What Makes Soil, Soil?

A profile of a vertisol soil

Soil has texture

The particles that make up soil are categorized into three groups by size: sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Although a soil could be all sand, all clay, or all silt, that's rare. Instead most soils are a combination of the three.

The relative percentages of sand, silt, and clay are what give soil its texture. A loamy texture soil, for example, has nearly equal parts of sand, silt, and clay.

Soil has structure

Soil structure is the arrangement of soil particles into small clumps, called "peds". Much like the ingredients in cake batter bind together to form a cake, soil particles (sand, silt, clay, and organic matter) bind together to form peds. Peds have various shapes depending on their “ingredients” and the conditions under which the peds formed: getting wet and drying out, freezing and thawing--even people walking on or farming the soil affects the shapes of peds.

Ped shapes roughly resemble balls, blocks, columns, and plates. Between the peds are spaces, or pores, in which air, water, and organisms move. The sizes of the pores and their shapes vary from soil structure to soil structure.

Texture + structure = soil behavior. A soil’s texture and structure tells us a lot about how a soil will behave. Granular soils with a loamy texture make the best farmland, for example, because they hold water and nutrients well. Single-grained soils with a sandy texture don’t make good farmland, because water drains out too fast. Platy soils, regardless of texture, cause water to pond on the soil surface.

Soil has color

Color can tell us about the soil’s mineral content. Soils high in iron are deep orange-brown to yellowish-brown. Those with lots of organic material are dark brown or black; in fact, organic matter masks all other coloring agents.

Color can also tell us how a soil behaves. A soil that drains well is brightly colored. One that is often wet and soggy has an uneven (mottled) pattern of grays, reds, and yellows.

What's Soil Got to Do with... Food?

Screen shot of video about soil and food

... Water?

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... Medicine?

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