Introductory Soils Lab 2. Soil Field Trip NRES 201 Page 1 of 8 LABORATORY 2 A: SOIL FIELD TRIP Examination of soils in the field provides information about the soil in addition to the profile description. Slope and landscape position often determines the soil's internal drainage, the thickness of the surface horizons relative to other soils in the landscape, as well as the sequence of horizons found in the soil profile. Soils that occupy the same type of landscape position and slope often share similar properties and are classified the same even when separated by great distances. In Illinois, somewhat-poorly drained soils are often found on very broad, flat, level areas that are basically in equilibrium with precipitation, that is these sites are not receiving nor losing large amounts of water due to runon or runoff of precipitation from other site positions. Soils forming on sloping sites that lose water due to runoff are generally well drained and moderately well drained, while soils occurring in depression areas or other sites that receive water that is running off other landscape positions are generally poorly or very poorly drained. Soils that occur in a regular pattern on a landscape that differ in drainage due to landscape position are referred to as a toposequence or a catena. Flanagan Catlin Drummer Somewhat poorly drained Well drained Poorly drained Aquic Argiudolls Oxyaquic Argiudolls Typic Endoquolls 2a.1 SITE POSITION This category describes the landform on which the site is located. Site position is often closely related to the parent material from which the soil profile developed, i.e., alluvium on a flood plain. 1. Flood Plain - The lowest level of a stream valley, often referred to as the first bottom, alluvium is parent material 2. Stream Terrace - Old bottoms of present or past streams higher in the landscape than the flood plain. 3. Upland – Generic term for other landforms on which soils are usually developed in loess, glacial till, glacial outwash, or residuum from bedrock.Introductory Soils Lab 2. Soil Field Trip NRES 201 Page 2 of 8 4. Foot Slope - This refers to the position at the base of a slope where colluvial material accumulates. 5. Lake Plain - Sediments deposited in the bottom of past lakes. Lacustrine material tends to be very fine and sorted with the coarse material near the old shoreline and the finer material near the center. 6. Till Plain – Nearly level to sloping deposit of glacial till 7. Loess Plain - Nearly level to sloping deposit of loess, may overlie glacial till, lacustrine, outwash, or residuum etc. 2a.2 SLOPE Slope is determined using an Abney level. It is usually determined by sighting from one stake to a second stake of the same height a few meters away. Slope relates to the potential for erosion of a soil, the amount of runoff, and potential for ponding. The better drained soils in the landscape generally occur on sloping sites, while the worst drained soils in the landscape most often occur at the base of slopes or in depression areas. 2a.3 SOIL TAXONOMY The modern soil taxonomy system (USDA) is based on the classification of soils from properties that can be observed in the field. Pedogenic processes effect the classification of soils in the system since they determine many of the soil properties, but they are not the dominate differentiating property that they were in older soil classification systems. Soils are classified by the presence or absence of special types of pedogenic horizons called diagnostic horizons. For example, soils are classified into different orders based on the type of A or B horizons present. Diagnostic Surface Horizons. Diagnostic surface horizons are called epipedons, meaning the surface of the soil. These are special types of A horizons. 1. Mollic epipedon – A thick dark colored high base saturated (> 50%) mineral surface (A) horizon. Specifically this horizon has sufficient soil structure that it is not massive nor hard when dry. The colors include chromas and values of 3.5 or darker when moist and values darker than 5.5 when dry. This epipedon is common in Illinois and is associated with prairie ecosystems. The mollic epipedon must be more than 10 cm thick if lying directly over hard rock. If the soil contains a B horizon (i.e., an argillic, natric, or cambic diagnostic horizon), fragipan, or duripan, the mollic epipedon must be at least 1/3 the thickness of the solum when the solum is less than 75 cm thick and at least 25 cm thick when the solum is thicker than 75 cm. The mollic epipedon must contain less than 18% organic carbon or it becomes a Histic epipedon. It must also contain less than 1500 ppm of citrate extractable P2O5 or it becomes an Anthropic epipedon. 2. Histic epipedon - Strongly developed organic (O) horizons of mineral soils. The horizon is from 20 to 60 cm thick. Usually formed on soils that under natural conditions are saturated with water at least 30 days per year. Organic carbon content ranges from 12 to 18% as a minimum depending upon clay content. 3. Umbric epipedon - Epipedons that meet all the requirements of a mollic epipedon except that they are too low in base saturation (i.e., too acidic).Introductory Soils Lab 2. Soil Field Trip NRES 201 Page 3 of 8 4. Ochric epipedon - Surface (A or O) horizons that are too light in color, too low in organic matter or too thin to be a mollic, umbric, anthropic, plaggen or histic epipedon. This epipedon is common in Illinois and is associated with forest ecosystems, particularly in the south. 5. Anthropic epipedon - Surface horizons with the characteristics of a mollic epipedon, but formed due to the activities of man. Extractable P2O5 is > 1500 ppm due to anthropogenic activities. 6. Plaggen epipedon - Man made surface horizon, produced by long term manuring, often thickened by the addition of soil. Artifacts are common. Common in parts of Western Europe. Diagnostic subsurface horizons. Mineral horizons that form below the surface, although they may be exposed by erosion or other processes. 1. Argillic – (Bt) An illuvial horizon (a zone of gain) in which the illuvial material is silicate clay minerals. Since these horizons form due to the movement of clay minerals, they often have coatings of clays (clay skins) on ped faces and pore surfaces. a. If any part of the eluvial horizon (zone