How to Amend Soil for Growing Herbs

Take a soil sample from 6 to 10 locations across your herb patch by inserting a hand trowel up to the handle into the soil to loosen it. Lift a slice of soil 18 inches deep from the sampling spot. Place each soil sample in a bucket and mix them. Pick out debris and scoop 2 cups of soil into a plastic food-grade bag, such as a sandwich bag, with a zippered closure.

Take the soil sample to your local county extension service. An agent with the service will provide you with paperwork for submitting the soil sample to the nearest USDA-approved soil-testing facility. Label the bag with the code provided by the paperwork. Box up the bag and the paperwork, but do not put the paperwork in the bag. Include a check or money order for the testing fee and mail the sample and paperwork to the soil laboratory. Most soil laboratories return the testing results within three weeks. The results will indicate the soil pH, nutrient structure and soil amendments you should use to improve the soil for growing herbs.

Obtain the soil amendments recommended by the soil test from a garden center, farmer's cooperative or plant nursery. Soil amendments that will improve the soil for herbs include compost, peat moss, pine bark and dolomite lime. These amendments will improve nutrient structure and drainage as well as raising the pH of the soil. Most herbs prefer a soil with a neutral to slightly alkaline pH of 7.0 or higher. When the pH rating is too high, you may also lower the pH rating using elemental sulfur. For most soils, elemental sulfur is not required.

Loosen the soil of the herb garden to a depth of 12 inches by inserting a spade vertically into the soil and rocking it back and forth. Rake over the soil with a garden rake to further loosen the soil. Spread your soil amendments over the soil as recommended by the soil test. In general, you should apply compost, peat moss and bark at a rate of 3 cubic meters, or a 4-inch-thick layer per 1,000 square feet of soil. These amendments will boost nutrients and aerate the soil to improve drainage. Do not apply fertilizer unless directed by the soil test. Many herbs are native to rocky areas with low fertility and are easy to overfertilize. Add lime or sulfur at the rate directed by the soil test. The amount will vary based on the soil's current pH, and the amount that you wish to change the pH.

Mix the amendments into the soil by turning them under with a rake or garden fork. Allow the amendments to sit for a minimum of six weeks before planting.