Some places have rich, loamy, fertile soil that can grow almost anything; some places have hard, clay-filled dirt that roots have to struggle to break through; some places have barely any dirt and are mostly exposed rock. What drives this difference? Is it possible to look at a place’s geography and predict its soil type?
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Like the saying goes: “on a geological scale, everything you care about will happen quite quickly.”
If you view soil types as the product of *thousands upon thousands of years* of process and progress, it’s much more clear to see why certain areas have certain types of soils. Glaciers, lake beds, the tops and bottoms of mountains. Water washes things downhill, so higher areas tend to be more rocky. Forests continuously generate new debris for soil. Clay soils are interesting, there’s (broad terms here) a bit of a cycle of clay soils resisting the drainage of water, so you get lakes, and then lakes generate silt, which yields new clay, and then the whole thing dries up when the water that caused the lake finds someplace else to go and the climate changes.
And there are plenty of other factors, but the key takeaway is: over the last hundred hundred thousand years, how water behaved in an area is a good indicator of what type of soil the area will have. Again, broad terms, but that’s the gist of it.
Soil type is defined by something scientists call ClORPT (Climate, Organisms, Relief, Parent material, Time).
Climate: More precipitation means more water moving through the soil. Water deposits minerals which make the soil more fertile. Temperature affects how fast that water evaporates which determines how dry the soil will be. Both of these together help make it easier for things to grow in certain areas.
Organisms: Of course, if plants grow, then herbivores will be attracted to the area and carnivores will follow the herbivores. More animals in the area means more organic compounds and, when those animals poop or die, those compounds are then sitting on top of the soil. In addition to water pulling things down further into the soil, animals help these compounds integrate with the soil through movement. Digging, clawing, running, etc all move the soil around which helps all of the stuff at the top get mixed in. Especially smaller animals like worms are a huge factor in soil composition.
Relief: The terrain affects where soil builds up which affects how deep it is and much it can absorb. Soil will naturally be deeper at the bottom of a hill than at the top due to erosion.
Parent material: Soil forms in different ways. Some soil is formed directly from eroded bedrock. Some soil is the result of materials brought into an area by wind, rivers, glaciers, gravity, etc. Where the materials come from determines what the base of the soil is and how it will develop in the future.
Time: Soil forming over bedrock will take longer to form because the bedrock has to erode into soil-sized particles. Soil deposited by wind will already be smaller and so will grow plants faster. Younger soils tend to be more shallow. Depending on the ecosystem, old soil can sometimes be less fertile because all the nutrients have been used by plants and animals and then deposited somewhere else.
All of these things work in tandem to determine what a certain area’s soil will be like.
Soil is made of two things: tiny pieces of rock, and biological stuff both living and dead.
Rock gets weathered (broken into smaller pieces) and eroded (the pieces are moved away) by many different processes, like wind and water. Which processes happen, as well as what the rocks are actually made of in the first place, determine the type of pieces they end up as. Clay is really small pieces, and sand is small but not as small as clay, for example. The grains can also have different shapes, like spiky or smooth. Size and shape determine how compact the dirt can be, how much space for other stuff there is between the grains.
The biologic stuff is even more complicated. Plants grow roots, the roots die and decay, bugs live and die, bacteria do all sorts of stuff to all that, burrowing animals move it around, large animals eat the plants and defecate; all of this biological matter goes into making soil.
Different places have different soil because they have different rocks, different climates, and different ecosystems. One can indeed look at all of these things and predict the soil type, but only in very broad categories. Details can change a lot over a very small area. Even small farms can have different soil types in different parts of their field.
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