County Extension Offices will often test soil samples for a fee, but not all of them do testing. Fortunately, digital meters and soil test kits that measure nitrogen levels are readily available online and at garden centers. With these tests, you can determine if nitrogen is depleted, deficient, adequate, sufficient or surplus prior to adding fertilizer. Excessive amounts of nitrogen will cause plants to grow poorly and wash into waterways.
When to Test
Adjust the amount of nitrogen added to the soil based on the results of the test. Where it is sufficient or surplus, no nitrogen should be added. Soil testing is usually done in the spring or fall but can be done any time. Test for nitrogen levels after adding soil amendments when testing garden soil. Soil amendments often contain nitrogen.
Topsoil typically has different nitrogen levels than soil down near the plant roots.
Take the samples from below the soil surface. Dig down 3 to 6 inches using a dirt shovel or hand trowel for annuals, herbaceous perennials, fruit and vegetable garden plants, and lawns. Dig down 1 foot for shrubs and trees. Set this soil aside. Gather a 1/2- to 1-cup sample from the bottom of the hole. Take at least five samples from different areas of the garden or lawn. When testing the lawn area of the back and front yards, take multiple samples from each. Mix the garden samples or lawn samples together. This will give an average for the entire garden or lawn. The amount of nitrogen needed could be slightly different from one area to the next even within a few feet, and it would be very difficult to determine the exact area that is slightly lower or higher. Using an average for the area will provide adequate coverage, even though the resulting amount of nitrogen may be slightly lower or higher than needed in a few small areas. Break up clumps and remove stones and debris. Spread the soil out on several layers of newspaper to let it dry naturally.
Put on garden gloves or rubber gloves before collecting soil samples. Natural oils on the skin could alter test results.
Preparing Soil Samples
Put a 1-cup sample into a container. Add 5 cups of distilled water. Put a lid on the container and shake it vigorously or stir the water and soil for a minimum of one minute. Let the soil and water sit in the container for 1/2 to 24 hours until the soil particles settle to the bottom of the container. Clay and silt soil will take longer to settle than sandy soil.
Testing Soil Samples
Pour the nitrogen test powder into the container first when using a digital meter or last when using a kit. Both are provided with test kits and digital meters. Use the dropper, also provided with the kit or meter, to add the sample water to the container. Take only the water after the soil has settled. Shake the container to mix the powder and water. Let the container sit for 10 minutes to allow the color to develop. Compare the water color to the test chart when using a kit or, if using a digital meter, insert the container into the meter to find the level of nitrogen in the soil.