Building Topsoil and Soil Fertility
- 6 hours ago
- 4 min read
By Mikl Brawner
“Where can I get good topsoil?” is a question I hear frequently at our nursery. I often look wistfully towards the plains and say, half-jokingly, “You can get good topsoil about 800 miles east of here.” That’s where I grew up, in Iowa, where two tomato plants feed a family of six. It’s not that local suppliers are trying to deceive us when they sell Colorado clay as topsoil; it’s just that glaciers didn’t dump three feet of loam on top of our clay.
Many people think, “I’ll just get rid of this clay or build a nice raised bed on top of that worthless dirt, and then I’ll have a good garden.” Unfortunately, that’s not a great idea. My father used to say, “There’s no royal road to learning.” The same can be said of good soil in Colorado; we have to earn it, build it, and grow it.
The old physical concept of soil is “clay, sand and silt.” Now a lot of us say, “Soil is physical, chemical, biological, and electrical.” Likewise, the old advice to make soil more fertile used to be, “Just add good strong fertilizer and watch the plants shoot up.” Many of us have seen the ill effects of strong chemical fertilizers and are more likely to recommend supporting soil life and the Soil Food Web.
In his book Soil Stewardship Handbook, Aaron Perry says, “In a world of seeming complexity, we find that hardly anything is as complex as the living web of interconnectedness found in our planet’s soil—trillions upon trillions of living creatures in each handful, interacting in beautifully sophisticated ecologies… We don’t have to totally understand the mechanisms in order to be expert soil builders, working alongside Mother Nature.”
The most fundamental basis of soil fertility is the mutually supportive relationship between plants and soil organisms. Because plants have the magical ability to photo-synthesize—to make food out of sunlight, water, carbon dioxide and minerals, they are life-supporting. They ooze 30% of their sugars into the soil surrounding their roots; fungi, bacteria, worms, and others are drawn to or multiply in the nutrients. Those soil beings bring water and minerals to feed and protect the plants. So whatever we do to support the microorganisms supports the plants and what we do for the plants supports soil life.
Why not just buy topsoil? Because it’s just clay, probably no better than your own clay. Why not just add planters mix? Because all the mixes I know are approximately two thirds topsoil (clay) and one third compost. Unless you are trying to fill a hole or raise the grade (for example, to get water to drain away from the house), just add compost to your own clay.
Raised Beds
A raised bed does make an edging that holds back weeds and some people do have successful raised beds. If you live in the mountains with very rocky soil you may have no choice. But they dry out quickly and it’s expensive to fill them with good potting mixes. Most mixes are designed for pots and chemical fertilizers. Devoid of nutrients, they are just peat, perlite, and vermiculite. Buy the best you can find but even good potting soils aren’t yet “living soils”; they need a few years of growing plants and adding nutrients - and maybe mycorrhizae - before they are really productive with crops like peppers and tomatoes.
Make raised beds no higher than 12”. Every inch adds to the expense and water use, plus it’s good to make use of your own soil. Dig, till (just once), or pick at least 4”-6” deep and mix your potting soil/compost into the turned soil. Don’t put down a permanent weed barrier; it will plug up and prevent worms from aerating your garden. A temporary barrier of cardboard or newspaper is fine because the worms eventually eat through it.
Fertilizing
A high nitrogen chemical fertilizer will consume your organic matter and make the plants grow too fast and too soft, leaving them more vulnerable to sucking insects and fungal diseases. Chemical fertilizers supply macronutrients (NPK), but lack essential micronutrients that strengthen cell tissues and support plants’ immune system.
Our clay soils are not bad, just incomplete. They are rich in minerals important to plant growth and they have a lot of surface area, which is great for holding water and releasing it slowly to plants.
Most soil tests in Colorado reveal deficiency in nitrogen and organic matter. To supply nitrogen add organic fertilizers, animal manures, animal wastes like feather or blood meal, crab and shrimp wastes; or add alfalfa, cottonseed meal or corn gluten meal. You can grow plants that fix nitrogen, like beans, peas, and vetches. To supply organic matter, apply and dig in 1”-2” of compost. Or you can plant a cover crop like buckwheat. Sheet composting (lasagna method) works well but takes time so is best started in fall. To supply micronutrients sprinkle rock dust like Rocky Mt. Minerals or Azomite or kelp.
Air is an often-overlooked ingredient in topsoil. Like animals, plants need air as much as water. People think they are being kind to plants by watering every day, but this drives out the air. Our Colorado clay soils are too often compacted; the lack of air slows down plant metabolism and often leads to fungal diseases.
Our most common method of getting air into our soils is by adding compost, but in our climate compost breaks down quickly. Microorganisms help aerate soil but need oxygen and food to do it. I advocate incorporating 10-20% expanded shale along with compost, especially if your clay is really tight. (Put 1”-2” on the surface and dig it in.) Expanded Shale is mined near Boulder and fired in a kiln. This makes the pieces ceramic and porous; they hold air and water for microbes and plants, and they don’t break down.
Other helpful additions are humate, rock minerals, seaweed and calcium. I believe it is better to err on the side of under-fertilizing. For plant health and nutrient density, it is best to support soil life and soil health as a long-range approach.
Mikl Brawner and his wife Eve co-own Harlequins Gardens in Boulder, specializing in organic veggie starts and herbs, natives, sustainable roses, xeriscape, unusual perennials, and products to build healthy soil.
