Soil provides food for thought as well as for plants

By Gary Raham
Nature Writer and Illustrator

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I never thought much about soil before I came to Colorado. But then I was a city boy from Michigan with brand new degrees in biology and a desire to teach. I knew worms, protozoans and bacteria galore lived in soil, as I sampled them from time to time to impress students, but if I didn't get stuck in muddy soil or track it inside the house, I had no quarrel with dirt. I walked on the stuff and forgot it. Much later I discovered what farmers and geologists learn right up front: soil shapes the character of everything that lives upon it.

I did notice pretty quickly that Colorado soil differs from the dark crumbly stuff I knew in Michigan. A person could actually insert a shovel into Michigan soil with little trouble and dig a respectable-sized hole without damaging the shovel. Not so in Colorado. Once the wind blew the loose grit off the surface and into your eyes, the remainder was suitable for making bricks or throwing pottery. A friend of mine who came from Iowa (not far from Michigan, after all), was never happy planting in his back yard until he had applied sufficient cow manure to at least give the soil the right color and texture--if not aroma.

A minor epiphany occurred one day when I read "Roadside Geology of Colorado." This book, one of a series published by Mountain Press Publishing Company in Missoula, Mont., describes the geology one can see while speeding by it on interstate and other highways. In the section describing I-25 between Cheyenne and Denver the authors note: "At milepost 281 (just a little north of Wellington) the highway descends to the top of the Pierre shale. Land use changes abruptly from grazing on the sandstone (west of the interstate) to farming on the shale. This change is so consistent it can be used in the piedmont area to map the formations, which are rarely exposed in outcrop but strongly influence soil chemistry and texture."

Imagine that. You can map rock formations by noting what grows on top of them. Of course moisture and overall climate play a role, but the soil either has what particular plants need to grow or not.

Pierre shale, by the way, is the geologist's name for Colorado's "pottery soil." Don't plan to dig your way through it anytime soon, as it extends a mile deep (at least in the middle of the Denver Basin). It was deposited over a period of 31 million years when Colorado was ocean bottom. The sandstone (Dakota) hogbacks to our west were once beach sand on Colorado's eastern shoreline.

Since reading "Roadside Geology," I've tried to pay more attention to the impact of soil on the character and type of vegetation it supports. Take locoweed, for example. This common name actually refers to several species in at least two genera of plants in the pea family (Astragalus and Oxytropis) named for the dizzying (sometimes lethal) effect eating it can have on livestock, especially horses. Locoweed grows in thin, rocky soils of the high plains and sagebrush steppe country. I've seen several varieties adorning the hogback east of Horsetooth Reservoir and in Lory State Park. Locoweed concentrates selenium in the soil into its tissues, which causes its physiological effects. The plant's impact must not be totally unpleasant at some level, as some animals actually come to prefer locoweed in their diet despite its deadly effects at high doses.

Locoweeds will thrive on a range of dry country soils. Other plants such as Bell's twinpod (Physaria bellii) have developed such restrictive soil requirements that, like Goldilocks, its food must be "just right." Bell's twinpods grow only on a certain limey shale called Niobrara sandstone, which is exposed in Colorado only between Boulder and Fort Collins just east of the higher Dakota sandstone hogbacks mentioned earlier. Where the Niobrara is thick enough to justify cement manufacturing, the twinpods have gone homeless. In protected areas like Coyote Ridge, just south of the Larimer County landfill, you can find them blooming now in all their four-petaled yellow glory only on the china-plate shards of sandstone erupting from north to south along our front range.

Sometimes I miss my younger self, emersed in that bliss of ignorance about so many things and thus much more self-assured about those few things I did know. Other times, like now, I'm thrilled with each new addition to my database and the new connections it brings to old experiences. In consequence, I can no longer blissfully ignore dirt--that rich and unique mixture of the organic and inorganic that serves as both the mechanical and ecological foundation of everything I hold dear.