One of the pleasures of spring is digging in the soil and that earthy smell but how much do we really know about the soil beneath our feet. According to Leonardo DaVinci, “We know more about the movement of celestial bodies than about the soil underfoot.” Although we’ve learned a lot since Leonardo’s time, we still have a lot more to learn.

At the core of any property, and critical for plant growth, is soil. Soils store and release nutrients and water to plants for growth, and secure plant roots. Colorado soils are about half mineral from decomposed or eroded rock, with the remaining half divided between air and water, and a small organic material percentage.

The mineral portion consists of clay, silt, sand and gravel/rock particles. The proportion of each determines many soil properties, such as water holding capability and tendency to compact.
Here’s a visual comparison of the three particle sizes; sand is a basketball, silt is a ping pong ball and clay is pea size. Sand particles, due to their larger size allow for more air spaces, do not compact readily, or hold water as well as soils made of silt or clay. Silty soils have smaller air pockets and hold water better than sandy soils. Clay particles are the smallest and can readily compact, driving oxygen out of the soil.

While most people understand that plants use carbon dioxide and give off oxygen, many do not realize that plant roots also require oxygen. If the soil is compacted, there is less oxygen for the roots, and plants can be stressed and vulnerable to disease and environmental disorders. Compacted soils do not allow good water infiltration or root growth. So why not just add sand to clay soils or clay to sandy soils to prevent compaction or improve water holding capability? Doing either of these will produce one of the earliest known building materials, adobe bricks. The best amendment for our soils, either sandy or clay, is organic matter, which helps the water holding capability of sandy soils and lessens compaction in clay soils and feeds soil organisms.

Beside the inert portion in soil there is a whole system of vertebrates, invertebrates, insects, arthropods, bacteria, fungi, protozoa and other microorganisms that call the soil their home and support plant life. Within one teaspoon of soil you might find 62,000 algae; 72,000 protozoa; 111,000 fungi; 2,920,000 actinomycetes; 25,280,000 bacteria; and 50 nematodes. This doesn’t even count the earthworms, insects, and larger mammals such as prairie dogs and gophers.

What do all these organisms do? The vertebrates mix the soil, moving subsoil to the surface and mixing it with topsoil. Insects, arthropods and earthworms mix the soil, ingest some of the organic material, and contribute organic material via their waste products and dead bodies. Algae cycle water and nutrients by producing organic acids that help make nutrients available to plants and organisms. Algae do not decompose organic matter but their growth produces additional organic matter (their dead bodies). Fungi actively decompose organic matter and can form symbiotic relationships with plants. Plants provide fungi with food and the fungi enhance the availability of various plant nutrients (P, Zn, Ca, Mg, Mn, Fe and Cu) to the plants. Bacteria are critical in altering soil chemistry. Some bacteria transform carbon dioxide and inorganic minerals and chemicals from either unavailable or toxic chemicals to nutrients available to plants. Other bacteria rely on organic material for their own nutrients that they then transform into plant nutrients. Protozoa control the soil bacteria populations.

So next time you work in your garden or go for a hike, think about the life that is taking place beneath your feet.

By Sharon Bokan, Colorado State University Extension Boulder County. Sharon is the Small Acreage Coordinator at Colorado State University Extension Boulder County. For more information call 303.678.6167 or visit