A Nifty DIY Soil Test

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“I don’t know what that gunk is in the jar, but can’t we get it off the kitchen table?”–This from my husband a few days ago.

“No!  That’s my soil test!”

The soil at our new house seemed sticky to me, and fearing that I would be dealing with clay for the next several years, I did a simple do-it-yourself test to determine what type of soil I would have in my new vegetable garden.

All soils are made up of tiny particles, each of which belongs to one of three categories:

  • Sand particles are the largest of the three.  Soils composed of mostly sand offer excellent drainage, but do a poor job of retaining water and nutrients.  They have to be watered often and fertilized carefully, lest the nutrients wash away.
  • Silt particles are smaller than grains of sand.  Silty soils are plant-friendly.  They make nutrients and water readily available to plant roots, but they drain freely and allow roots to “breathe”.  Loam, the stuff that gardeners dream about, is composed mostly of these medium-sized particles.
  • Clay particles are microscopic, and they bind together tightly.  Clay soils are rich in nutrients, but dense.  They are gooey when wet and as hard as a clay pot when dry.  They are no fun.

The test I did is described by Steve Solomon in both his Growing Vegetables West of the Cascades and Gardening When It Counts.  The goal is to separate the particles in your soil and determine the proportion of each.

box of dirt for soil test
Box of dirt–Check!

Per his instructions, I gathered two cups of soil, picked clean of roots and stones as best I could.  (There are tons of rocks in my soil, ranging in size from bb’s to cantaloupes.  The reason for this will be explained in a little bit.)

I put the dirt in a clean quart sauerkraut jar and marked the soil line with a piece of tape.  Then I added a squirt of dish soap (this helps the particles to separate), filled the jar with water up to one inch from the top, and screwed the lid on tightly.

If you are following along at home, now is when you shake the jar vigorously until your arms fall off, or for about ten minutes.

Set it down.  After two minutes, you’ll see how much sand has settled.  Mark that line with another piece of tape.  After two hours you’ll see how much silt has settled on top of the sand.  Mark that, too.  The remaining portion is clay.  It may take several days for the clay to settle completely and for the water to clear, but you already know how much clay is there, because you marked where the original soil line was.  All that is left is to measure the layers and calculate the sand, silt, and clay fractions of your soil.

Voila!
Voila!

It was a little difficult for me at first to see the distinction between sand and silt.  Turning the jar around and shining a flashlight on it made it more clear.  The sand had settled unevenly along the back side, and it clearly appeared more coarse than the silt.  The clay layer was quite obvious.  It looks like peanut butter:

sand silt and clay fraction of soil

I approximated the layer of silt to begin at 15mm, the clay began at 54mm, and the total soil sample rose to 62mm.  Hence, I have a lovely loamy (though rocky) soil, composed of 24% sand, 63% silt, and 13% clay.  Hooray!

After looking further into the geology of the region (Portland, Oregon), this makes perfect sense.  Thousands of years ago, the Missoula Floods, fueled by the melting glaciers of the last Ice Age, washed through the Willamette Valley, depositing gravel, silt, and tumbled boulders.  I have alluvial soil–soil that was once a riverbed.

I’m not looking forward to picking up wheelbarrows-full of Missoula Lake rocks.  But I think that once I do, my vegetable garden will be very happy here.

Anybody need some river rock?


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9 Responses

  1. linda
    linda at | | Reply

    Fantastic …I’ll try this out. Looks like you got some good soil to work with after all .

  2. Loree / danger garden
    Loree / danger garden at | | Reply

    I learned about this test years ago when I took the Master Gardener training course. I’ve never done it though…perhaps it’s time.

  3. google.com
    google.com at | | Reply

    Awesome article.

  4. Phillip
    Phillip at | | Reply

    Oh gosh, doing this right away. And yes, I can use river rock. Believe it or not, from what I little digging I hhttp://ave done, I have seen not a single rock.

  5. Laurie Beard
    Laurie Beard at | | Reply

    Is the % of sand, silt and clay you have the ideal mixture? Or what is the ratio of ideal soil? The soil around my yard was compacted with rock and some clay & soil about 4 years ago when they built my house, which is built down a couple levels into a forest floor. So it is not the original soil completely.

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