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?

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?

 

 

Garden Science: Nitrogen Fixation

IMG_6528Back in the day, I worked for a large agricultural supply cooperative in Minnesota, which sold seed to farmers. To say that the co-op’s seed guys were fixated on nitrogen fixation would be an understatement. Nitrogen fixing is the process by which certain legumes take nitrogen from the air (N2) and, with the help of bacteria, transform it to nitrogen plants can use (NH3). Farmers love this beneficial habit because they can plant corn (a huge nitrogen user) after soybeans (a nitrogen fixer) and not have to add as much fertilizer. The co-op’s seed guys had a special soybean seed that was a nitrogen fixing machine — but that’s another story.

Recently, a fellow gardener brought up the issue of whether green beans in Minnesota were in the ground long enough to fix nitrogen. I have always assumed so — but his comment got me wondering. This weekend I pulled up one of my bush bean plants to check and there they were — those warty little nobs on the root of the bean that signal nitrogen fixation. How much nitrogen is fixed depends on the soil, the bean, and a number of other factors that are beyond my pay grade. But it is a good idea to rotate beans into garden beds in which the soil may have been depleted by heavy nitrogen using plants, such as tomatoes. It’s also a good idea to simply cut the beat plant down to soil level at the end of the season, leaving the warty, nitrogen-rich roots in the soil to decompose.

Nitrogen fixing is one of the reasons many gardeners will plant a cover crop of legumes on new garden beds. The University of Minnesota has a good article about nitrogen fixation and another one on cover crops that you might want to read for additional information.

Dirt Matters

For a visual reminder of why soil matters so much to the health and vigor of plants, see the two photos below. These are identical sunflower plants. I planted them on the same day late in the spring. The plants are located less than 20 feet apart in identical full-sun exposure. Why are some of the sunflowers only waist-high, while others are over my head? One word: Dirt.

IMG_6500The short sunflowers are planted in the little meadow between our yard and the path around the city storm-water drainage ponds. I’ve never improved that soil — it’s just what the city and the builder left behind after building our house. It is, as soil scientists might say, lean.

IMG_6505The tall sunflowers were planted on the edge of my vegetable garden. A few years ago, a load of landscaper’s black dirt was dumped there for use in another garden. I didn’t need all the dirt I ordered, so I left it there. This spring, I installed a lasagna garden for planting tomatoes, a rich mix of dirt and compost. The soil near that bed is very close to garden perfection: well-drained, humus-filled soil.

Not every plant thrives in rich soil — nasturtiums, sedum, and many herbs prefer a slightly lean soil — however, if you’re plants aren’t as tall or robust as you would like, consider giving your dirt a boost.

Lasagna Garden, Half-Baked

Lasagna gardening is a no-till method of starting a garden — usually one for vegetables — that produces humus-rich soil, the ideal environment for “heavy feeders” such as tomatoes. The basic idea is that you layer materials that normally would go into compost to create your garden bed. Like lasagna, the garden has several types of layers and, after it cooks, the layers shrink and blend into each other.

IMG_4793
Bottom layer: Wet cardboard
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Rough compost stuff on bottom

For my newest raised bed, I decided to try the lasagna gardening method. In a perfect world, the bed would have been installed and filled last fall, so it would have all winter to sit and percolate. Since that didn’t happen, I added some black dirt and plenty of finished compost to the lasagna.  I followed the instructions from the organic gardening columnist at About.com. I mowed the area where the bed was going to go — it’s near a meadow at the rear of my yard — then set the raised bed in place. Next, I covered the inside of the bed with brown cardboard and wet it thoroughly. On top of that, I added a bunch of partially finished compost from my pile, then a layer of finished compost from the city compost pile, then a layer of leaves, spent perennials, and other material I collected during garden clean-up this spring. In between each layer, I hosed it down more. So far, I have not spent a cent.

First layer of finished compost
First layer of finished compost

Because I will be planting this bed shortly, I added a few bags of garden soil. This cost about $15. On top of that, I added another few inches of compost from the city compost pile. I built the bed off-and-on over two weeks starting in mid-April and have been letting it sit since then. Yesterday, I added some marigold starts around the edge of the bed for color and to encourage good pollinators to frequent my garden. Around June 1, I will plant my tomatoes in the bed. At the end of this season, I’ll add some additional layers to continue to build the soil in the bed.

Will tomatoes grow as well here as they have in my regular garden beds? I’ll let you know.

When the Question is Soil, the Answer is Compost

Last week, a reader commented on the post on Best Bets for Beginning Vegetable Gardeners that she wanted to start a garden but her lot was very sandy. This reader lives in Sherburne County, just north of the Twin Cities, which is known for its sandy soils. I faced a similar dilemma when we first moved to our current home, except that the underlying soil was clay rather than sand.

Whether your soil is sandy, clay, or essentially barren (as some new home sites are), the answer is to add compost and other organic matter. Compost improves soil in several ways.  It adds nutrients and is a near perfect slow-release fertilizer.  It also is filled with micro-nutrients which means that compost is better than synthetic fertilizer in the same way that it’s better to eat salmon than take fish oil pills or to eat a lot of vegetables rather than take vitamins.

Compost also improves soil structure. So, if your soil is sandy, like those in Sherburne County, it helps the soil retain water better. If it’s clay, it loosens the soil, helping water flow through more efficiently, creating passageways for plant roots. Finally, compost and other organic matter create the right environment for all of the bacteria and organisms that make your soil a self-regulating eco-system. In other words, your garden will be less susceptible to diseases.

In the meantime, use raise beds

If you have less than ideal soil and want to grow vegetables, build a raised bed. These are very easy to build and then you can add your own soil. I usually use part black dirt from my local landscaper and part compost. This creates a pretty rich mix — good for tomatoes and other heavy feeders.

Credit where credit is due

I’ve been thinking more about soil lately in part because of two columnists in Northern Gardener — Don Engebretson and Bud Markhart — who have written a great deal about soil. If you go to Don’s web site, click on “Care” on the front page, and in the first section of the Care page you will see an article called Soil Sacrifice, which gives much more detail on all the bad kinds of soil people can have and what to do about them, usually add compost.  (I would link right to it, but Don’s site doesn’t allow that.) Bud Markhart is a U of M professor and our new Sustainable Gardener columnist, whose recent writings have given me a whole new appreciation for all the bugs, worms and wigglers that live in the soil.