“Where can I get some good topsoil?” That’s a question I hear frequently at our nursery. And I often look wistfully towards the plains and say, only half-jokingly, “You can get good topsoil about 800 miles east of here.” That’s where I grew up, in Iowa, and 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 the glaciers didn’t dump three feet of loam on top of our clay.
Many people, especially those who come here from places with real topsoil think, “Yuck, 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. Why not? My father used to say, There’s no royal road to learning; and the same can be said of good soil in Colorado. We have to earn it, build it and grow it.
The answer to What is soil? Used to be “clay, sand and silt.” That was the old physical concept of soil. Now a lot of us are saying, “Soil is physical, chemical, biological and electrical.” Likewise the old answer to How do I make my soil more fertile? used to be “Just add that good strong chemical fertilizer, and watch the plants shoot up.” Now many of us have seen the ill effects of strong chemical fertilizers and are more likely to recommend supporting the 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….The good news—essential news—is that we don’t have to totally understand the mechanisms in order to be expert soil builders, working alongside Mother Nature.”
I certainly don’t have all the answers here, but I will try to be helpful by first giving you the basics, then some of the nuts and bolts. 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. And when 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. And those soil beings in turn 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 the soil life.
The new formula for soil health has four or five aspects:
1) Minimize soil disturbance because turning the soil exposes the organic matter to the air, which oxidizes (burns up) the carbon, releasing carbon dioxide. Organic matter is important in soils because carbon is the “key driver of the nutrient-microbial recycling system.” This is why we add compost to our gardens, and how we can keep carbon out of the atmosphere.
2) Keep the soil covered. A covering of mulch or plants helps keep the soil moist by reducing evaporation and it moderates soil temperature. Mulch is also a source of slow-release nutrients. This is Nature’s own system of supporting soil life.
3) Cultivate a diverse population of plants. This naturally supports a diverse population of microbes which creates a stronger capacity to improve nutrient cycling, improves water infiltration and reduces pests.
4) Maintain a continual living plant community that feeds the beneficial soil life. This is accomplished with cover crops and evergreen shrubs, perennials and ground covers. I would like to learn a practical approach to cover cropping in a home veggie garden.
5) Integrate livestock. This supplies manures for nitrogen and organic matter and more. Or apply well-composted manures like dairy cow or chicken.
Now some nuts and bolts of soil building. First, the Why nots? Why not just buy topsoil? Because our topsoil is just clay, probably no better than your own clay. And why not just add planters mix? Because all the planters mixes I know are approximately two thirds topsoil (clay) and one third compost. So 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. And why not a raised bed? A raised bed does make an edging that holds back the weeds, and of course some people do have successful raised beds. But it is expensive to fill a raised bed with good potting mix, and most potting mixes are just peat, perlite and vermiculite, devoid of nutrients. They are designed to fill pots and be fertilized with chemical fertilizers. Even good potting soils are not living soils and need a few years growing plants and adding nutrients and maybe mycorrhizae before they are really productive with crops like peppers and tomatoes.
If you do make a raised bed, don’t make it higher than 12” because every inch adds to the expense and water use, and because it is good to make use of your own soil. Dig or pick the soil at least 4”-6” deep and mix your potting soil/compost into the turned soil. Do not put down a permanent weed barrier which will plug up and prevent worms from aerating your garden. A temporary barrier of cardboard or layers of newspaper is fine because the worms will eventually eat through it.
Why not add chemical fertilizer? A high nitrogen chemical fertilizer will consume your organic matter, make the plants grow too fast and too soft, making them more vulnerable to sucking insects and fungal diseases. Chemical fertilizers supply macronutrients like nitrogen, phosphorus and potassium, but lack the essential micronutrients which make cell tissues strong and support the immune system of plants.
On the positive side, our clay soils are not bad; they are just incomplete. They are rich in minerals important to plant growth, and because they are very fine textured, they have a lot of surface area which is great for holding water and releasing it slowly to the plants. But how do we improve clay soils? Most soil tests in Colorado reveal that our soils are deficient in nitrogen and organic matter. To supply nitrogen add organic fertilizers, animal manures, animal wastes like feather meal or blood meal, crab and shrimp wastes; or add alfalfa, cottonseed meal or corn gluten meal. You can also grow plants that fix nitrogen, like beans, peas and vetches. To supply organic matter you can add composts. Put 1”-2” on the surface and dig it in. Or you can practice sheet composting (lasagna method), plant a cover crop like buckwheat, or add a mulch (not redwood or cedar which resist composting). To supply micronutrients, you can sprinkle rock dust like Rocky Mt. Minerals or Azomite or kelp.
One often-overlooked ingredient in topsoil is air. People think they are being kind to their plants by watering every day, but when water fills all the pore spaces in soil, it drives out the air. Like animals, plants need air just as much as water. Our Colorado clay soils are too often compacted making them less capable of absorbing water, and 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 arid climate, compost breaks down fairly fast in the soil. Microorganisms can help aerate soil but they need oxygen and food to do it. That is why I am an advocate of 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 rock pieces ceramic and therefore porous so they hold both air and water for microbes and plants. They don’t break down so they hold the air spaces after the compost has broken down. Then you can mulch to continue the carbon cycle.
Other additives that are helpful are humate, rock minerals, seaweed and calcium. I believe it is better to err on the side of underfertilizing. For plant health and nutrient density, it is best to support the soil life and soil health as a long-range approach.
References: USDA Natural Resources Conservation Service
ACRES USA magazine, Christine Jones, Ray Archuleta,