Wayne Schmidt's Soil Testing Page Comparing seven different soil test kits and devices.



As an avid gardener the fertility of my soil is of upmost importance. To insure that it contains all the nutrients needed by the plants growing in it, the soil is constantly tested and the results used to select which amendments to till in. The problem is deciding if home test kits are accurate enough to be useful and if so, which is the best. To answer these questions I purchased seven test kits or fertility-measuring devices and put them through their paces.

To check the accuracy of each kit, its results were compared to the gold-standard for professional laboratory soil testing: the soil analysis test with micronutrient audit provided by Peaceful Valley Farm and Garden Supply.

A single large soil sample was collected and thoroughly mixed before being divided into individual samples for testing.

Here are the results:



Settling Test:

This self test is something anyone can do. Take a cup of soil and water, mix them in a glass and let everything settle for several hours. Measure the height of each main component (sand, silt, clay and organic matter) and divide each by the overall height of the settled column. The sample above is 63-percent sand, 27-percent silt, 4-percent clay and 3-percent organic matter. The following soil chart indicates that this is sandy loam soil, perfect for growing almost anything:

Compared to the Peaceful Valley report my estimate for the amount of organic matter was almost twice what they found. Upon checking my test I noticed that the organic matter tended to be drawn to the edge of the glass, creating the impression there was more of it than there actually was.



Rapitest PH/ Soil Fertility Meter:

This device has two metal prongs that are inserted into a soaking wet soil sample. The electric characteristics of the soil are displayed on the meter, which, by flipping a switch on the side, indicates the PH or overall fertility of the soil. In this case a PH of 6.6 and middle-low fertility. (I checked this meter against a buffer standard calibrated to a PH of 7.0 and got a reading of 6.8. This suggests that the meter reads low, making the 6.6 value actually closer to 6.8.)

The most attractive feature for this type of detector is that it tests a large sample of soil at one time. All the other tests, and I suspect even the Peaceful Valley system, use very small amounts of soil for each test. This means that even with carefully mixed soil, a very small sample may have an extra bit of something in it that throws the test off so it doesn't represent the average soil.

The device was easy to use and provided quick results with little or no mess. Its PH reading was 0.4 lower than the 7.0 results from Peaceful Valley when testing solutions close to 7.0. However, when I used it to test pure peat moss, which has a PH of 3.8, it only went down to 6.0. Therefore I doubt the one I purchased is any useful.

La Motte PH Test Papers:

These simple test strips indicate the soil PH is 6.5. In actual use the color boxes for 6.0 and 7.0 are very close and hard to differentiate. This is 0.5 lower than the Peaceful Valley result.

Oakton PH Meter:

I've used this tester for years. After making sure the batteries are fresh and calibrating it against a buffer reference solution, it provided a PH value of 7.0. This is an expensive tester and acts up from time to time. Its sensors also have a limited lifetime, usually just a few years. It agreed with the Peaceful Valley result.

Ferry Morse Soil Test Kit:

Simply add a sample of soil, some water and the test powder from one of the capsules and you can find out what are the PH, nitrogen, phosphorous and potassium levels. In my case: PH = 6.5 (none of the color boxes really matched the color of my sample so this is a best guess), N = very low, P = very low and K = very low.

The problem with this test kit is that the actual amount of soil used for each test is so small that it's impossible to get one that reliably represents the entire soil sample. Each test uses less than 0.1-teaspoon of material. This is such a small quality that you can almost count the individual grains making it up.

The results were significantly different than the Peaceful Valley report. The PH was 0.5 lower whereas the nitrogen, phosphorous and potassium values weren't even close. I'd rate this tester as a failure.

Minor problems with this kit were: The instructions were in such a small font that I needed close-up glasses to read them; The colors of the caps to two of the test tubes didn't match those on the comparison boxes; And it was very difficult to open the capsules and get all the test powder into the test tube.

Rapitest Soil Test Kit:

The colors and testing techniques were identical to the Ferry Morse kit. I suspect the both kits were made using the same materials. The test results were PH = 6.5, N = 0, P = 0, K = 0.

Like the previous test kit these results are so far off the mark as to make this tester a poor choice.

One very annoying thing about this test kit is that the rectangular test boxes tend to tip over backwards very easily.

Hanna Soil Test Kit:

This test kit used packets instead of capsules. They are easily to open but it's hard to completely empty them. I tried the PH test twice and both times failed to get a reaction. I suspect the reagent was faulty. The other tests were: N = trace, P = high, K = low.

The instructions at one point tell you to measure 0.5 ml of a sample but don't explain how to do it. I suppose they assume everyone has a 0.5 ml spoon in their kitchen drawer.

While it got the phosphorous right, it failed the PH, nitrogen and potassium tests so I'd rate this a poor soil test kit.

The last three test kits were all about the same in ease of use.

La Motte Soil Test Kit:

By far the largest, most elaborate, complicated and expensive test kit. It was also the most fun. The liquid reagents were much easier to use than the powders of the previous test kits. Laying everything out and working with this kit took me back to the days of high school chemistry lab. Great fun.

The basic process is to mix an extraction liquid with a soil sample, let it settle, pipe off a sample of the clear liquid and add a reagent to this clear liquid to get the results. In my case: PH = 8.0, N = trace, P = high, K = high.

The PH was 1.0 too high whereas the nitrogen was much lower than the Peaceful Valley result. The phosphorous and potassium values were correct. I'd rate this tester as a failure.

The Common Flaw Shared by all the Test Kits:

All these tests used tiny quantities of soil for testing. The problem is that these samples are so small that no matter how much the main sample is mixed up the small test samples taken from it aren't uniform. The expensive solution is to make the test samples larger and scale up the reagent accordingly. The problem then becomes that a single test would use up a lot of reagent.

Hand in hand with the small-sample problem is that the instructions never comment about whether the test samples should be packed down into the measuring spoon or left loose. This is a critical point because compacting would double the amount of soil in the sample.



Conclusions:

None of the test kits I purchased provided accurate results and I'm going to throw them all away.

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