Soil Formation (A Goldilocks Theory – the best soils form in places that are not too hot, too cold, too rainy, too dry, or too swampy)

What makes a great soil? Start your answer by realizing that soil is not something that comes directly from rock. On the contrary, soil is the result of long-term interactions between rock, climate, slope, and living things in a place. As a result, the right question to ask is: What are the conditions that produce a great soil?

If a place is really cold, the processes that break rock down into smaller pieces do not work very rapidly, and there are not living things. As a result, there really is no soil. ice-covered mountains in Greenland

If a place is cold, but occasionally goes above freezing, the processes that break rock down into smaller pieces are barely able to work. The soil is thin and rocky. high mountain in Montana

If, on the other hand, the place is really hot and rainy, the chemical processes that decompose rock work very fast, and they make tiny particles of iron and aluminum oxide. This soil is hard to work and not very fertile. field in Alabama

Soil in a hot and rainy place tends to be fine-textured (tiny particles), red (iron oxides), acid (leached) and hard to plow or fertilize. In the U.S., soils like this are usually used to grow pine trees or, occasionally, pasture for cattle. ultisol in southern Mississippi

Under the right conditions, a soil in a hot and humid place can actually form a dense layer of iron and aluminum oxide. This is called plinthite – it gets worse closer to the Equator. plinthic soil in Georgia

Plinthite looks like a lumpy mixture of white and red clay. When it dries in the sun, it forms a brick-like layer (“plinth” actually comes from an old word for brick). This is good for making bricks or pottery, but bad for farming! plinthic soil in Georgia

If a place is too dry, salts tend to accumulate over long periods of time. The salt comes in dust storms or very rare rains. The salts are especially common in the dry beds of temporary lakes that form after storms. salt flat in southern Arizona

Excess salt is common in soils in dry places – one of the benefits of a flooding Nile River was to reduce chemical fertility (salt content) to tolerable levels. Inadequate irrigation can make things worse – as in the Indus Valley aridisol in Arizona (yes, both words come from arid, meaning dry)

At the other extreme is a place with adequate temperature but too much rain (or a coarse, sandy rock). Under these conditions, nutrient salts are leached downward in the soil, leaving the topsoil nutrient-poor and infertile. leached soil in Michigan

If you have gotten the hint that “just right” is in the middle, not too cold or hot, not too wet or dry, that’s exactly right. But terrain is also important - low places tend to have more water, which interferes with some soil processes. low, wet soil in Minnesota

Wet soils tend to accumulate organic matter (plant and animal remains that are only partially decayed). Up to a point, this is good, but too much is as bad as not enough – notice how roots refuse to go into the acid, wet subsoil. low, wet histosol in Minnesota

Floodplain soils are wet during and shortly after floods, but if they dry out in time, floodplain soils can be some of the most productive soils in the world. wet floodplain soil in Mississippi

Floodplain soils often have distinct layers made of material deposited during extra large floods. This new material can be very fertile and easy to work, or it can be unusable, depending on where it came from! floodplain inceptisol in Mississippi

So, here’s the first picture again. It’s actually an exception that proves the basic rule. This fertile soil formed on soft limestone rock in a hot, rainy climate. This is the kind of climate that is likely to make a red, infertile soil with most kinds of rock, unless flooded with water. Bottom line: soil is the result of long-term interaction of many factors in a particular place. In other words, it’s a geographic product! Black mollisol that gave the Black Belt of Alabama its name – see the multimedia unit on plantations.