DIY Soil Test

I’m pretty new to this whole gardening thing, but I already knowthat there are different kinds of soil and they get treated differently by gardeners depending on their type. In my new gardening Bible, Growing Vegetables West of the Cascades, the author describes a very simple test that one can perform to determine what kind of soil you’ve got. I did this test yesterday and was surprised, and pleased, by the results.

Our first opportunity to take a good look at the soil around us was when we cleared land for our new pasture and had to rake it before seeding. I noticed variations in the composition of the soil as I moved around the area. While we are overloaded with rocks (thanks to the glaciers that shaped this land during the last ice age) I could still tell that some parts were thicker and softer than others, and some areas had way more insect life than others lurking under the surface. When we were looking for temporary spots for growing Husband’s heritage wheat and corn test plots, as well as a place to put the heirloom tomato transplants we’d picked up on a whim from a local organic farm, we chose sections of the new field that we knew had thicker, richer soil. However, it still didn’t look like the dark, rich loam that you get out of bags from the garden centre, and I wondered exactly what we had here.

Now that I’ve decided where my garden plot will be next year I’ve been reading more seriously in preparation for digging up my beds this fall. I had become quite convinced that we had clay soil because I’d seen it clump up, I’d seen it retaining water and puddling, and I’d seen it form a crust over the surface. Just goes to show you that a little knowledge can be a dangerous thing, or at least send you off in the wrong direction. Thankfully, I am an optimist and wanted to make doubly sure that I wasn’t going to have to deal with this, the most troublesome of soils. So I did the soil test.

You take about a half-litre of dry soil, remove as much organic matter as you can (leaves, twigs, etc) and rocks/pebbles, then crush it up as finely as you can. You put the dry soil in a 1-litre glass jar and mark a line on the jar where the top of the soil is. You then fill up the jar with water to about 1/2 inch below the top, add a few drops of low-sudsing dish detergent (natural or eco-friendly brands are low-sudsing), put the lid on and shake vigorously until it’s thoroughly mixed up. Put down the jar and exactly two minutes later mark a line where the first bits have settled out. Exactly two hours later mark another such line, and then two days later mark the final line. These will represent the fractions of sand, silt, and clay in your soil. The proportions of each together will allow you to identify your soil.

As you can see from the photo at the top, our sand fraction (line 1) is about 1/3 of the total soil volume (top line) and silt (line 2) makes up about 1/2 the volume. The clay still hasn’t settled out although it won’t be two days until tomorrow. Still, based on what has settled out over the last 24 hours I’m guessing it will take a lot longer (Solomon says that it could take up to a month for that fraction to settle if it’s very fine). Nevertheless, I already have enough information to go on.

I wasn’t too sure about the sand part because the water was so murky it was hard to tell what had settled and what hadn’t. I just gently wiggled the jar and drew the line between the part that swished with the movement and the part that didn’t. How’s that for scientific? You can see a line of demarcation between lines 1 and 2 in the photo, and I wondered if maybe I’d marked line 1 incorrectly. However, even if the true location of line 1 is where that demarcation is it doesn’t change the results that much. Sandy soils are defined as being more than 70% sand (according to Solomon) and that demarcation is around 50%. The good news is that the clay fraction is definitely less than 1/3 of the total soil volume, which means we don’t have Clay Soil.

What we have is Loam, classified as soils with mostly sand and silt and less than 1/3 clay. Ours trends towards Silty Soil due to the large silt fraction relative to sand, although if I’m mistaken about the true location of line 1 then it trends towards loam or sandy loam. Either way we are dealing with good news. When I read the section on Fine Loam and Silt Soil (page 66 if you have the book) the description of these soils fit ours to a tee which further convinced me of the validity of the results: soils with greater density due to the clay being fine, with smaller pore spaces and thus slower drainage; they also tend to form surface crusts and contain enough clay to form clods easily if tilled up when wet. The good news is they tend to be more fertile than other types of soil. The challenges with this type of soil is the surface crust which can be managed by adding thin layers of compost and only working it in the top layer (my new colinear Rogue Hoe, which I will write about soon, makes this a breeze, not to mention every time I weed with it I will be breaking up that surface crust) and by regular green manuring which I plan to do anyway (this means planting a cover crop any time the bed it not in use and tilling/mulching it into the soil when done).

While my gardening book is specific to the Pacific Northwest region of North America this soil test can be done by anybody and his explanations of the different soil types and their challenges would also apply. I also found this handy diagram for determining soil type based on your fractions:

When you use this diagram don’t make the mistake I made and eyeball your lines vertically (I’m used to reading graphs with vertical axes; note that the left and right axes on this diagram are angled). Follow the angled lines. So for example, if I was correct with my soil test and I have about 30% sand, 50% silt and 20% clay then I can start at the 30% mark on the bottom axis (sand) but as I follow it upwards I am moving to the left. I stop where it intersects with the line that angles to the right and ends at the 50 on the right axis (silt). I end up right on the border between Loam and Silt Loam. On the other hand, if I was inaccurate and I actually have close to 50% sand and 30% silt I end up in the right corner of the Loam section. Either way I have loam, not clay, and that makes me a happy camper!

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One response to this post.

  1. Whoa, brings back memories of Soil Lab in college. I learned all this stuff way back when but of course have not retained the details. If our soil ever dried out, I’ll do the test although I already know we’ve got lots of sand. Unfortunately, a bit of our soil got swept away in a flood yesterday as we got 2 1/2″ of rain fairly quickly.

    Hope you have fun gardening in your new soil!

    Reply

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