Types Of Dirt
A large-scale dirt screening machine
There are different kinds of dirt, it turns out:
Topsoil is the very top layer of dirt. It has lots of organic matter in it, from the plants and dead bugs and such lying on the surface. This makes it nutrient-rich. Topsoil is also called black dirt because the organic matter makes it darker. It often has some clay in it, so it will be “sticky.”
Fill dirt is from the next layer down. It has no organic matter in it – plant roots don’t grow that deep, and bugs don’t dig that deep. Fill dirt is lighter in color than topsoil, and because there’s no organic matter to break down, it’s more stable; it will settle less over time.
Screened dirt is fill dirt that’s been fed through a screen to make it finer and more granular by breaking down the chunks (a screening machine is pictured). Screened dirt is easier to contour, and it will settle even less than fill dirt which hasn’t been screened.
Notably, “potting soil” is not dirt. It’s mostly peat moss and pine bark. It’s often mixed with fertilizer and is designed to retain moisture.
Why I Looked It Up
I had a new sidewalk installed, and I had to fill in the trenches cut around it. I went to an aggregate materials place, and they told me “the fill dirt is out back, and the screened dirt is further back.”
I had no idea what they meant, but when I got back there, it was clear that fill dirt and screened dirt were two different things. The fill dirt was big and chunky, and the screened dirt was finer, almost like sand. (This is when I took the picture at the top of this article.)
Once I got the trenches filled in, I planted grass seed. I needed some dirt to go over the seed, so I went to Menards and bought some bags of “top soil,” utterly ignorant that it was different than fill or screened dirt. However, when I placed the dirt over the seed, I did notice that it was darker and stickier and had more stuff in it.
I normally have terrible luck growing grass. But – to my amazement – the grass grew wildly. This got me thinking that I did something different this time, and I set out to figure out what it was.
I’m convinced it was the topsoil, which I now know has a bunch of organic matter in it that’s optimal for growing things.