SOIL PROFILE DESCRIPTION Soil – 206 Soil EcosObjectivesIntroductionSoil TextureSoil StructureSoil ColorHorizonsSOIL PROFILE DESCRIPTION Soil – 206 Soil Ecosystem Lab Objectives After completion of this lab a student should be able to: 1. Define the terms soil profile, horizon, texture, structure and concentration. 2. Describe how textural class is determined using the feel method. 3. Understand how to use the soil textural triangle. 4. Identify and describe four structure types and indicate their probable location in a soil profile. 5. List 5 soil colors and give a possible cause for each color. Introduction The word soil, in a general sense refers to all of the unconsolidated material occupying the earth’s surface. Soil is a mixture of varying proportions of inorganic mineral and rock particles, living and organic matter, and voids or pores which contain variable amounts of air and water. It develops at the interface between the atmosphere and lithosphere (bedrock), forming a blanket ranging in thickness from a few centimeters to two meters or more. Soil is the medium from which most plants derive mineral nutrients and water. Soil also provides physical support for both plants and animals including humans and the structures they build. As you proceed through these lab exercises, keep in mind that a soil is not an inert, unchanging material. Rather, at any one time, a soil may be undergoing many simultaneous physical, chemical, and biological changes. A distinction may be made between the soil (in the general sense) and an individual soil body. An individual soil body, called a polypedon by soil scientists, is a three dimensional body with definite recognizable boundaries. Its upper boundary is the earth’s surface, and its lower boundary is the lower limit of biological activity and weathering. A polypedon is bounded laterally by other soils with properties different from those of the polypedon being considered. Thus, the general term “soil” is actually a collective term for a large number of individual soils, each having its own distinguishing characteristics. Refer to p. 198 in Gardiner and Miller, 10th edition, for further discussion of the pedon/polypedon concept. The concept of the polypedon comes from the need to study and communicate information about soils in a systematic manner. To establish a polypedon, soil scientists first determine the kind and range of soil properties that characterize each soil. Second, each soil is assigned a name, such as Palouse silt loam, or Jory clay. This system permits division of “the soil” (again in the general sense) into many separate and individual units. In a manner similar to plant identification, a specific soil may be referred to by a common name or by a taxonomic name. For example, the taxonomic name (genus and species) for a tomato plant is Lycopersicon esculentum. Similarly, the taxonomic class (at the family level) for the Palouse soil loam is Fine-silty, mixed, mesic Pacheco Ultic Haploxeroll. As just implied, an individual soil body occupies a certain definite section of the landscape. Soils vary from one another in their properties and each has a unique internal organization. A soil profile is a Spring 2006 1single vertical cross section of soil extending from the surface into the underlying unweathered parent material. The soil profile is composed of horizons (horizontal layers of soil) which may be characterized by physical, chemical, and biological properties. Horizons may be divided into major categories corresponding to the surface soil (O and A horizons), the “subsurface” soil (E and B horizons), and the substrate (C and R horizons). Three of the most basic physical properties used to describe soil profiles are texture, structure, and color. The following exercises are designed to give you some experience in describing these properties. Soil Texture Soil texture is defined as the relative percentage of sand, silt and clay in a soil sample. Hence, soil texture is concerned with the size of individual mineral particles. A major division in the size of soil particles is made at a particle diameter greater than 2 mm. Soil particles with a diameter greater than 2 mm belong to the coarse fraction while soil particles less than 2 mm belong in fine earth fraction. The fine earth fraction itself is divided into three main size classes, according to the U.S Department of Agriculture: Classification of Soil Particles by Maximum Diameter US Department of Agriculture 0.002mm 0.05mm 0.10mm 0.25mm 0.50mm 1.0mm 2.0mm >2.0mm Very Fine Fine Medium Coarse Very Coarse Clay Silt Sand Gravel It is unlikely that a soil will consist of mineral particles of a single size category. Normally a soil will contain some combination of sand, silt and clay in addition to other organic and inorganic constituents. Soils having similar proportions of sand, silt, and clay are grouped into one of the twelve textural classes. The textural triangle is designed so that any combination of sand, silt and clay can be placed in a textural class and assigned a name. Texture is probably the single most important physical property variable determining such fundamental soil properties as fertility, water-holding capacity and susceptibility to erosion as well as its influences on drainage, aeration, plant available water, ease of tillage, and the chemical and physical condition of the soil. Differences in many of these properties among soils can be attributed to the strong dependence of texture on soil mineralogy. Quartz, feldspars and micas dominate the sand and coarse silt fractions, while the much more reactive oxides and clay minerals dominate the clay and fine silt fractions. The importance of texture as a fundamental soil property is further emphasized by its relative permanence. Soil texture can change only over very long periods of time through erosion, mineral weathering or translocation of particles through the soil profile. Spring 2006 2The following table indicates the effect of texture on some soil properties (Soils, Dubbin, 2001). Property Textural Class Clay Silt Sand Water-holding Capacity High Moderate Low Drainage Rate Slow (unless cracked) Moderate Fast Water Erosion Susceptibility Moderate High Low Wind Erosion Vulnerability Low High Moderate Cohesion, stickiness, shrink-swell High Moderate Low Inherent Fertility High Moderate High Ease of Pollutant Leaching Low