The Twelve Soil Orders

At its most basic level, the world’s soil falls into just twelve groups. Gelisols, Histosols, Spotosols, Andisols, Oxisols, Vertisols, Ardisols, Ultisols, Mollisols, Alfisols, Inceptisols, Entisols—the Twelve Soil Orders sound to me like an alliance, sitting about a round table and dividing up the territory of the earth. Of course, as political battles tend to go, the soil order could not be kept simple. The dividing lines between soils are meandering; soils mix and change, becoming more complex and requiring finer levels of description.

Our vegetable plot is of the Vertisols variety; our orchard amongst the Entisols. And in just the five mile radius around the farm, four of the twelve soil orders are represented. Their boundaries look topographic. And despite the simple breakdown of twelve soil categories, a few hundred feet can mean the difference between farmable and un-farmable land.

Our vegetables grow in a Vertisol called Capay Clay, a name specific to this region. The farm sits just outside of the Capay Valley, the soil’s namesake. Typical of the vertisols, our soil shrinks and expands. In the rain, the soil opens up, holds in water and stretches out to make room. As the water slowly drains, the top of the soil dries first, leaving a crust full of cracks and crevices. After heavy rains we must wait over a week to drive a tractor through the fields. We worry that our tiny seeds will not break through the crust. But new plants are stalwart. If they can break through the cement sidewalks of Los Angeles, they can withstand our Capay Clay.

Learning about my land has been a crash course in soil science over these past few months. Impromptu lessons in reading a soil test, closely watching the soil to see how long it takes to drain from heavy rains, and calculating the nutrients that need added to our soil to grow its general health.

A new application can help farmers, and those making land use decisions in general. I recently learned that the USDA’s Natural Resources Conservation Service, the organization charged with mapping soil in the U.S., made a commitment to map all of the soil in the country by this year. They are close, with only a few holes remaining mostly on public lands. And we can look at all of it! In collaboration with the NRCS, Toby O’Geen, a professor at UC Davis and a group of his students developed Soil Web, an application for Google Earth, Google Maps, iPhones, and Androids (and soon all smart phones). In the field, the app shows your soil profile based on your gps coordinates, so you can wander about your land and watch the soil change. Whether at your computer or in the field, Soil Web lets you explore the makeup of your soil at the field level.

To a soil scientist, the family my soil belongs to is Fine, montmorillonitic, thermic Typic Chromoxererts. It is astounding how little this means to me. But with the help of Soil Web, I can learn the secrets of the Soil Orders a bit more, and reveal the possibilities and limitations under my feet.

Admittedly, some of the information on Soil Web is still above my head, but with a little help from one of the app developers, Toby O’Geen, I gained some quality insight into my soil. I’ll share my new knowledge on the next post. But for now, go explore some soil.

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