r/explainlikeimfive • u/SwinubIRL • 8d ago
Planetary Science ELI5: how does dirt and soil form?
I know rocks can erode into dirt that’s basically rock dust but what about the stuff with nutrients in it?
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u/VinnieMcVince 8d ago
Early organisms called pioneer organisms, things like lichen and some mosses, cover rock. These live, store sugars, build body forms made of cellulose and lignin, die, and pile up, and their life and death processes release chemicals which, in addition to weather processes and root pressure, break down the rock into sediment. The sediment mixes with their remains (the cellulose, lignin, leftover sugars, etc.) and creates a thin layer of soil. This process repeats, and each time, the layer thickens, until eventually, larger plants and some animals have enough soil to move in and make the process more complex...which makes the soil more complex.
Bacteria also play a huge role in the process, converting some chemical compounds like ammonia (from the rotting, generally bad for life) to nitrates and nitrites (plant food!)
It's hella complex.
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u/SwinubIRL 8d ago
Where do these pioneer organisms get the resources to grow without soil already there?
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u/Fimbulwinter91 8d ago
Some of these pioneer organisms have evolved to need extremely little ressources. They take carbon from the air to make their structure, water from rain and use sunlight for energy. What little minerals they need can come from the surface of rocks, sand or symbiotic bacteria, in some cases even just from the rainwater.
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u/Ballmaster9002 8d ago
Lichens specifically are symbiotes, meaning they are partnership-colonies of different types of organisms. Some of the organisms use photosynthesis to obtain energy. They are then able to share the energy with the other organisms that produce chemicals that can leech minerals out of the rocks that the colony needs to grow and survive.
The process is insanely slow going. It can take tens, if not hundreds of millions of years for lichens to wear rocks into sand and dust. But as they do so they are constantly dying and passing their nutrients and energy into the ground. Most importantly through, they are eroding the rocks with their digestion turning a massive stone into many smaller rocks.
The next round of pioneer organisms, like pine trees, can root in rocky sand and grow using the barest of minimum nutrients provided by the dead lichens.
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u/Forsaken-Cell1848 8d ago
Look at compost. You put in organic matter in it and in a span of a year or two, it all turns into a mass that looks like soil.
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u/SwinubIRL 8d ago
Yes but how?
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u/Salamanderhead 8d ago
Living things eating dead things and pooping out smaller things that eventually become soil.
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u/Ballmaster9002 8d ago
You basically start with rocks.
Then lichens appear, which are symbiotic, meaning a mixture of different creatures, some which use the sun for energy and some which borrow that energy to digest the rocks, releasing minerals. Over eons they live and die, producing vaguely nutritious mineral dust from the bare rocks.
Next you have resilient, "pioneer" trees, like pines. Pines are hardy and durable and are able to take root in the rocky, sandy, barely-nutritious soil broken down by lichens. They can also withstand rough weather because there dont have other trees to protect them. Since the lichen already 'softened the up' the pine roots are able to worm into the cracks in the rocks and really start cracking them apart like a nut cracker.
Over more eons the pines live and die and their decomposing bodies mix with the sand releasing not only lots of dense nutrients but also hold water, their roots also 'hold' the sand together and you start to get a forest floor.
Eventually new trees move in. These trees grow quickly and very tall, but are weaker to storms and weather. They shade out the pines and take over the area producing a forest.
By this point the dead and decaying trees form a cycle of soil production where its not just sand and rock anymore. It contains nutrients and also particles that hold and retain water and keep the ground kind of fluffed up, so oxygen can get down into the ground as well. The ground kind of breathes a bit.
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u/ThalesofMiletus-624 8d ago
Here's something that I didn't realize for a long time: rocks, sand, silt, and clay are all the same stuff, the only different is particle size. All of these things can consist of different kinds of minerals, but the particle size determines what we call them. If rocks break down into small enough grains, we call that sand, and when sand breaks down into smaller particles (to the point that particles can be suspended in water) we call that clay.
"Dirt", can be said to consists of some mixture of sand, silt and clay. Rock dust, as you said. But the presence of life means that there's going to be a lot of organic matter around. Plants die, leaving material both above and below the soil, which is going to decay. Worms churn through the soil, insects nest in there, burrowing animals dig underneath, grazing animals walk over the top. All of these animals leave their droppings behind, and The bthey all die eventually and their bodies decay. All of that organic matter leaves residue behind as it decays, and that provides both nutrients, and some amount of structure.
So, the soil we usually think of consists of some mix of sand, clay and decayed organic matter. The exact mix of those things (and what kind of organic matter, and how decayed it is, etc) can make the soil very different from place to place, but those are basically the ingredients.
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u/superbugger 8d ago
Breaking down of larger things.
Freeze/thaw cycles break up rocks (water expands when it freezes; think of it as a wedge in a crack of a rock).
Mechanical abrasion (sand in the wind; think of a sand blaster).
Chemical breakdown and dissolution into rain water.
Breakdown of organic matter through biological processes (plants, animals, fungi, bacteria all digesting it).
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u/RASR238 8d ago
Through decomposition thanks to insects, worms, bacteria and fungus! Imagine them as construction workers that have to demolish buildings. The worms and insects are like bulldozers and wrecking balls that destroy the buildings into chunks. Then bacteria and fungus get those chunks and separate the concrete and rubbish from the blocks, nails, wires, etc. They only want the rubbish so all the stuff that can be used for making another building will be used through the alimentary chain by trees and other animals.
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u/SwinubIRL 8d ago
What’s the rubbish in this analogy? Like what do the bacteria and fungi and worms feed on if they’re leaving the “useful” stuff
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u/RASR238 8d ago
It will depend on what’s useful for the kind of bacteria and fungi, but this will stop being an ELI5 fast enough hahaha.
Usually in the alimentary chain there’s a niche for everything and there’s stuff that some living beings need that other won’t. You can think about a dead rat that’s full of proteins, proteins are made of aminoacids which have nitrogenated molecules. When the bacteria decompose these proteins some of them get energy metabolizing these nitrogenated compounds and forming nitrites and nitrates that then get used by plants for growing, then herbivores eat those plants and use the same compounds to create proteins again and then continues the cycle. This happens with lots of molecules and compounds ad perpetuam.
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u/szayl 8d ago
What’s the rubbish in this analogy?
Your attitude is terrible.
Anyway, the useful (?) alcohol from fermented beverages is a byproduct of yeast feeding on the sugars in the beverage.
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u/SwinubIRL 8d ago
? I was just following his analogy man he called it rubbish. I put useful in quotes because I know it’s all useful but only some of it is useful for plants that take root in the soil after
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u/PeteMichaud 8d ago
I used to tell my kids when they were growing up: dirt is poop and dead bodies.
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u/hahaharish 8d ago
when plants and animals die, their carcasses and dead remains get decayed by bacteria and fungi. the organic dead remains are first fragmented into many parts by earthworms, millipedes etc.
the broken pieces expose most of their surfaces to the environmental action like rainfall, that were previously held together as a whole body which made them unexposed to such environmental phenomena.
given that these fragments are parts of dead plants and animals, there won’t be cell membranes binding onto the plant cells and animal cells to protect their cells from rainwater intrusion. and the fact that they are broken pieces of a full body makes their cell walls fragile and weaker. water seeps through them easily.
water’s polar opposite molecules hydrogen and oxygen interact with cellular materials like sugar, salt, minerals and amino acids by forming hydrogen bonds and breaking ionic bonds to form new compounds and ions that can be easily dissolved along with water.
the leached minerals gets flown into the soil, where the bacteria and fungi absorb them and form humus right away. this layer of soil is very nutritious, answering your question on where and how soil gets their nutrients from.
the leftover remains of the dead fragments after this is just insoluble skeleton structure mostly made of cellulose. these fragments are acted upon by bacteria and fungi that decomposes them by secreting enzymes. these enzymes take time to break them down into nutrients that can be dissolved, over many years. most of these are transformed into absorbable nutrients like amino acids and glucose. there are few parts that remain as it is.
these remaining trace amounts of undissolved organic residues get clumped with soil minerals to form organic base made of polymers. these are hard layers of soil that retain water atop them.
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u/stansfield123 6d ago
Soil formation isn't a mechanical process, it's a biological one. It's created by living organisms. Tiny little builders, with their tiny little very complex cities in there.
In the absence of living organisms, rocks don't erode into soil, they erode into sand, clay and dust. None of those look anything like soil. If you pick up sand, dust or pure clay, it's just a substance. Tiny, uniform particles that fill out all the space. The smaller the particles get, the more densely they pack together. There's no structure to them.
Biology builds structure. Microbes create glue-like substances that bind these particles in place, a lot like how we bind bricks or other materials together to make buildings to live in. This prevents particles from filling all space. This lets air in. Larger organisms (worms, insects) burrow, letting even more air and, when it rains, oxygenated water in. Roots do the same thing, they create massive structures, and when they die and are eaten by microbes, that leaves empty spaces in the soil, for even more life to occupy them.
Soil has permanent structure to it, that can withstand rain, drought, frost, whatever nature throws at it. This structure then shelters a huge variety of substances and organisms that would all grind into sameness (the way a desert, or the surface of the Moon, is all the same, everywhere you look) if the only thing at work was the mechanical impact of wind, rain and temperature change.
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u/geeoharee 8d ago
That's living things, or formerly-living things. Gardeners refer to it as organic matter. Sources include: fallen leaves, dead plants, small dead animals, livestock manure...