New Advice on Soil Amendments

compost-in-bags-no-label-lowes-cs_lah_5090-1You’re gotten your test results back from the soil lab, telling you to add some organic matter. What’s the best thing to add?

In the past, I’d just bop on down to the local garden center and load up a few bags of… something. Soil amendment, composted manure, planting mix, potting mix, top soil, compost… there are hundreds of products, and the names are pretty random.

So are the ingredients. Since there are no legal standards, these bags can contain whatever the manufacturer wants them to. There’s no labeling law, either. If there’s a label at all, often you’ll see something like, “Contains (peat, forest products compost, and/or compost), wetting agent, fertilizer.” You have no idea if this particular bag has peat or compost, much less what went into that compost. And what’s a forest product? Bark? Sawdust? Squirrels?

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Too Much Compost?

 

veggie-garden-student-run-csu-perc-lah-094-1When I first encountered this concept—that a gardener could use too much compost—I immediately thought, “Is that even possible?” As an organic veggie gardener dealing with soil comprised of decomposed granite punctuated by lumps of sticky clay, too much compost seemed an impossibility. Isn’t compost the answer to all our gardening problems?

It’s true that “add compost” (or other organic matter, such as peat moss or leaf mold) is the best advice for gardeners dealing with either clay or sand. Organic matter opens up the solidly packed clay particles, allowing air and water—and therefore roots—to penetrate what would otherwise be an impervious substrate. In sand, organic matter acts as a sponge, holding on to both water and nutrients that would otherwise quickly drain away.

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Earthworms

Photo: Joseph Berger, bugwood.org
Photo: Joseph Berger, bugwood.org

We hardly notice them most of the time… they’re out of sight, underground, aerating the soil, creating humus, increasing fertility. It’s only after a rain storm, when the ground is saturated, that they come up for air. Then we see their desiccated carcasses strewn across the pavement. Robins eat them, anglers use them for bait, and little kids bring them home in their pockets as pets. Most of us dissected one in biology, carefully counting the five aortic arches while debating the coolness of being squeamish. Yet, for all their inconspicuous habits, earthworms play a major role in both our gardens and in the wild.

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Improving Your Soil

Soil is the foundation of your garden. It pays to invest in creating the best possible soil for your plants to grow in. Living along the Front Range of Colorado’s Rocky Mountains has many benefits. However, no one would move here for the black topsoil! Instead of the optimal 5% humus content, most of our soils have little or none. It’s up to us to improve on Mother Nature.

shoveling-manure-home-plhYou can easily increase the percentage of organic matter in your soil by adding compost or another organic amendment. This added humus will act as a sponge, increasing water retention in sandy soils. On the other hand, in clay it acts to improve drainage by increasing the size of air and water spaces. Plus, organic matter works with your fertilizer by holding nutrients in a form that is available for absorption by roots. As you can see, organic matter is an important component of healthy soil.

It’s best not to add too much organic material at once. Many organic amendments are based on manure, and could contain harmful amounts of salt, as well as weed seeds. Plus, the nitrogen in fresh manure can burn tender roots. Make sure to let manures age before adding them to your garden. Decomposition requires nitrogen. Any form of organic matter that isn’t completely decomposed will steal that essential element from your plants.

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