Garden soil can easily become depleted of plant nutrients unless you pay attention to its health. At least once a year, successful gardeners do a soil test for pH (acid-alkaline balance) and plant nutrients and add the recommended amendments. Fall is a good time to do this.
Soil test kits, sold at most garden centers and in mail order catalogs, can give accurate results if they are correctly designed and carefully used. The most reliable and consistent results, however, are obtained from a soil testing laboratory. Call your local County Cooperative Extension agent, listed in the phone book, to locate a laboratory in your area.
Test Each Garden
If you have a single garden, you will need to take only one sample. Additional samples should be taken from any separate garden areas. Let's say you have a vegetable garden at the end of your backyard, a flower garden in the center of the lawn, and a border with shrubs and other perennials in your front yard. Because the soils can be quite different on those areas, and the nutritional needs of the plants vary, you should plan on doing three tests.
Collecting a Soil Sample
The soil to be tested can be dry or damp, but should not be wet. You will need a clean plastic bucket, a garden trowel (not brass, bronze or galvanized), and a small zip-lock bag to contain the soil sample. Some soil test labs provide a bag or container and instructions.
Take a narrow slice of soil, from the surface to about 5 inches deep, and place it in the bucket. Depending on the size of your garden, take from six to 12 samples at intervals across the garden, putting each one in the bucket.
Mix the samples well so that the top surface layer is completely combined with the lower layers, and all the samples are thoroughly mixed. From this mixture, scoop about 1 cupful and place it in the plastic bag.
Reading the Results
A good soil testing laboratory will provide a report that lists the plant-available nutrients found in the sample, along with the pH reading and suggested amendments to correct any imbalances or deficiencies. If you are unsure about how to read the results, call your Cooperative Extension service for advice.
The Importance of pH
The acid-to-alkaline pH scale runs from 0 (acid) to 14 (alkaline). Pure water is said to be neutral at pH 7 on the scale. Most plants like a soil that is slightly on the acid side of neutral--about 6.5 to 6.8. (A few, such as potatoes, strawberries, hollies, rhododendrons and azaleas, prefer acidic soils in the 5.0 to 5.5 range.) Adjusting the pH of your soil is extremely important, because if the pH is wrong plants may not be able to take up nutrients from the soil, even if those nutrients are there in abundance.
Along with the soil amendments recommended by the soil test lab, add about 2 inches of good-quality compost all over the garden. This helps to retain moisture and nutrients, and increases the organic vegetative content of the soil, which benefits plant roots by supporting soil life. Lightly combine the compost and the soil amendments, mixing them into the top 4 or 5 inches of soil. Do not dig them in deeper, because then they will be beyond the reach of most plant roots.