It turns out that soil is almost all rock (trust me, I did the research).
Even the good stuff - dark brown, thick, and fluffy, is still about half rock. What a scam! All those bags of garden soil you bought from Home Depot over the years have, this whole time, been mostly just rock. Isn’t that my problem though? The plants won’t grow in the backyard because the ground is too rocky. But Home Depot then sells me more rock to fix my rock problem? Seems like a racket, but it does work so I shouldn’t complain.
The nerds with funny glasses and microscopes tell us there are three types of rocks that can make soil: sand, silt, and clay.
The first type of soil is sand. This too is insulting because I’m an adult and I know that sand is sand and not soil. But we all have to pick our battles, and they are the nerds.
They also say that silt is soil, which sounds way more right than sand because silt has an “i” and “l” just like “soil” so it’s a lot more like the same word. Unlike sand, silt is much finer. If you remember your Prequel Star Wars, Anikan Skywalker hates sand. Like a lot. Sand is all coarse and rough and irritating and gets everywhere. But not like silt, silt is softer and smoother.
But even silt’s got nothing on clay. You may think of clay as heavy stuff you make pots out of or really dry cracking stuff, but clay is actually the cooler brother to silt. Clay is super duper smooth and really fine. The most fine. Clay gets all the ladies. If clay is silt’s cooler brother, sand is the weird cousin with his glasses on upside-down, too thick for his own good.
To summarize, rocks from biggest to smallest are: the earth (obviously the biggest rock we have), then maybe Africa which is also pretty big, then boulders, then stones, then pebbles, then sand, silt, and clay.
Soil is therefore classified as the smallest forms of rocks, just the sand, silt, and clay. And soil can be any combination of the three. You can have ½ silt and ½ clay, or ¼ silt and ¾ clay, or ⅓ each sand, silt, and clay, or all sand. I still think sand included as a type of soil is generous, again, because we are all adults here and we know that crops don’t grow on sand dunes or sandy deserts. But, it also turns out to be true that very little will grow in pure clay either. Clay turns out to be too fine on its own just like sand is too coarse. Silt is the goldy-locks of soil size but even 100% silt is not the ideal form of soil.
What home depot is actually selling you (even though it is about half rock) is the culmination of hours of funny glasses pushed up against microscopes. They (Home Depot) have crafted the perfect combination of sand, silt, and clay to “Get More Done” in your garden. About half of the rock ends up being sand with another quarter each of silt and clay. This combination takes advantage of the unique characteristics of each size soil and combines them together for optimal performance. Sand is big enough to create air pockets and spaces for water to seep into. The silt and clay pack tighter together and fill in some of the spaces between the sand grains keeping water from moving too fast through the soil.
It also turns out that healthy soil is about 20% water and 20% air, which, if you think on it, makes good sense. Soil without air would have to be very very compressed - kind of like when your recipe asks for compacted brown sugar. The volume of brown sugar in the bag is partially air until it is compacted in a measuring cup and reduces in volume. the same operation applied to soil would leave very little room for water. When soil is not compacted, it acts like a sponge and can absorb water into all the interstitial spaces.
The final critical ingredient in healthy soil is organic material: dead plants and animal waste. This is the dead stuff that makes soil alive. Any soil with a large amount of organic matter will attract decomposers. To compose, like composing a symphony, is the process of putting together. Decomposition breaks things apart and decomposers are the breakers. Earthworms are a classic example, if your soil has worms it’s probably doing well. Many other decomposers are microorganisms that fall into the category of bacteria or fungi. In general, healthy soil needs plenty of dead stuff and an army of decomposers to break it all down.
The last thing I’ll say about soil is that order matters. Maybe it is similar to baking, you can have all the right ingredients in all the right amounts and still mess up your cake if you don’t separate the wet and dry ingredients or if the butter is too hot. With soil, you need the right components in the right proportions and they all have to be mixed together properly to be useful for growing crops.
A simple example would be if you can imagine standing on the bank of a small creek. Obviously water is in the creek, and you are standing there breathing the air. You can also see that you are standing on a sandy part of the shore while the bank on the other side is wet and compacted, rising up above the water line: clay and silt. There are plants growing in places, there are also broken tree branches hung up on rocks in the water. You hear a bird chirp and see it fly across the water with a worm in its beak. And, to top it all off, you realize you just stepped in a tall, warm pile of deer crap. This scene has all the necessary components of good soil, but they aren't mixed.
Or imagine a farm where the farmer has perfectly apportioned his sand, silt, clay, and crap. The field is laid and tilled, and right before seeding he drives an earth compactor across the whole surface. All the right stuff is there, but it is doubtful any seed will take root.
That’s it for today folks.
No abstract analogy, just a good old fashion science lesson.
Have a great week.