Piece of Cake

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Late-autumn soil building is a birthday gift to your garden.


I love chocolate cake; it’s my go-to birthday treat. And the best is not heavy or dense, neither is it too light or so springy that it bounces back off your fork. How does a well-made cake relate to gardening? Think of the cake when you’re caring for your soil.

Going into the dark, short days of winter, your thoughts may not turn to garden soil. Isn’t that something we address in the spring, right before we plant? Well … yes and no. If you have pretty good soil, perhaps just a bit of spring prep is all that’s needed. But most of us should pay attention to our soil year-round, and especially consider its needs right after a growing season. If you’re a summer gardener, that would be now.

Late autumn is a great time to take a few simple steps that will preserve and enhance your soil for next spring. To preserve what you have, get it covered for protection. To enhance a sub-standard soil, mulch and cover-crop.

What is good soil?

For a home gardener, good soil is a garden that grows good plants. Evaluate how well your garden performed this year. If your veggies were the envy of your neighborhood and kept the family well-fed, your garden soil must be productive, for there is no bounteous harvest without good soil. Of course, soil quality is just one reason that your garden might be less than optimally productive, but for the purposes of this article, let’s stick to the soil.

A great soil is called “loam” and contains a balance of clay, silt, and sand. Most urban gardeners don’t have a quality loam because our natural topsoil has been displaced by building and cultivating, so we need to amend the soil. If it’s too sandy, it won’t hold moisture and nutrients; if it’s too clayey, it won’t have enough aeration for roots and soil microbes to thrive. Either of those imbalances can be improved with the addition of decomposed organic matter, such as compost. However, if the imbalance is extreme, it might take years and incredible amounts of organic matter to improve it; in those cases, just build some raised beds and start fresh with a soil mix that is ideal.

Adequate nutrient levels are another hallmark of a productive garden. However, if you’ve grown crops intensively for years without regular inputs of fertilizer or organic matter, you might have low soil nutrition. A professional soil test will answer this question for you and give guidance as to how to improve it. While fall is a great time to get a soil test, you wouldn’t be adding fertilizers now. That is something done shortly before planting. (Note: King County residents can get five free soil tests from the King Conservation District. Details at kingcd.org.)

Robust microbial activity is needed for healthy soil too, and this is not as easy to detect as nutrient levels. Sure, you can dig a hole and look for earthworms, but the worm is to the soil as the whale is to the sea. While there might be one visible whale, there are surely millions of smaller swimmers, some microscopic. It’s the same in the soil. The “soil foodweb,” to use the term coined by Dr. Elaine Ingham, a leading soil scientist, is a complex community of microorganisms like fungi, bacteria, and insects that are necessary to break down organic matter and make it available to the plants.

The good news is that you can begin to correct imbalances in soil type, nutrition, and microbial levels by taking a few simple steps right now.

Three garden gifts

To move the soil scale to a loamy balance, add a composting layer on top. This low-effort method is sometimes called sheet-mulching. Cut down the spent vines, stalks, and flowers; layer them on the soil; then cover the whole thing with cardboard and a top layer of finished compost (like the municipal yard-waste product made by Cedar Grove). Take care not to add diseased plants, such as tomato vines that show late blight, or the ripe seed heads of weeds. Also don’t add “running” weeds like bindweed that will just re-root and snake out from under the pile.

By next spring, the soil decomposers will have broken down the layered material, and you can plant into it. With this one action, you will have added more loam, increased the nutrient level, and fed the soil foodweb.

If you already have pretty good soil but just want to ensure good nutrient levels for next year, grow your own fertilizer.

Plant a cover crop in your vegetable beds now, and you’ll be growing a “green manure” that you can cut down and dig into the soil in the spring. This will break down and add nutrition to the soil, also feeding the soil critters in the process.

Although most cover crops should be planted by mid-October to get good germination and initial growth before the short, cool days of late fall, you can plant fava beans up until the second week of November. Choose the small-seeded type typically grown as a cover crop, and you won’t be tempted to let them grow to the harvestable stage for eating; they should be cut down when flowering.

One final step that can be taken to protect and enhance even the most productive vegetable garden: Shield the soil from winter rain compaction. If you do nothing else, cover your empty vegetable beds with a top mulch that will soften the effects of heavy or regular rain. Cover-cropping serves this purpose, but instead, apply a thick (3–4 inches) layer of straw mulch or a layer of burlap bags.

As the weather starts to warm and the rains get softer in spring, pull this covering off the bed to let the bed breathe and warm up, which will trigger the activity of the soil foodweb.

Take some of these steps now, and reap the rewards next spring at planting time. When you plunge your garden fork into the dark, rich, renewed soil, it will surely seem like your birthday.

Bill Thorness is author of Cool Season Gardener and Edible Heirlooms.

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