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.