Adventures in Soil Testing

So I’ve been taking a class called “Soil Basics” at the Chicago Botanic Garden. The instructor is Ellen Phillips, who has many years experience as a soil scientist in the US and overseas. (She’s an excellent teacher, and I recommend the class for those of you in the area.) As part of the class we each brought in a soil sample to be tested.  My sample was from the raised flower bed along the path to the front door. I’ve often suspected that the soil in this bed is actually too rich, because plants tend to grow like gangbusters.

So here’s what I learned:

  • This flower bed has one heck of a lot of organic matter: 14.7%. Usually 5% is considered pretty good. Guess all that compost and mulch didn’t go to waste. Actually, though, turns out that soils with too much organic matter can be hard to rehydrate once they are thoroughly dried out. I was given an explanation for this that completely baffled me. Fortunately that problem has not yet occurred in this bed. My plan is to continue mulching, but no more compost for you!
  • The nutrient levels ranged from “Medium” (calcium) to “Very High” (phosphorous). It’s good to have enough nutrients, but too much of anything can cause runoff or even toxicity. Given the performance of plants in this bed, I don’t think anything is at a toxic level. I have read, however, that very high phosphorous can inhibit mycorrhizae (the critters that help roots absorb nutrients).

  • The testing company recommended that I apply three pounds of nitrogen per 1,000 square feet, even though the test can’t measure nitrogen. Isn’t that odd? My instructor explained that this recommendation was based on research showing that in general, comparable soils in this area need this much nitrogen. Okay … if you say so, but I think I’ll pass on adding more nitrogen.
  • No big surprise, but my soil is alkaline, with a pH of 7.5. The testing company recommends that I till in 10 lb. of sulphur per 1,000 square feet. But why should I, since most of the plants I’ve tried in this bed are perfectly happy with the pH and everything else. Seems like tilling in sulphur could mess up my perennials, so again I’ll take a pass.
  • The test indicated that the soil in this bed has a low cation exchange capacity (CEC). What is the CEC? Basically, it’s the ability to hold nutrients that take the form of positively charged ions. What is a positively charged ion? When you’re older you’ll understand. However, Ellen explained to me that the results of this test were probably skewed by the high level of organic matter, so not to worry.

What I conclude from this: Soil tests can be very useful, but don’t run off to the garden center to buy stuff based on the recommendations until you really understand what they mean.

15 Comments on “Adventures in Soil Testing”

  1. Thanks for the interesting post. I’m one of those feed-the-soil-not-the-plants types, and as long as the plants are happy, I am happy. That said, my veggie garden has *not* been happy in recent years – a simple soil test revealed not enough nitrogen, hence the addition of horse manure this year. From this year’s experience with overly peat-y raised beds, I believe the part about high organic matter being hard to rehydrate.

  2. I took a similar class and although it was very informative, was not very practical for me either. Our instructor delved into nutrient dense soil–adding percentages of greensand, magnesium, etc. to achieve more nutrients in the food. that was a little too much for me. Just give me the basics, please. That’s all I can handle!

  3. I never did a soil test until this year. I just assumed that by adding compost and rotted manure and a little greensand and rock phosphate from time to time, my soil couldn’t help being ‘good enough’. I was surprised by the results. I was told No More Nitrogen! But my pH was higher than I expected – no more lime for you! I also had a good percentage of organic matter, but not as much as you have. I do have a friend who had problems with her large garden because she had too much organic matter. The downside of living across the street from a horse farm. Generally, I learned that my soil management was pretty good which was nice, and I will watch the amounts of chicken manure I apply.

  4. This is what we do most of as Master Gardeners for clients and I have done hundreds of them. It is not my favorite part of the job either. What we find out is that most samples are very similar in a given area, so it is very repetitive. It is good to know this information and you should be glad you took the class.

  5. Good info here. Most of the soil in the county I live in is dark, rich silt loam. I’ve never had it tested, but the area is generally considered to have the best soil in the state. I do compost and mulch, but not excessively. I think my main problem is that I sometimes use the wrong mulches. I’ve had the best luck with marsh hay as mulch.

  6. There you go with your dirty talk again! The class and test sound interesting & I’ve thought about getting a soil test done but never have. I just assume things about my soil based on geographic location, large amount of rainfall, and digging around in the stuff.

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