Almost every garden book starts with the admonition to “get a soil test.” I hate to admit it but in nearly two decades of active gardening, I have never had one — until this summer.
Some of my vegetable garden boxes have not been performing as well as I thought they should be over the past couple of years. When one area of the garden looks bad and the rest look OK (or better than OK), then soil may well be the problem. I had a coupon for $2 off the standard University of Minnesota soil test courtesy of the Hennepin County Master Gardeners’ Learning Tour, which I went on in July. So, I got out my trowel and collected samples of the soil from a couple of places in the boxes, and took it down to the soil test office at the U’s St. Paul campus. Within a week, I got the results back in the mail.
The results were both surprising and not. In the “not surprising” category, I found out that my soil is a bit alkaline. It has a pH of 7.1, which is slightly high. The ideal pH for growing vegetable is 6.0 to 6.5, some say 7.0. It might be hard to lower the pH much because the water in our area is very alkaline (like 7.5 to 8.0) and that’s the water I use on the garden. Plants generally grow well up to a pH of 7.5, so I likely won’t try to adjust this much. I may see if I can find some more acid mulches (such as pine needles) and use those in the vegetable garden.
Also “not surprising” is that the soil has adequate levels of nitrogen and a high percentage of organic matter — 10.5 percent, which is pretty good though lower than the 19 percent required to have “organic soil.” My potassium levels are in the normal range at 158 parts per million.
What struck me as surprising was the extremely high levels of phosphorous in the soil. The report did not list an exact number but my soil has more than 100 parts per million of phosphorous. A “very high” reading is 25 parts per million. What does that mean? Well, according to this university article, it may mean the composts and manures that I have added to the garden were high in phosphorous. I do use a lot of compost and it generally comes from my own yard or the city compost pile. I’ve also added aged chicken manure to this garden in the past. This University of Wisconsin article on soil tests says that high phosphorous readings are not uncommon in urban soils and that it’s best to avoid “balanced” fertilizers, which most organic fertilizers are.
The U of M recommended that I use a fertilizer with no phosphorous and more nitrogen than potassium. (The exact ratio recommended for me was 30-0-20—that’s nitrogen, phosphorous, potassium or NPK—for you fertilizer geeks.) I’ve done a bit of online searching and most fertilizers with that rating are commercial fertilizers designed for golf courses or other turf-heavy spots. I’ll be looking over the winter for some low-phosphorous options, preferably organic.
Phosphorous is not bad per se. It’s vital for root growth, for instance, but too much phosphorous can promote weed growth (yep!) and lead to stunted plants. Apparently too much phosphorous can also affect plants’ abilities to take in zinc and calcium, which are essential nutrients for vegetable crops.
My plan was to spread a lot of leaf compost that I made this summer over the vegetable gardens this fall. I’ll be doing some more research to see if that is still a good idea. I’ll also be taking soil samples from some of my other garden beds. Knowledge is power, as they say, and the more you know about your garden, the better you can tend it.
Have you ever had a soil test?