“Soil” vs. “Dirt”

As a soil scientist I often hear non-soil scientists refer to soil as “dirt”, or people even ask me if there’s a difference. In my mind there is. The simple difference is this: “dirt is soil where it’s not supposed to be”. For example, if your dog goes outside and plays in your backyard, he’ll be frolicking, running, walking, and rolling over soil. In this ecosystem (your yard) soil is a crucial component of the system providing a medium in which plants can grow, worms can crawl, kids can play, etc. Once your dog is all dirty and runs onto your kitchen floor before you have a chance to wash him off, he’ll leave a trail of paw prints behind him. The stuff that is all over the dog, and now the floor (which was once soil in your yard) is now dirt, because it’s not supposed to be there.

Clear as mud, right?

Here’s the scientific definition of soil, courtesy of the Soil Science Society of America:

soil (i) The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of plants.

(ii) The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows the effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and micro-organisms, conditioned by relief, acting on a parent material over a period of time. A product- soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.

Part ii of that definition gets a little “sciency”, but it refers to the five soil forming factors (CLORPT: CLimate, ORganisms, Relief, Parent material, and Time) which I will discuss in a future post. The important thing to remember (definition i) is that it is unconsolidated material (not completely solid material like rock) on the surface of our planet that can sustain plant life.

I gave an example of how soil becomes dirt. Dirt can also become soil. Let’s say you wash your dirty dog in a big tub. When your done, all of the dirt (it wasn’t supposed to be on the dog, on your floor, or in your tub) has now collected at the bottom of the tub. So you dump it out onto your back yard. Now this dirt is part of a functioning ecosystem again where it is on the surface of the earth and able to sustain plant life and feed worms. It is now soil.

One more comparison: you wash off dirt, while you use soil. For example:

Dirt — When your pants are dirty from gardening all day you throw them in the washing machine which washes off the dirt.

Soil — When you grow some tomatoes in your backyard the tomato plant uses the soil as a source of water and nutrients.

Thus, unlike “dirt”, soil is useful.  Personally, I am not offended by the use of the word “dirt”. However, it is a bit like calling a clean, well behaved dog a “dirty mongrel” in my opinion… just food for thought. To reinforce that point, compare the origins of the word “dirt” versus the origins of the word “soil“:

Dirt — 1250-1300; Middle English dirt, drit; cognate with Old Norse drit excrement; compare Old English dritan

Soil — 1300-50; Middle English soile < Anglo-French soy < Latin solium seat, confused with solum ground

In other words, “soil” is soil, while “dirt” is crap!