What is compost? Why do we love it?
In an earlier post on mulch, we explained the difference between compost (a material made of decomposing organic matter) and mulch (a protective covering on the surface of soil). I confess that we in the garden industry often confuse newer gardeners by using the terms interchangeably since compost often makes an ideal mulch, as well as an excellent soil amendment (what we mix into soil when we plant).
We also probably tend to ramble on about the virtues of compost, as other folks would talk up the latest messaging app or superfood. We, however, are justified in our zeal. Real gardeners love the stuff. Compost adds and preserves air space, moisture, and nutrients, and fosters the web of life in the root zone. It improves soil fertility and structure. And it does it all by reusing “waste” material. What’s not to love?
Our ancestors knew all this, the cycle of returning the bounty to the earth, but we’re learning more about the science behind it.
A little about soil
All soils consist of some mix of four basic elements: minerals (clay, silt, sand, rock, etc.), organic matter, water, and air. That’s not to mention beneficial microbes, microfungi, earthworms and other organisms, whose essential roles we are gradually discovering. Plants vary with what proportions of the four elements they prefer, but most need a balance. Typical, healthy soil consists of about 45% minerals, 25% water, 25% air and 5% organic matter. Within the mineral portion, equal percentages of clay, silt, and sand are ideal, but of course, they may vary.
Why use compost?
Planting soil should be amended when it’s either too sandy or, more commonly, too compacted. Much of our “native” soil, especially in urban settings, has been compacted over time by human use. Compaction may be intentional (such as for building construction) or unintentional (such as repeated foot traffic during play or landscape maintenance). Or the soil has been leached of many nutrients by years of Northwest rain and its organic matter has been broken down to where the mineral percentage can reach 75% and above. Not much air space, and nowhere for water to flow or for organisms to circulate. We tend to label such soil “heavy clay” but not all compacted soils are necessarily clay-dominant.
Adding organic matter to compacted soil helps aerate, foster the microbial network, and restore the balance of the elements. This is what we mean by “breaking up clay” with compost. Even simply mulching the surface of the soil over the root zones with compost can go a long way toward restoring healthy soil, as it naturally works its way down. Some compacted soils may make need several years of gradually adding compost to be restored to fertility. Adding organic matter to sandy soil helps the microbes and increases the soil’s water-holding capacity.
Not all soils are this bad, of course. There are settings that have avoided compaction and where heavy plantings of trees, shrubs, and/or perennials have naturally provided a steady and nutritious mulch of leaves, twigs, and other material over the years. But many of our soils indeed benefit from adding compost.
Please note that it’s fine to amend (till in) soil for planting new plants or starting the vegetable garden each season, but for established plantings, it's best to mulch the soil surface but not till it in. Tilling can damage existing roots and their delicate microbial network.
Is it possible to add too much?
Yes, as we described in our mulch post. We want to restore the balance of the soil elements. Making it too moist or too rich can increase the chance of soil-borne diseases. It’s hard to prescribe a quantity or frequency, but adding smaller amounts once or twice a year is better than adding too much at once and/or too frequently.
Where do I acquire compost?
You can, of course, make it at home from yard and kitchen waste. The benefits of "closing the loop" through home composting are well-documented. There are many ways to make compost if you have the materials, space and time (see some resources at the end of this post).
If you lack the time or space to make your own compost, there are commercial versions. The bagged soil amendments we sell at Swansons are mixes of organic ingredients specifically formulated to mix into garden soil. They have certain advantages: they lack weed seeds that sometimes infiltrate our home compost, many include mycorrhizae to help the microorganism network, and they're very easy to transport and apply. Some compost mixes also include composted manures, which add nutrients and are especially suited for vegetable planting. See our list of recommended soil amendments for specific garden projects.
If you need bulk quantities of commercial compost, we can direct you to sources around the region.
When should I add it?
Any time of year as you plant something, or to mulch if the surface soil needs cover. There are some advantages, however, to mulching in early spring, when the microorganism network increases its activity, or in late fall, to protect delicate plants from winter cold and dryness.
So close that loop! Keep feeding and building your soil with compost and it will feed you back and beautify your world for generations to come.
Resources for home composting:
Here is a comprehensive collection of articles on compost from Washington State University, Whatcom County Extension.
Also, see our care sheet, Gardening 101, for basic mulching and amending information.