A proper sample is critical
Proper sampling technique is an important part of soil testing. Even in a modest-size garden of 1,000 square feet, 1 cup of soil—the amount typically used for a test—represents only about one one-thousandth of one percent of the top 6 inches of ground. So that 1-cup sample had better be representative of the whole area.
To get a truly representative sample, dig in a few random spots around the test area and mix the soils together. Avoid sampling any anomalous spots such as near a fence or where you fill your fertilizer spreader or once had a compost pile. If the test area itself seems insufficiently uniform because of, say, a large, wet, sunken portion, then divide the area into two or more separate test areas. Areas devoted to different kinds of plants, such as vegetables and lawn, require separate samples. Vegetable and flower gardens, though, may be sampled together.
Collect soil to a depth of 6 inches, which is approximately the depth of most plants’ feeder roots. Before you dig, remove any surface debris such as wood chips, compost, plant residues, or sod, then make a hole to the required depth. Discard the first shovelful of soil. It’s a cone-shaped slice, so it contains a greater proportion of soil from the surface than from lower down. Take another slice, uniformly thick from top to bottom along the edge of the hole you just made.
Throughout your sample preparation, avoid contamination from dirty hands or utensils. Gather together samples from each test area into a clean, plastic bucket, then mix and crumble them, discarding stones, sticks, insects, and other debris as you mix. Spread the soil on a clean baking pan to air-dry for a day, then remove about a cup for testing.
If you are sending your samples to a lab for testing, you will get a recommendation for fertilizer and for amounts of lime or sulfur needed to adjust the pH level. Fertilizer recommendations are based on what is in the soil and the kinds of plants you intend to grow. Follow these guidelines closely because too much of any nutrient can be as harmful as too little, causing nutrient imbalances, even death, to plants.