Let's take a step back before I answer that. First of all, what is soil? That question stumped me when I first thought about it. "Dirt? Like, the ground. Rocks? Stuff." It's funny, because we've been playing in the dirt since we were toddlers, but it's hard to say what it actually IS.
First, it's about 45-50% rock particles, meaning rock, sand, and inorganic matter. Second, it's about 45% air and water, or what we call pore space. Finally, it's 5-10% organic matter, meaning anything that was once alive (plants, animals, manure, etc). Notice that this organic matter accounts for a very small percentage of your average soil. But that small amount is extremely important, and it's not necessarily better to have more past a certain point. Small-scale organic farms may have up to 12 or even 15% organic material in their soils, but that's about as high as you'd want to go. Any more would make the soil too acidic and would not be beneficial for the plants. So what about buying a bag of compost, and planting your potted tomato in pure compost? Nope, not a good idea!
Seattle Tilth website says about what compost is:
The natural biological cycle of growth and decay creates compost. A plant that has grown by gathering nutrients and water from the soil, energy from the sun, and carbon dioxide from the air, dies and becomes the raw material of the decay process. Microorganisms, worms, fungi and insects recycle materials from the decaying plants into their bodies and eventually back into the soil. Compost is the material that results from the decay process and is similar to organic matter in the soil.
Compost has many benefits for the soil. It improves drainage and aeration of clay soils, preventing water logged plants. It increases the moisture and nutrient holding capacities of sandy soils, and reduces drought damage to plants. Compost helps keep nutrients in the soil near plant roots, and it can immobilize and degrade pollutants. By preventing crusting on the top of the soil, compost can help seeds to sprout and water to soak in more easily.Wowzers! That a lot of benefits. So not only does the compost provide nutrients, but it also makes the nutrients more available to the plants to use! It filters toxins! It invites beneficial insects and bacteria!
Compost also improves soil structure by moderating two extremes. On one end you have a really sandy soil- Perhaps you've watered a potted plant, and water just rushes out the bottom. You would have to water that plant more often, because the sandy soil is not holding on to moisture. That flush of water is also washing away nutrients more quickly. So adding a little bit of compost actually helps with both moisture and nutrient retention.
On the other extreme you might have a heavy clay soil- perhaps you've watered a plant, and the water hardly soaks in, just sits on the surface or runs off. When it finally soaks in, the soil stays wet for a long time. This soil type has the opposite problem- it will hold on to water for so long that a plant could get water-logged. Also, because clay tends to be very compact soil type, it is not as aerated as it should be, and plants aren't able to germinate or develop their root structure as easily as they could. So what's the solution? You guessed it, add compost! You can add compost to the top of the garden bed as a mulch, or work it in. Either way will ultimately provide similar benefits, and your garden will be much more water efficient, vital, and nutrient-rich if you do.
Another benefit of using compost is that it helps you keep materials from your yard on-site, and it turns a waste material into a resource. Here in Seattle we are lucky enough to have a large commercial composting facility, but that still means your leaves, grass clippings, and branches get hauled away in a truck, only to get brought back to the city in a truck, in a bag of compost that you then have to purchase. If you compost at home, you take out the middle man. Even more importantly, if you live somewhere with no commercial compost, then yard waste gets thrown into the garbage and eventually a landfill, putting valuable organic material into an anaerobic dump where it will break down and release carbon dioxide and methane gas. A simple yard-waste compost pile in the backyard can minimize that problem.
"Well," you might be thinking, "I'm not really into making my own compost, and I'm not a gardener. Why would compost matter to me?" Great question! It turns out that compost/organic matter in soil is critical on an urban planning level. Almost every winter there are heavy rains all around the country, and every year there is massive property damage as a result. There are more floods now than 50 or 100 years ago, but why?
It has to do with permeable surfaces in the city. Think about western Washington a century ago. There was little pavement compared to today, fewer buildings, and a lot more trees. When rain fell on our precipitation-happy corner of the world, it first hit leaves and pine needles, hanging out on branches for awhile before falling down on the ground. That forest floor was full of organic matter- fallen leaves, pine cones, decomposing logs, etc- that acted as a sponge, and held on to the water for awhile. In short, a couple inches of rain had a lot of area where it was absorbed, and it happened s l o w l y.
In contrast, a lot of land today that was once farmed or forested is now paved or roofed. A couple inches of rain will fall over square miles of impermeable surfaces, so where it can soak in gets overloaded. Plus, most urban soil that rain does hit is likely to have had much topsoil (with its spongy top humus layer) removed in the construction process, and be a lot of more compacted and less absorbent than it used to be. Both of these factors lead to run off and massive flooding in urban areas.RainWise through Seattle Public Utilities. Being "RainWise" means doing your part as a resident to help manage storm water runoff, for the purpose of "reducing flooding, protecting property, and restoring our waters for people and wildlife". Their website has great little PDF fact-sheets, on all sorts of ways to be RainWise, including: planting trees, increasing permeable paving, disconnecting downspouts, installing cisterns, building rain gardens, and my favorite, improving soil with compost. I think any amount of increased compost use on a residential level is a great thing, though it's important to note that compost is also used on a commercial scale. Various blends of compost are available from Cedar Grove and used by commercial construction companies as well as the WS Department of Transportation in building projects, erosion control blankets, living walls, mulching, meeting soil quality regulations, and many other things I'm sure.
There is a ton more I want to say about the benefits of compost (we didn't even touch on compost biology!) but that's all for today. Stay tuned for the next compost post- "What's the difference between hot and cold compost?" Coming soon!