Hydric Soils

If you can squeeze a fist full of soil into a ball, you've got soil with no room for much oxygen. And that's bad news for most plants, because oxygen is an essential part of their respiration. They absorb the oxygen from the soil through their roots.

Cracked soil often indicates that it was once wet soil. Photo: © Carolyn Duckworth 2004

These oxygen-starved (anaerobic) soils often smell like sulfur or rotten eggs due to the bacteria that thrive under anaerobic conditions. The dampness also causes chemical reactions in the elements of the soil; for example, iron will oxidize and mottle the soil with orange.

Scientists recognize two major types of wetland soils: organic and mineral. Organic soil has an obvious amount of decomposing plants; this kind of soil is often black or dark brown. Mineral soil contains few decomposing plants; instead, it is comprised of materials such as clay, sand, or silt. Wetland mineral soils may be gray, greenish, or bluish-gray; they also might be mottled with orange or red streaks.

Lucky for scientists, color charts exist that key soil color with the amount of water in the soil (such as the Munsell Soil Color Charts). The U.S. Natural Resource Conservation Service (NRCS, formerly the Soil Conservation Service) also publishes a list of hydric soils http://soils.usda.gov/.

Soil content also determines the speed of draining. For example, water seeps through sandy soils faster than clay soils. Sand particles are large and irregularly shaped; they have more air pockets through which water can move. Clay particles are smaller and they can compress when wet; their smaller air pockets fill more quickly and completely.

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