Soil Test Contamination

Tom McCutcheon
Monongalia County Extension Agent
WVU Extension Service
10/1998

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

# Substance pH P K Ca Mg
1 Corn - 1 tsp. crushed cracked corn 6.7 71 367 1700 218
2 5-10-10 - 1 tsp. 5-10-10 crushed 7 120 2752 4350 313
3 10-20-20 - 1 tsp. 10-20-20 dust 6.9 120 1156 1950 244
4 Coffee - 1 tsp. crushed coffee 6.7 68 480 1720 178
5 Dog food - 1 tsp. crushed dog food 7.4 79 413 2086 181
6 Tobacco - 1 tsp. smokeless tobacco 6.4 71 625 2320 238
7 Wood ash - 1 tsp. wood ash 8.2 91 1553 6730 414
8 Oil - 1 tsp. 30 SAE oil 6.9 67 276 1810 174
9 Worm castings - 1 tsp. castings 6.8 75 338 2550 177
10 Lime - 1 tsp. ag. lime 7.5 82 291 4950 1382
11 Soluble fertilizer - 1 tsp. dilute 1 tsp./gallon 6.7 71 302 1900 169
12 Iron chelate - 1 tsp. iron chelate 5.5 0 295 1740 162
13 Vegetable juice - 1 tsp. multi-vegetable juice 6.5 73 362 1840 170
14 Dried blood - 1 tsp. dried blood 12-0-0 7.6 82 367 1720 159
15 Swine pellets - 1 tsp. crushed pellets 6.9 75 410 1810 204
16 Milk - 1 tsp. 2% milk 6.1 67 290 1660 159
17 Bread - 1 tsp. bread crumbs 6.2 66 297 1820 169
18 Soap - 1 tsp. dilute 1 tsp./2cups water 6.3 69 279 1730 164
19 Tomato juice - 1 tsp. tomato liquid 6.6 75 366 1850 172
20 Urine - 1 tsp. urine 5.2 67 290 1750 173
21 Manure - 1 tsp. shredded cow manure 6.5 73 325 1940 184
22 Sulfur - 1 tsp. sulfur 6 73 278 1830 154
23 Heated stove - 20 min. at 500oF 6 77 236 1830 140
24 Microwaved - 5 min. on high 6.6 71 294 1870 173


Controls were very similar showing almost no difference in the test results.

25 Control - Nothing added 6.4 69 264 1660 150
26 Control - Nothing added 6.4 71 278 1690 155
27 Control - Nothing added 6.4 68 282 1750 161
28 Control - Nothing added 6.4 69 244 1650 146
29 Control - Nothing added 6.4 69 287 1710 167


Statistics of Control Samples

avg. 6.4 69 271 1692 156
std 0 0.98 15.52 36.00 7.52
+/- CI n/a 1.22 19.26 44.69 9.34
upper confidence limit 6.4 70 290 1737 165
lower confidence limit 6.4 68 252 1647 146

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.