First of all, let’s get one thing straight. If you want to speak in agronomic terms, you’ve gotta use the term "soil." "Dirt" as a term is okay if you are speaking of cleaning house or trying to start a rumor about someone. But, if you want to impress agronomists, you must use "soil" as when referring to the stuff in which plants grow and develop into cash crops.
Now, why is soil so important? Well, it’s just the medium into which plants put their roots. You probably are already aware that there are different types of soil, but here’s the technical explanation. (You knew it would have to come to this sooner or later. After all, this is science.)
Soil type is defined by the amount of sand, silt, or clay present in a given sample as well as the texture or how it feels. It is produced by weathering of rock over a period of time. Five main types of soil may be identified and are defined by composition.
Humus is the dark, moist layer found on the top of a soil profile. This is because it is made up of dead and decaying matter. It is fairly fertile in that the decay process adds nutrients to the soil that plants love to soak up.
Gravel is made of, well, gravel. Particle size is anything larger than 2.0 mm. In other words, it contains highly visible rock particles or pebbles
Sand is the stuff you see at most beaches. In terms of texture, it is made of large particles, 0.02 mm - 2.0 mm in size. Sandy soil has less than 20 percent silt and/or clay. Water drains through sand very quickly.
Silt has particles that measure from 0.004 mm - 0.006 mm. The grains in silt look like tiny pieces of rock. Silt will generally float on the top of a layer of water and will take time to settle out of the mixture.
Clay is the stuff that pottery is made of. In terms of texture, it is made of particles smaller than sand, less than 0.002mm in size. Clayey soil is made of at least 30 percent clay particles. Since it is composed of the smallest particles, it will take the longest to settle out of solution. Also, because of the small particulate size, water tends to puddle on clayey soils. (Don’t you just love that word, "clayey?")
A soil profile of any given position may contain one or all or any combination of the above. (Check out Soil Recipes for an activity that shows this diversity in your area.) Since there is no single soil type in all fields, even within one field, agronomists must know their growing area in terms of soil type, water retention, and nutrients available.
In the Southeastern Coastal Plains of South Carolina, agronomists must learn to coax the best potential from each crop they plant. Soil properties must be understood along with the needs of the plants in order to produce crops. There are people that are paid to research what it takes to do this. They work for the USDA in the Agriculture Research Service. Really, folks, I’m not making this up. The science of agronomy has become so specialized that these research scientists are incorporating the latest technology in the effort to allow farmers to be more effective in the growth of their crops. Questions that farmers want answered as to type of soil in the field as well as how well that soil is holding onto available water can now be answered with the use of new technologies in global positioning systems (GPS), variable-rate (VRT) controllers, on-the-go yield monitors, geographic information systems (GIS), and PC’s. This effort is being termed site specific agriculture or precision farming.
USDA-ARS. Nitty Gritty Soil Facts: The Dirt on Dirt. June 2009.
Krisanna Machtmes, LSU AgCenter