Most soil tests measure the pH of a soil to determine whether it is acidic or alkaline. The critical pH number will tell you if your soil will allow plants to receive the nutrients they need to flourish. Some soil tests also measure the nutrients present in the soil as a guide to how much and what kind of fertilizer you may need to add.
When to Test
Testing soil in the spring is best to determine what level of nitrogen and other minerals will be needed for the growing season. Test one or two weeks before planting to allow time to get test results and buy the necessary fertilizer.
Since soil ordinarily loses some nitrogen during the winter, fall is not the best time to test garden soil. If you test your soil in the summer or fall, make sure it has been at least two months since you last applied fertilizer.
Collecting Soil Samples
Remove mulch before collecting samples. Collect samples with a hand auger, bulb planter, spade or shovel. Hand augers are useful for sampling soils at different depths. Bulb planters are available at most garden supply centers. Augers and bulb planters are more accurate than spades and shovels.
To get a representative sample of your garden, mix samples of soil taken from at least 10 locations. Collect samples up to 6 inches from the surface. Deeper samples are sometimes taken to test for soluble nitrogen, sulfate and chloride that can move deeper than other nutrients.
The pH scale ranges from 0 to 14. Soils with pH below 7 are considered acidic; soils above 7 are alkaline. Neutral soil has a pH of 7. Each number on the scale measures a tenfold difference from the preceding number. A pH of 5 is 10 times more acidic than a 6. Most garden vegetables do best in a soil with a pH of 6 to 6.8.
Adjusting Soil pH
If you have acidic soil, you may have to add lime to raise its pH. If you have alkaline soil, you may have to add sulfur to lower its pH. Most soil tests will tell you how much lime or sulfur to add and in what form to achieve an ideal soil pH.
Once you have achieved the best soil pH for your garden, you can maintain it by regularly applying manure and compost.
Some soil tests also report the nutrients in your soil. The three main macronutrients are nitrogen (N), which plants need for growth of stalks, stems and leaves; phosphorus (P), which plants need to produce blossoms and fruit; and potassium (K), which helps plants use water effectively. Other important macronutrients are calcium, magnesium and sulfur. Labels on fertilizers list three numbers that indicate the ratio of the principal nutrients. A 10-10-10 fertilizer, called a balanced fertilizer, contains 10 percent by volume each of N, P and K.
Where to Get Soil Tested
Most state agricultural extension service offices will test your soil for a small fee and sometimes for free. If your extension services does not test soil, you can buy soil testing kits from garden supply centers that are ordinarily less accurate and limited in what they measure. You can find soil-testing laboratories online. Plant scientists at Montana State University recommend using independent laboratories that are members of the North American Proficiency Testing Program.