10, I know this is a percentage of active nutrients within the bag. I have a 10 lb. bag and it contains 10% Nitrogen, 10% Phosphorus, and 10% Potassium or 1 lb. of each nutrient. Everyone knows how to figure out a 10% off sale at their favorite store, this is the same thing! If the recommended analysis isn’t available, you know how much of the nutrient you need to apply so you can use another product.

Don’t Guess, Soil Test!

If you have a question to submit, please email Gene Fox at gene_fox@ncsu.edu. Check us out on the  Beaufort County Master Gardener Facebook page. Until then, happy gardening!