Vegetable Garden: Understanding Soil

Day 9 of Garden Tribes’ Boot Camp for novice gardens. Today it’s all about soil–what it is, what it isn’t, and how you make it work for you and your harvest. Before going any further you should know that if you are in a newly constructed house, the “material” surrounding the house may bear little resemblance to soil. Between the time the old house (or whatever) was removed and you moved into the new house, most of the site was probably changed significantly. You may even have the joy of finding MaDonald’s wrappers when you stick a shovel in the ground, or pieces of concrete, or great rocks that had to be either buried or disposed of some other way. So what is said about “soil” here may not actually relate to your situation. I suggest you dig a few good big holes in various not-too-conspicuous places to determine what is down there.

As the expression goes, “Feed the soil, not the plants”.

  1. Soil is a complex structure of organic and inorganic things. The quality of soil is dependent on the size of the particles, the amount of air space around the particles, and the percentage of organic matter mixed in among the particles.
    1. Clay is the smallest particle of soil. The more clay, the denser the product, the less air space, the wetter, the heavier it is. But clay is wonderfully full of all the macro- and micro-nutrients and trace minerals needed for most plant growth, and is generally slightly alkaline. More on pH later.
    2. Sandy soil is the opposite: large particles, it holds on to little nutrient, but drains very well. Sandy soil is considered “light”–because it’s not heavy, since it doesn’t hold much water.
    3. Silt is mid-sized particle, easier to work with, but still doesn’t hold many nutrients.
    4. Every ground space has some combination of the above three materials. And once some organic matter is added you have loam–either clay loam, silty loam, or sandy loam.
  2. In coastal BC because of our rainy climate, some of the nutrients that contribute to the pH of soil get washed out of the reach of plant roots. So in general we have acidic soil here, which is why rhodos, azaleas, heather, oregon grape, and huckleberry do so well and are found in almost every garden. If you have any of these, and they thrive, you can be fairly confident your pH is acid. That’s not a bad thing (don’t think sulphuric acid here), in that more nutrients are available to plants in an acid environment than in an alkaline environment. So acid is better than alkaline. And if your pH is really low, then you can grow better blueberries than I can!
    • Changing the pH of soil is pretty much a fool’s job. It takes several years of continual application of either a sulphur product (to acidify) or lime product (to alkalinize). Better to just appreciate what you have, and it you want to grow something that prefers a different pH, go with containers/raised beds. This is how many vegetables do well in acidic soil.
  3. The key to the good soil that you want to grow your plants in, is organic matter. Material that was once alive, when added to the soil, creates food for all the soil-based organisms that will increase the fertility of the soil. It’s those organisms that take the organic matter (last year’s banana peels and coffee grounds, or the sheep or mushroom manure, or Sea Soil…) and work on it to release the macronutrients (Nitrogen-Phosphorus-Potassium–NPK) and micro-nutrients (Mg, Ca, Fe, etc) that your plants need to grow and fruit.
  4. So the last thing you want to do is destroy the community of organisms that are doing the work of providing nutrition to your plants. You want to maintain the soil structure–not too much vigorous tilling, and certainly not every year. You want to avoid synthetic fast release fertilizers which can act almost like antibiotics against your beneficial bugs. And you want to continue to nurture them by giving them a supply of organic material ever year.

Here’s a crappy little video on feeling the quality of soil, and the post that goes with it: the handful of soil sort of sticks together, so it has a little clay, is obviously not that heavy, and not very wet, and doesn’t have much organic matter–it looks pretty grey.

A Primer On Soil Characteristics–Part 1

What’s your soil like?

Here are some of the possibilities:

Clay-ish: rich in nutrients, but very fine particles, so it clumps together so much that there’s no air spaces between particles. Doesn’t drain very well.

Sandy-ish: almost no nutrients, but very large particles, so drains very well. So well in fact it’s hard to keep plants hydrated.

Loamy-ish: this is your preferred quality, a combination of both clay and sand, so enough fine particles to hold onto water, and enough course particles to let it go.

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Notice the happy little worm in the middle of the pic enjoying the combination of loam and organic matter.

(All pictures are enlargeable by clicking on them.)

I’ve posted two short videos here to demonstrate nice loamy soil that sticks together weakly, and slightly too sandy soil that I bought as garden soil amendment.

Then there’s organic matter.

Organic matter bulks up soil making it “fluffy”. It is also what attracts earthworms and microorganisms, both of which do the real work of making soil healthy. Earthworms eat small portions of food and plant material, leaving behind “castings”–earthworm poo, aka “black gold” (not to be confused with “black gold–Texas Tea”). And bacteria and fungi are able to take molecules of nitrogen that are unavailable to plants and convert it to ammonium and nitrate that is available to plants.

Optimal soil is loamy and rich in organic matter. But really, we have what we have, and trying to change it can be an exercise in major frustration!

Wendy tells me she has clay soil.

Her backyard is flat, at the bottom of a hill, and in the winter the turf is VERY soggy. In the summer it’s so dry it’s like straw; she figures that because there isn’t enough depth of topsoil on top of the clay so it’s difficult for the grass to put down deep roots.

Really sounds like she’s got it right. Unless you’ve got at least 6″ of topsoil, preferably 8″ of topsoil, grass will not do well. There just isn’t enough depth for a reservoir of moisture and nutrients deeper than the roots, so they don’t reach downward. And if underneath that not-quite-deep-enough layer of topsoil is predominately clay, the clay creates a barrier underneath the layer of topsoil allowing a pool of water to collect when it’s wet. Roots of plants that aren’t really “bog-tolerant” will either rot, or just stay shallow. When the rains stop the pool of water eventually dries out, but the grass roots are still petty shallow and suffer from the relative drought. Even if you water the lawn regularly, there’s still that clay barrier just a few inches away preventing proper drainage.

Wendy thought she would like to  dig it all out, put a better layer of topsoil, and seed a new lawn.

What do you think? What would you advise Wendy to do?

For those who are interested, here’s an excellent short article on managing turf on clay soils.

There seems to be no one solution to improving the site. If you scrape and replace, you’ve still got the layer of clay underneath, and sooner or later end up with the same results. If you regularly add layers of organic matter on top, eventually the worms will do a great job of incorporating the OM with the clay without the damage that rototilling would do. Do you want to work on this project for 5 years or so? (I would.) If you already have large shrubs or trees, they have adapted to the current level of the ground, and their superficial roots would surely not appreciate several inches of material on their heads, effectively suffocating them. So if you do choose the “top dressing” option, keep those layers less than about 2-3″.

Now here’s a couple other options, and both involve abandoning any attempt to have a grassy lawn:

1. Substitute your grass turf with another lawn alternative.

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Thanks to A Small Green Space for her picture of a clover lawn.

Dutch White Clover will grow from seed on your clay patch, and if you already have grass “growing” there, the clover will happily coexist. If the site is bare, sow a lot of clover.

Sedges are another option, such as Carex “Hime Kawasuga” or C. pennsylvanica, short growing, underground spreading, infrequently mow-able clumping grasses.

I’ll do another post exclusively on lawn alternatives, so stay tuned…

2. Raised beds. Instead of trying to manage the whole yard, build several raised beds with pathways in between. These can be your typical 2×8″ boards filled with compost and soil, or something a lot more attractive and creative-looking like dry-stack stones in less rectangular shapes.

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Dry-stack bricks, not dry-stack rocks, but still very cool.

This has been an overly long post, so I’ll stop here and save the rest for a future post.

Do you have soil that is less than Ideal? In fact, how do I even know if it is less than ideal, or if it’s my less-than-optimal technique that’s responsible for under-performing plants?

Leave a comment, share this to your favourite social network site, ask questions. You don’t need to suffer with a garden space that doesn’t meet your needs. Let’s get things working better!