Reading Soil Test Results

Since I’m gardening in a new yard, I sent a couple of soil samples to the U of M Soil Testing Lab earlier this summer. Most horticulturists recommend a soil test as the first step in planning a garden because it helps you decide what additions, if any, you need to make to the soil.

For years, I did not have soil tests, but instead relied on the mantra: When in doubt, add compost. But I was concerned about the soil in my new garden, so I got the tests done.

A Solid C

The results came back within two weeks and, if my soil were getting a grade, it would be a solid C, maybe a C-.  While not completely surprising, the tests show I have a lot of work to do to build organic matter and soil fertility. I had two different samples tested because I have two distinctly different garden areas in the backyard: one that had been planted with turf and hostas previously and one that had been the site of an old garage. We scraped off some of the garage-area soil and added 2 inches of black dirt, but clearly that was not enough.

The better of my two soil test results is pictured above. This is for the turf/perennial area. It shows a coarse texture—more sandy than clay. The soil has a pH of 7.3, which is not terrible for Minnesota, and an organic matter percentage of 6.7 percent. My previous garden had a similar pH, but organic matter of over 10 percent. To be considered “organic” soil, a garden should have 19 percent or better organic matter. These garden beds will be getting a layer of leaves and compost over the winter and next spring to improve the soil fertility.

As with my previous garden, the phosphorus levels are sky-high. This garden has 51 parts per million of phosphorus, compared to a “very high” level of 25 ppm. My previous garden, however, had over 100 ppm of phosphorus. The potassium level is 76 ppm, a medium score, compared to more than 300 ppm on some soil I was sold for raised beds—which I think is too much.

The two key pieces of information from a soil test are the organic matter percentage and the fertilizer recommendations. The U suggested that any fertilizer I add to these beds have a ratio of 7-0-10—so no phosphorus, but some nitrogen and some potash.

Sandy, Clay or What?

The soil on the site of the former garage was also labeled as coarse by the U, which shocked me considering how difficult it has been to dig in. If the soil is wet, it actually makes a sucking sound when you pull a shovel of dirt out of it. It does have a lot of rocks, so I decided to do the low-tech test to find out if your soil is clay or sand.

This is the test where you put a bunch of soil in a container, shake it up with water and let it settle. Because of the relative weight of the different types of soil (sand, silt and clay), the soil will settle out in layers, with the heaviest layer (sand) on the bottom and the lightest layer (clay) on top. For a better explanation of how this works and how to use a soil chart, check out this fine video.

It took forever for all the sand and silt and clay to settle out of my jar. The photo above was taken 48 hours after I started the test. Most of the videos/pictures I’ve seen show clear water on top and clear layers. My jar has some layers, but there is still a lot of soil floating around in the water.  The soil did not fully settle out until about a week later.  My best guess on composition is it is 50 percent sand, 20 percent silt and 20 percent clay. According to the soil chart (below), this kind of soil is considered loam or clay loam, which should be decent garden soil.

Perhaps the problem isn’t the rocky (inorganic) part of the soil, but its complete lack of organic matter. According to the U, the organic matter level was an abysmal 2.7 percent. This is the reason I’m growing vegetables in raised beds.

But here’s the funny thing about this potentiall atrocious soil–stuff is growing in it! I planted some cosmos and they seem to love it. I put in five plants of ‘Blue Heaven’ little bluestem and they’re happy as can be, as is a ‘Little Henry’ sweet black-eyed Susan plant that I got at a garden writers event earlier this summer, some Russian sage and even allium bulbs. Not everything likes that soil, of course, and three honeyberry plants that I thought might do well there, up and died in just a few weeks. Very sad.

My plan is to buy additional native and prairie perennials for the areas around the raised beds, which will be good for attracting beneficial insects. I’ll also add leaves and compost, but getting this soil to the “organic” level is going to be the work of many years.

Have you had a soil test on your garden? How does your soil measure up?

Growing Vegetables in Raised Beds (and What’s Going on With This Soil?)

One of my backyard gardens is on the site of a former garage. We removed the one-car garage last summer and replaced it with a larger building to store cars and tools. The site of the former garage is now where I grow vegetables in raised beds and am trying to grow some perennials, shrubs and vines on the extremely poor soil.

The tomato has a fruit, but it’s so small, you can tell its struggling. Photo take July 15.

Raised beds can be a terrific way to grow vegetables, but as I am finding out, your beds are only as good as the soil in them. Witness the photo at left. The bed this sad tomato was planted in was one of two that were filled with a soil mix that was labeled as being specifically for raised beds, including those with vegetables. I planted beans, parsley, tomatoes, squash and marigolds in the two beds—in late May/early June. For weeks, they have sat there. And sat there.

With the exception of marigolds, which seemed to be growing a tiny bit and are flowering, none of the plants were thriving—or even growing much. On many of them, the leaves turned yellow. Nothing has up and died yet, but they sure have been struggling. Witness the photo below, a small raised bed (with less sun than other raised beds) where I used an organic bagged soil mix and some manure. These tomatoes and basil plants were roughly the same size as the ones in the other beds when they were planted a few weeks ago, yet they are growing, producing flowers and fruit and generally doing what a plant should do. (Update: Since I wrote this post a week or so ago, these tomatoes have grown so much I’ve had to lash their trellis to the fence.)

Planted in a bagged soil mix, these tomatoes have grown so much I’ve had to tie them to the trellis. Photo taken July 15.

So what’s up? It could be the plants have gotten too much water, but given the size of the beds and the dryness of the top 3 inches of the soil, I doubt that. I checked out this article on what yellow leaves on plants means and my yellow leaves don’t perfectly match any of the pictures—though they are close on a couple of them.  A couple of weeks ago, in absolute frustration, I decided to add some more nitrogen to see if that helped. One bed got composted manure; the other got liquid fertilizer. The plants have grown more since then—one of the beans has finally latched onto the trellis I want it to climb and a few bean flowers have emerged.

I also sent a sample of the soil to the U of M Soil Testing Lab to find out exactly what kind of soil I’ve got here. (I contacted the landscaping firm that sold me the soil, but have not heard back from them.) The U turned around the soil test results quickly and I found out that while the mix had a good percentage of organic matter (12.5 percent), it had sky-high levels of potassium (that’s the K in the N-P-K ratio on most fertilizer bags.) Potassium’s main role in plant growth is to regulate how other nutrients are taken up by the plant and to regulate certain processes. Too much potassium in the soil will interfere with up-take of nutrients such as calcium and magnesium. The U recommended I add nitrogen but nothing else to the soil to improve plant growth.

One of the main reasons to grow vegetables in raised beds is that you can control the soil better. In my case, the dreadful soil that was already on the site made growing vegetables impossible without raised beds. If the beds are tall (mine are about 14 inches tall), they should be treated like a container, with regular watering and fertilizing to enrich the soil. Needless to say, come fall, I will be adding leaves, compost and manure to all my beds in hopes of getting the soil in better shape for next year.

Do you grow vegetables in raised beds? What’s your favorite soil mix?

Soil Test Results

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?