If the WVU Extension Service was a religion, soil testing would be a foundational doctrine, and I, as a zealous priest, would include soil testing in the answer to every question. As I tirelessly preach the good news of soil testing to lost farmers and gardeners I realize that some of what I say is based on faith and not on my knowledge of facts. Since the WVU Extension Service isn't a religion and most people who ask me questions prefer that the answers I give are facts and not faith, I took a closer look at soil testing.
One of the things that I and all Extension agents explain in detail is how to correctly take a soil sample for a soil test. With much trepidation we warn of contaminating the sample with substances that will change the results of test and cause the lime and fertilizer recommendation to be incorrect. This could lead to wasting money by over applying lime and fertilizer or causing less than optimum plant growth due to incorrect pH and nutrient levels. To avoid contamination, we recommend using a clean container when collecting a soil sample.
How much of a problem is contamination? Last winter I surveyed 30 farmers and gardeners concerning what type of container they use when sampling soil for testing. One-third said they used plastic bags and another third used plastic buckets. The rest used bread bags, metal buckets, milk jugs, coffee cans, a glass jar, a box, and anything clean. They were also asked what other uses they had for the containers. Most answered that soil sampling was the only use, but some listed using them for carrying ashes, feed, nails, lunch, and other items.
Contamination could come from bread crumbs, ash, feed or anything left in an unclean container. One could come up with many possible contaminates from a variety of different scenarios, and that's what I did.
I took a couple shovels of soil from one location and divided it into 29 samples, one cup of soil per sample. Then I randomly took 5 samples as my controls (no contaminant added) and added one teaspoon of a different contaminant to each of the rest. The contaminants I use were everything from dog food to fertilizer. The results follow.
Soil Test Contamination
Note the dramatic effect wood ashes had on the results. The control pH was 6.4 and the sample contaminated with wood ash had a pH of 8.2 It also had a much higher phosphous and potassium levels. I didn't do this to show the specific changes each contaminant made, but to show the necessity of properly taking a soil sample by using a clean container.
One other note: When taking a soil sample, it's recommended not to artificially dry a sample, but to allow it to air dry. I went ahead and tested this, too. One sample was dried in an oven for 20 minutes at 500o F and another sample was dried in a microwave for 5 minutes. Both samples had test results different than the controls.