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?

Composting on a Grand Scale

See the pile of compost in the image at left? That pile represents all of the food waste from St. Olaf College from a year. Amazing.

I was one of a dozen or so University of Minnesota-Extension Master Gardeners from Rice County who had a chance to tour St. Olaf’s composting facility, its student farm, and its food-service operations last week. We were led by St. Olaf Facilities Director Pete Sandberg and Peter Abrahamson, general manager of the food service and former executive chef at the college.

With about 2,650 students plus faculty, staff and visitors (St. Olaf has a well-deserved reputation as one of the best places in Northfield to eat), the school serves about 32,000 meals a week and generates 700 pounds a day of food waste, including meat, dairy, napkins and other compostibles.  The system that St. Olaf uses to take leftover pizza, hamburger buns and banana peels and turn them into garden gold is efficient, high tech and helps the college in its efforts to reduce its impact on the environment.

OK, not the prettiest picture ever put on this blog, but this is what the food waste looks like as it goes to the composter.

When students return their trays to the college dishwashing area, all the compostibles are collected and ground to a pulp. The liquid is taken out of the waste and every day St. Olaf facilities workers take the pulp out to the compost facility, which is located on St. Olaf land near the Northfield Hospital.

As Pete Sandberg unlocks the door, Peter Abrahamson explains the food to compost cycle. That pile of wood chips provides dry, brown matter to compost with the food waste.

At the facility, the food waste (a green for composting purposes) is mixed with wood chips (a brown), and over the course of 14 to 21 days, it moves slowly through a large composting machine. The interior of the composting facility has what Sandberg called “a barn smell,” but other than that the process is relatively odor-free.

Not quite done. Pete Sandberg shows the Master Gardeners the inside of the composting machine.

After its time in the composter, the mostly finished compost is moved outside to piles. It’s still very hot inside and the composting process continues in these piles, where the compost will sit for several months to a year as it finishes cooking. The facilities workers turn the compost regularly using backhoes to keep the piles going.

Once it is finished cooking, the compost is used all over St. Olaf’s campus — in the flower beds and containers, as well as on the St. Olaf student farm, STOGROW. Bon Apetit, the food services provider at the college, buys all of the vegetables produced at the student farm and uses them in meals during the growing season.

Tomato seedlings at the STGROW farm.

St. Olaf has received national attention for its efforts to reduce its impact on the environment. The school’s composting, farming, and food service programs allow it to “close the circle,” as Abrahamson said, using waste from food to grow more food. While these kinds of systems are not available to home gardeners, increasingly institutions are taking composting seriously and finding better ways to make compost on a grand scale.

For more information on St. Olaf’s composting efforts, check out this video that the college’s PR department put together.


Compost Tips from a Pro

While the compost piles here in Minnesota are frozen solid and buried under a couple of feet of snow now, it’s fun to imagine the time when they will begin churning and decomposing and creating delightful humus for the garden. And, after the Master Gardener class on soils and composting, held a week or so ago, I have a better idea of how compost works and how to get compost quicker. University of Minnesota professor Carl Rosen led the class, which covered everything from what soil tests measure to how to improve your soil’s fertility: basically, add compost.

Here are six tips from Rosen on making compost.

  1. The optimum size for a compost pile is 4 feet by 4 feet by 4 feet. At those dimensions, the compost will heat up quicker and decompose faster. That said, compost happens, almost no matter what you do. If your pile is smaller, it will take longer for the leaves, grass clippings, and vegetable waste in the pile to decompose (think years, not months), but it will decompose eventually.
  2. Compost starter is not necessary. Most compost starters provide microbes and nitrogen to get the material you are composting going. While microorganisms and nitrogen are needed for compost to get started, you can add a couple of shovels of garden soil and maybe a handful of fertilizer to get the same benefits at a much lower cost.
  3. Compost piles like shade. While you can make compost anywhere, Rosen likes shady locations because they protect the pile from drying winds and sunlight, which also dries the compost out and slows the process. Another factor to consider in placing compost bins are the bins’ impact on neighbors—compost that stinks (it shouldn’t) or obstructs a neighbor’s view probably isn’t going to make you the most popular person on the block. Consideration is always a virtue.
  4. Add air. Most gardeners know that to make compost you need brown materials (dried leaves, etc.), green materials (plant debris, grass, vegetable peelings), and moisture. But you also need air. You can add air by turning your pile or by adding bulkier items. If you want to add air with bulky materials, Rosen recommends wood chips, which keep air in the pile because of the spaces around the chips. The only disadvantage of wood chips is that you will need to sift your compost through a screen to get them out.
  5. Is it done yet? When your compost pile is finished, it should be about half the original volume of the pile and have an earthy smell. A well-managed compost pile will be ready in 4 to 9 months. A poorly managed pile will take 1 to 3 years, according to Rosen.
  6. Gardens need compost! While not exactly news, it’s important to recognize how many ways compost benefits gardens. To improve fertility and tilth, add 1 to 2 inches to the top of the soil and work it in 6 to 8 inches, if possible. Compost also makes a great mulch — you need 2 to 4 inches to suppress weeds — and it’s a wonderful amendment to potting soil for containers. Make compost 30 percent of the volume of soil in your containers for healthy plantings.

One other note from Rosen: Compost piles in frigid climates (like Minnesota’s) are dormant in winter. Another reason to hope for spring.

Adding Layers to the Lasagna Garden

Ready for winter

Last spring, I added a new raised bed to my vegetable area and filled it using the lasagna method. Despite not having a winter to percolate and meditate and otherwise breakdown, the soil in the garden was humus-rich and fertile. A trowel would easily sink 10 inches into the “dirt” in this bed. I grew some large (if slightly out of control) tomato plants in the bed, and I have a freezer full of the tomato sauce I made with the fruits. So, all in all, a success.

When I cleaned the bed up about two weeks ago, it was clear the lasagna had shrunk. This is to be expected. Lasagna gardening basically involves making compost in your vegetable bed. So, over the past few days, I’ve been layering on the fresh materials: slabs of sod taken out of the lawn, vegetable scraps, a thick layer of finished compost from my two compost piles, and on top, a layer of chopped leaves.  These will have several months to breakdown and renew the bed. I’m not sure what I will plant there next year, but it’s a long winter here, so I’ll have plenty of time to think about it.

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.

Bottom layer: Wet cardboard
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 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.