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Soil taxonomy is the practice of describing, categorizing, and naming soils. Like the taxonomy of living organisms, soil taxonomy is designed to make it easier for people to communicate information about different kinds of soils, how they are used, their properties, and where they are found. The United States Department of Agriculture (USDA) has developed a complex soil taxonomy system which is widely used, and the organization publishes keys which can be used to identify soils, as well as mediating when disputes about the taxonomy of various soils arise.
Under the USDA's soil taxonomy system, soils are organized into orders, suborders, great groups, subgroups, families, and series, with orders being the broadest category, while series are the smallest. Some examples of the 12 orders in the system include: Gelisols, Oxisols, Vertisols, Aridisols, and Inceptisols.
When soils are taxonomized, their composition is a key feature, but scientists also evaluate their location, and factors like the climate where the soil is found. Features like permafrost can be important to soil taxonomy, for example, as can extreme dryness or humidity. Soil composition is based on a number of factors, including minerals in the area, decayed organic material, underlying geology, and so forth, and these influences can be quite diverse, with thousands of soil types recognized under the USDA system.
A number of functions are served by soil taxonomy. The ability to use taxonomic nomenclature is critical to people when they want to communicate with each other about soils, as rather than using a term like “loose, loamy soil,” they can select the appropriate series using a key, precisely communicating the details of the soil type in a name. This is useful in the preparation of environmental reports and a wide variety of other documents, allowing someone anywhere in the world to immediately understand the soil conditions in a given area when they are described taxonomically.
Soil taxonomy is also important because it creates a framework which people can use to understand soils. The hierarchical organization can be used to examine the relationships between different types of soils, for example, and researchers can use this information to explore geology, agricultural techniques, and a wide variety of other topics. Soil scientists use soil taxonomy extensively in their work, to do everything from describing the soil in someone's back yard and discussing the implications for gardening to exploring the loss of topsoil as a result of heavy winds, desertification, or flooding.
@umbra21 - There are a lot of little tests you can do to identify what kind of soil you have on your property, but if you are serious about growing things you might have to send samples away to a lab.
This is usually not that expensive and it will give you real results, including levels of soil nitrogen, the soil acidity and so on.
You might also want to ask around your area, or even take a look at the different plants that seem to be growing well. There's no guarantee that you have the exact same soil profile as your neighbors but it's pretty likely.
They will be able to tell you which plants they have had no success with and which ones flourish without any attention at all.
I remember in my geology class they had a couple of field trips where we had to go out and classify different kinds of soil in the landscape around the university. Although true classification needs special equipment, they had us do it like we could at home. Like, if you squish a bit of soil in your fist and it sticks in a tight ball, it contains more clay. The more loosely it sticks the more sand it contains.
You could also tell things from the color of the soil, how gritty it felt between the fingers, what it looked like when a bit of turf was cut away and so on.
Once you know a few of the more simple tests it can help you to identify the soil profile in your backyard and the best plants to grow there.