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O horizon: An organic layer of fresh and decaying plant residue. <br />L horizon: A layer of organic and mineral limnic materials, including <br />coprogenous earth (sedimentary peat), diatomaceous earth, and marl. <br />A horizon: The mineral horizon at or near the surface in which an accumulation <br />of humified organic matter is mixed with the mineral material. Also, a plowed <br />surface horizon, most of which was originally part of a B horizon. <br />E horizon: The mineral horizon in which the main feature is loss of silicate clay, <br />iron, aluminum, or some combination of these. <br />B horizon: The mineral horizon below an A horizon. The B horizon is in part a <br />layer of transition from the overlying A to the underlying C horizon. The B <br />horizon also has distinctive characteristics, such as (1) accumulation of clay, <br />sesquioxides, humus, or a combination of these; (2) prismatic or blocky <br />structure; (3) redder or browner colors than those in the A horizon; or (4) a <br />combination of these. <br />C horizon: The mineral horizon or layer, excluding indurated bedrock, that is <br />little affected by soil-forming processes and does not have the properties typical <br />of the overlying soil material. The material of a C horizon may be either like or <br />unlike that in which the solum formed. If the material is known to differ from that <br />in the solum, an Arabic numeral, commonly a 2, precedes the letter C. <br />Cr horizon: Soft, consolidated bedrock beneath the soil. <br />R layer: Consolidated bedrock beneath the soil. The bedrock commonly <br />underlies a C horizon, but it can be directly below an A or a B horizon. <br />M layer: A root-limiting subsoil layer consisting of nearly continuous, horizontally <br />oriented, human-manufactured materials. <br />W layer: A layer of water within or beneath the soil. <br />Humus <br />The well decomposed, more or less stable part of the organic matter in mineral <br />soils. <br />Hydrologic soil groups <br />Refers to soils grouped according to their runoff potential. The soil properties <br />that influence this potential are those that affect the minimum rate of water <br />infiltration on a bare soil during periods after prolonged wetting when the soil is <br />not frozen. These properties include depth to a seasonal high water table, the <br />infiltration rate, and depth to a layer that significantly restricts the downward <br />movement of water. The slope and the kind of plant cover are not considered <br />but are separate factors in predicting runoff. <br />Igneous rock <br />Rock that was formed by cooling and solidification of magma and that has not <br />been changed appreciably by weathering since its formation. Major varieties <br />include plutonic and volcanic rock (e.g., andesite, basalt, and granite). <br />Illuviation <br />The movement of soil material from one horizon to another in the soil profile. <br />Generally, material is removed from an upper horizon and deposited in a lower <br />horizon. <br />Custom Soil Resource Report <br />43 Index #10