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 trial.
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 polyhedron 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 polyhedron is bounded laterally by other soils with properties different from those of the polyhedron being considered.
Thus, the mineral 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 Gardener and Miller, 1 10th edition, for further discussion of the Pedro/ polyhedron concept. The concept of the polyhedron comes from the need to study and communicate information about soils in a systematic manner. To establish a polyhedron, soil scientists first determine the kind and range of soil properties that characterize each soil. Second, each soil is assigned a name, such as Applause silt loam, or Gory clay.
This system permits division Of “the soil” (again n 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 Electronic esculents. Similarly, the taxonomic class (at the family level) for the Applause soil loam is Fine-silly, mixed, music Apaches Ultimo Haplology. 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 rifle is a spring 2006 single vertical cross section of soil extending from the surface into the underlying unwatched 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 mom 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. Mm 0. Mm Clay Silt 0. Mm m 0. Mm 0. Mm 1. Mm 2. Mm Very Fine Medium Coarse Very Coarse Sand >2. Mom 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 ND 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, feldspar and micas dominate the sand and coarse silt fractions, while the such 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 translation of particles through the soil profile. The following table indicates the effect of texture on some soil properties (soils, Dubbing, 2001 property Textural Class Water-holding Capacity Drainage Rate Water Erosion Susceptibility Wind Erosion Vulnerability Cohesion, stickiness, shrink-swell Inherent Fertility Ease of Pollutant Leaching Ease of Compaction High Slow (unless cracked) Moderate owe (unless cracked) Low Fast Two methods exist for textural analysis, particle size analysis (mechanical analysis) and the feel method (hand texturing).
One type of mechanical analysis, called the hydrometer method, involves dispersing a soil sample in water and determining the sedimentation rate of the sand, silt, and clay particles. This method will be addressed in a later laboratory session. The second method of textural analysis allows for the determination of the external class of a soil without the aid of laboratory equipment. This method, the feel method, is commonly used to estimate soil texture in a field situation.
An experienced person can accurately estimate the sand, silt, and clay content of a soil sample with this method. A soil containing large quantities of sand will feel gritty when rubbed between your fingers. Silt has been described as having the feel of flour. A soil high in clay will be somewhat sticky (depending on the type of clay and the moisture content) and can usually be molded like modeling clay. Sample number 3 sand % silt % clay Textural class Soil Structure Structure refers to the arrangement of sand, silt, clay and organic matter into larger units called aggregates.
Aggregates that form naturally are called peed, whereas those that form artificially, as during plowing or digging, are known as clods. Peed, which may range form one to several hundred millimeters in size, develop through soil-forming processes over decades and centuries as the soil matures. However, human interference can very quickly modify or destroy this structure. Soil Structure greatly influences water infiltration, susceptibility to erosion, and ease of root penetration and seedling emergence.
For these reasons, much effort has been directed towards understanding the factors that promote and maintain good soil structure. We will examine the chemical (biotic) and biological (biotic) characteristics of peed in future labs, so this lab section is focused on the identification and location of peed. The individual soil particles that comprise peed are held together by binding agents including organic matter, clays, calcium carbonate, and iron oxides and are described in terms of their shape or type. The following table is a brief introduction to the classification of peed (Dubbing, 2001, up 20-21).
Type Size Horizon Description Granular 1 to 1 Mom Also know as spheroid’s, rounded, common in soils with high organic matter, loosely packed Platy A and B Thin, horizontal, plate-like, common in leached horizons or at depth, maybe from compaction Blocky 5 to Mm Prism-like 10 to mm Roughly cube-shaped, found in sub-surface, facilitate good aeration, drainage, root penetration Vertical oriented columns, found in sub-surface, height and shape vary, often too dense for root penetration, tops an be rounded (columnar) and often associated with high sodium Soil Color Color can provide soil scientists and land users with many clues about the genesis and mineralogy of a soil, provided the observers understand the cause of the various colors and are able to interpret the colors in terms Of soil properties.
Information concerning organic matter content, mineralogy, drainage, and aeration may be discerned from color and this information applied towards the management and potential uses of a particular soil. It should be stressed however that this information must be used with educated caution. The colors of soil are derived largely from organic matter and minerals. Dark brown to black colors at or near the surface of a soil profile generally indicate an accumulation of organic matter (more specifically called humus). The oxidation state of iron and manganese also influence soil color. Red, yellow and reddish-brown colors in soil are often the result of oxidized iron (Fee+) and manganese (Man+). Thus, yellow and reddish-brown colors may indicate the soil is well drained and well aerated.
When the oxygen availability is limited, such as in saturated soils, iron and manganese usually exists in their educed states-? Fee+ and Mn+. Under these conditions, the soil color will be more subdued shades of gray and blue. This condition is referred to as gelled soil color. A grayish-blue coloration in the lower profile may indicate that the soil is poorly aerated. Poorly drained soils may also exhibit flecks or spots of orange and yellow. This mottled soil color indicates a zone of alternate oxidation and reducing conditions caused by seasonal fluctuations in the water table. The water table is usually at its highest point during late winter causing saturation of the