What Is Dirt, Anyway?

By: Julie Day

“What is dirt?” – Joshua

This was one of the first questions I ever received on “Ask Julie,” and for a long time I sat it aside. But recently I went to my local landscape supply yard for some blended topsoil and asked about the ingredients, and I was told: “It’s a mixture of leaf compost, manure compost, mulch compost, creek sand, and screened dirt.”

I drove away thinking, “If all those things are NOT dirt, then what exactly IS dirt, anyway?” Turns out, dirt – or soil – is a mixture of:

  • Rock fragments: of varying sizes, from stones down to sand and silt granules.
  • Clay: made up of sticky mineral particles that tend to hold water.
  • Humus: decomposed plant and animal matter.
  • Living creatures: such as worms, insects, fungi, and microbes—that consume and contribute to soil makeup.
  • Water: from rain along with dissolved nutrients.
  • Air: mostly nitrogen and oxygen.

The ideal soil composition is 50% solid matter, 25% air, and 25% water. The makeup of the solid matter depends on the region, which is why some areas have:

  • Clay soil, which holds water and can drown some plants. It also hardens to a bricklike consistency during drought.
  • Sandy soil, found near the coast, which drains water rapidly and is best suited for drought-tolerant plants.
  • Loamy soil, which is crumbly, humus-rich, and considered to have the ideal proportion of ingredients for planting.

The native soil in my area tends to be high in clay, so the landscape supply yard blended in some sand and compost to give their topsoil a more loamy texture.

For more information and examples, check out our video on Soil Evaluation.

Julie

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