For the most part from broken and crumbled-up rock, for this earth was once nearly all of rock.
But dirt or soil is not only rotten and broken and crumbled-up rock, as you will see in one minute.
Do you remember how the leaves fell off the tree last Autumn, and how the grass died down? What became of the leaves and grass?
They died, yes, and turned into leaf mold, which is one form of humus.
You have guessed right, Mary Frances. Humus is decayed vegetable matter.
What Humus Does
Humus mixed with water makes humus soup, which is the very best kind of plant food, and the plant babies love it. They drink it through their roots, you know.
Not only does humus help with the matter of food, but it holds moisture in the soil, and in some almost magic way makes other plant food into a form which the plant can use.
Of course, you wish to give your plants the best kind of food, and of course you will want humus.