Basic desert soil horizons

A dry wash that has cut into the soil creating a cut bank on the left of the picture.  The cut bank exposes caliche which is common to old desert soils. 

Walking over the surface of the land not many of us think much of what we are walking on.  Most people probably only think they are walking over a disorganized pile of dirt.  However, as we saw in our last entry, soils are a lot more complicated, containing organized O, A, B, and C soil horizons.  Odd as it might soils, soil is organized, and organization is exactly what differentiates a soil from a pile of disorganized dirt.  Dirt becomes a soil when the dirt becomes organized into horizons.  When initially dumped on the ground by a glacier, wind, water, or falling off the side of a cliff, dirt it is disorganized.  After siting for many hundreds to thousands of years horizons develop and the dirt becomes a soil.  Generally the longer the soil sits in place the more developed the soil horizons become.

Most people are very surprised to find out that deserts have soils.  Amazing but true.  In-fact deserts have some of the most organized and interesting soils in the world.  Being deserts lack rain and haven't been glaciated, their soils can range from very young to very old.  Other places further north that receive more rainfall and have been glaciated have much younger soils.  The wide range of ages in desert soil results in a wide range of soil types.  Deserts have the same O, A, B, and C horizons the rest of the world has.  However, they have some features that make them distinct from soils from other regions.  We will cover desert soils horizon by horizon.

The black and green stuff covering the ground between the rocks is a cryptobiotic soil crust.  This type of soil crust is common in desert areas that have not be disturbed.

O horizon:  Due to sparse vegetation O horizons are pretty much absent in any desert.  Any organic material that does fall to the ground seems to disintigrate into nothing over a very long period of time, resulting in no O horizon.  Wind does however cause dead leaves to accumulate under shrub and tree canopies, resulting in a very thin O horizon.  For this and other reasons, soils under tree and shrub canopies are richer than surrounding desert soils.  In many circumstances desert soils if left undisturbed for very long periods of time (years to decades to centuries) will form a living crust as an O horizon.  This living crust is called a cryptobiotic soil crust and is made of algae and fungi.  These microbes help water absorb into the soil, increase soil nutrients, and help retain soil moisture, all very similar to how other types of O horizons work.

A horizon: Nothing really special about the A horizon in deserts.  This horizon is usually the sandiest and often covered with a layer of rocks and or cryptobiotic soil crusts (the O horizon).  The rocks and soil crust both serve to slow water runoff, increasing water penetration into the soil.  Soil crusts also enrich the soil with nitrogen which aids plant growth.

B horizon:  This is where things get interesting.  Clay and calcium deposited by wind on soil surfaces is carried deeper into the soil by rain water.  Over thousands of years, clay accumulates into a dense and highly organized argillic horizon, generally one to two feet under the soil surface.  On the bottom side or just below, about two to three feet deep, calcium accumulates into caliche.  Caliche is a rock-like accumulation of calcium.  The older a soil is the large and more developed argillic and caliche horizons become.  These horizons prevent plant roots and water from penetrating, therefore also preventing the growth of deep rooted plants.  Many shallow rooted plants do take advantage of water that accumulates above these horizons, and will proliferate in these soils.

Caliche remains along a dry wash bank after the A, C, and parts of B horizon have been washed away by flash floods.

A relatively accurate way of knowing if a soil has developed caliche and or an argillic horizon without digging a hole, is to look at soil color.  Tan to grayish soils typically do not have caliche or an argillic.  Reddish brown soils will have caliche and probably an argillic.  Soils become increasingly red with age as more iron is deposited by dust fall.  Calcium and clay content also increases as a soil ages forming caliche and argillics.  So as a soil turns red, caliche and argillics also form.  The more red a soil, the stronger the caliche and or argillic horizons. 

Grayish-tan soil, indicating this soil does not have an argillic or caliche.

Reddish-brown soil, indicating this soil does have both an argillic and caliche.

C horizon: This is the originally disorganized pile of dirt, generally three or more feet under the surface.  Often, if you are digging, this layer can be quite difficult to get too if you have to dig through dense argillics and rock-like caliches.  Interestingly, nearly all desert C horizons are water deposited.  They were simply washed off a mountain or hillside, deposited, and then developed into the A and B horizons we discussed above.  Odd as it might sound, water is one of the most important soil forming factors in lands severely deficient in it.  In-fact, nearly the entire desert landscape is sculpted by water.