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What is Soil, really?

It sounds like a dumb question, doesn’t it? But it’s actually much more complicated than you think.

The Soil Science Society of America (although I’m simplifying this a bit) defines soil as the “unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.”

Every type of soil has the capability to grow plant life. Look at the cactus and succulents growing in extremely arid climates. Look at the alpine plants that grow above timberline where the soil is nearly non-existent. Because plants don’t have feet and can’t move from place to place to get the sustenance they need, they must adapt to the conditions they are sewn into in order to thrive. But we have to remember that plants have had millions of years on this earth to adapt. We can’t expect to transplant water lilies into a desert and expect them to live. In essence, we have to consider a) the plants we grow, and b) the medium in which we grow them.

It may surprise you to know that soil conditions are responsible for 80 percent of all plant problems. It’s the most important factor in your success in growing plants. It may explain, in fact, why giving your lawn extra fertilizer and/or extra water may actually kill your lawn - not make it greener. That’s why we have to understand what’s below the surface.

When we talk about soil, we’re really talking about the earth’s crust - or skin, which is only one percent of the earth’s volume.

Let’s look at the typical makeup of soil in the western United States.

Well managed soil here generally comprises 47 percent mineral (rock, sediment, broken down granite of the parent material). In Colorado we see varying compositions of clay, sand, and granite. The rest of the solid material is composed of five percent organic matter. This organic matter comes from leaves and decomposing plant materials, animals, and soil organisms. That all accounts for only half of the makeup of soil. The rest is 25 percent air (or “pores”) and 25 percent water.

But, when we talk about “urban landscape soil,” we’re now talking about the ground our communities are built upon. This soil has been heavily graded by large machinery, moving and compacting this top layer of earth so that we can build houses, roads, and infrastructure. This soil looks quite different. It comprises 69 percent mineral, 20 percent water, 10 percent air, and only ONE PERCENT of organic material. What’s more, the soil in your backyard is going to be different - maybe very different - than the soil in your neighbor’s backyard. This is to say that disturbed soil is unpredictable across urban landscapes and must be examined before planning a garden of any kind.

Soil Composition

To understand why it is important to understand the composition of your soil, let’s look at the way plants actually take up water. We have to get a little sciency now. Very simply put:

Plants take up nutrients and water through its roots through cohesion and transpiration (learn more, here) by capillary action. This means that water can move against gravity to move upwards to cells throughout the plant. The composition of soil must have adequate pores for water and air for this to happen. Soil structure refers to how the various particles of sand, silt and clay fit together creating pore spaces of various sizes. Soil texture refers to the size of the particles that make up the soil. The terms sand, silt, and clay refer to the sizes of the individual soil particles.

Clay is the finest of texture. Clay particles lay flat like long plates, which creates very small, if any, pores. Water and air cannot easily move upwards. Think of your stacked stone fireplace and the groutlines being the pores. Water and nutrients move sideways and get trapped under particles. Fine sand acts the same as silt or clay, allowing little to no pore space between particles. Larger sand is the opposite and is granular, creating large pores so that water can move easily - especially down. Sandy soil does not hold water or nutrients well and therefore plants have a harder time taking what they need before heat and evaporation takes place. A mixture of these particles is what makes soil a good medium for various plants.

Soil structure is not the only factor in plant health, though. In one cup of soil you will find more organisms than humans who have ever lived. Soil also contains the most diverse food web on the planet. Billions of bacteria, protozoa, fungi, nematodes, and arthropods work symbiotically (for the most part) to feed plants by producing water soluble nutrients essential for cell structure. These organisms also break down organic matter and create pores in dense soil, which in turn helps plant roots take up those nutrients. Organisms need time and undisturbed landscape in order to do their magic. Urban Landscape soils often lack adequate soil life because the soil has been compacted and moved around.

Soil lays in horizontal layers of topsoil (which has the greatest organic matter), subsoil (which tends to hold leached minerals from topsoil), and finally, the bottom horizon of unconsolidated material that has been minimally affected by the soil-forming factors of wind, water, etc. Colorado soil has a very small topsoil horizon - if any at all.

Ever wonder why your lawn continues to stay brown no matter how much you feed and water it? Let’s look at why that might be:

  1. We have very little organic matter. The beneficial soil food web is often missing or often disturbed in urban landscape soil. If you don’t have the plant and animal decay to feed these organisms (like when you rake up all of your leaves or bag your lawn clippings), these little guys won’t stick around. If they are not eating and creating rich nutrients for your lawn - you’ll never create or keep quality, life-giving topsoil.

  2. The soil profile (the way in which sand and clay fit together) can also cause problems. Clay on top of soil means water and nutrients have a difficult time penetrating from the surface. Water is slow to leave the small pore space of the clayey soil due to the water properties of cohesion. Sand on top of clay creates a barrier where water will drain easily and quickly from the surface down to the layer of clay where it will sit and sit and sit - suffocating the plant’s roots from oxygen. The line between these profiles can cause a perched water table that can also cause salts and nutrients to leach out of the soil. The more you water in these conditions, the more leaching will occur.

  3. Adding chemical fertilizers is often used in an effort to improve plant health. However, fertilizers will only assist growth if the plant is deficient in the nutrient applied and other growth factors are not limiting plant growth. If you have poor soil, lack of water, and other inhibiting factors, fertilizer will simply compound leaching of nutrients.

Understanding soil properties is essential in getting plans right. And here in Colorado, we’ve been fighting against our soil since the early 1900s, trying to till, amend and grow crops that just can’t thrive here. Beth Chatto, the most influential British gardener of the last half-century, is famously quoted as saying, “Right plant, right place.” Through extensive experience and research she and her husband, Andrew Chatto, developed a way of gardening that took into consideration not only the climate, weather, and topography plants thrived in naturally, it also examined the soil in which these plants were able to grow. Shallow rooted plants are better adapted to take up nutrients with fine hairs and aerial roots (roots found above ground). Deeply rooted plants tap through heavy soils in arid climates in order to take up water and nutrients hidden well below the topsoil. Understanding the soil is essential in understanding the plant - and visa versa.

I’ll be exploring and sharing ways you can evaluate your soil health in the coming weeks. We’ll look at ways you can test your soil’s composition and ways in which you can improve your soil’s health. We’ll explore aeration, tilling, fertilizing, composting, and amending so that you can get the most out of your yard.

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