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Figure 3. Example page from Munsell Soil Color Charts (2000). Boxed areas indicate soil colors that may indicate wetness. (note: The colors presented here are not true to the publication and are here to illustrate one use of the book. Be sure to use the actual Munsell Soil Color Charts. 2000. GretagMacbeth, 617 Little Britain Rd., New Windsor, NY 12553.)
Oxidation and reduction processes in the soil associated with variable drainage and aeration are evidenced by the accumulation and depletion of materials such as organic matter and metal oxides. Accumulations and depletions can take many forms; as well as mottles or patches of color, minerals and oxides can accumulate as nodules, concretions, and surface coating and other forms. These accumulations and depletions can be used as indicators of soil wetness. In particular, depleted soil colors with a chroma of 2 or less and a value of 4 or more are the primary morphological indicators of wetness for assessing onsite wastewater suitability. (Figure 3)
The solid portion of the soil is made up mostly of particles of soil minerals and organic matter. Soil texture refers to the relative proportion of the three major size classes of primary soil particles. Texture generally is assessed on the fine earth fraction of the soil (particles less than 2 mm in size). Solid particles larger than 2 mm are considered coarse fragments and include pebbles and cobbles.
Clay is the finest of the particle size classes. Clays exert a great influence on soil chemical and physical properties, largely due to their large surface area and surface charge. Clay particles are chemically active and influence the movement, exchange and cycling of minerals, ions and organic constituents in the soil. The proportion of clay-sized particles also greatly influences soil physical properties, including aggregation, porosity, water movement and storage, aeration and workability of the soil.
Soil texture can be assessed in the field by an experienced soil scientist, who can judge the proportion of sand, silt and clay-sized particles by the feel of the soil when moistened and manipulated. A handful of soil is moistened and worked into a ball; the behavior of the material in terms of the amount of water needed to wet the soil, the coherence and stickiness of the material, and the dominance of coarse, medium or finer particles that can be sensed provides a series of clues to the texture of the sample. A flowchart illustrating the process is shown in Figure 4.
Figure 5. Soil texture triangle.
The soil textural triangle (Figure 5) is used to classify the sample according to the proportion of the primary particle size classes. Within a named texture class, soil materials share some common traits or ranges of physical behavior. For example, materials in the sand class have very low proportions of silt or clay size particles, and generally transmit water quickly, but do not have a great capacity to store moisture. At the other extreme, clay soils are dominated by the finest particles, and while able to store considerably more moisture, are often only able to transmit water very slowly. Medium textured soils, such as loams, have a mixture of particle size classes, and may provide a moderate degree of moisture retention and moderate permeability, and hence are most useful for efficient treatment of wastewater. More precise laboratory measurements of texture can complement field assessments.
In most soils, solid particles are not present as separate individuals, but are aggregated into larger clusters of various sizes and shapes to form the structure of the soil. Soil aggregates (or “peds”) are separated at zones of weakness when subjected to force. Structure is effectively the architecture of the soil.
Structure is an important property of the soil, because it influences the movement and storage of water and air, the growth of plant roots and the workability and stability of the soil when tilled or disturbed by machinery. Soil structure is a property best observed in the field and is described in terms of the degree of aggregation, and the size and shape of visible soil aggregates in each soil horizon.
Some soils are considered to be structureless. In these soils, individual particles are either not aggregated and separate readily into individual grains (single-grained condition), or are held together in such a way that no preferred planes of breakage can be induced (massive condition). Loose beach sands are an example of single-grained materials. Massive structureless materials occur commonly below the developed soil profile, and may be present in compacted soil horizons.
Figure 6. Soil structure shapes.
Common names have been given to various ped shapes as an aid in identification and communication (Figure 6). Granular peds are approximately spherical and are commonly found in surface horizons. Blocky peds have a more cubic shape with more or less planar faces. Angular blocky peds have mostly planar faces and sharper edges, while subangular blocky peds have more rounded faces and more irregular edges. Platey peds are flat or platelike in shape, and are longer in the horizontal direction. Prismatic peds are longer in the vertical direction.
Soil pores form between the solid structural units and within them. Pores are pathways for the movement of water, air, roots, nutrients, and even contaminants. Larger, connected pores are transmission pathways for fluid flow, so that connected pores influence soil drainage. Finer pores frequently store soil water. Pores are generally described simply in terms of their abundance.
Consistence is a complex property related to the strength of soil materials. Consistence refers to the degree of cohesion or adhesion of soil materials and the resistance to applied force. Aggregates or clods can be made to deform or rupture when subjected to force. The resistance to applied forces depends on soil moisture content; moister soils generally fail more easily. Consistence is often described according to the force needed to crush soil between the thumb and forefinger. Strongly coherent soils, with high resistance, may indicate poor conditions for water movement, root growth and the growth of soil organisms. Layers or zones of high consistence may occur naturally (e.g., fragipans or dense glacial till) or be induced by imposed forces, as is the case with compaction caused by wheeled vehicles. Dense layers in the soil are considered restrictive for the purposes of wastewater treatment.
Soil evaluators look at a variety of other evidence to judge the ability of soils to transmit water and air and to treat wastewater. When describing the soil profile, the following additional observations may be noted:
Plant roots penetrate the aerated pore spaces of the soil to extract air, water and nutrients. An abundance of plant roots indicates conditions well suited to biological activity and hence to wastewater treatment.
Reaction is a measure of the acidity, alkalinity or lime status of the soil. Effervescence of the soil when acid is applied indicates the presence of carbonates, frequently derived from the soil parent materials under normal soil formation conditions in Ohio. Since these carbonates are relatively easily leached, their presence can be a useful indicator of the depth to which leaching frequently occurs, and hence the depth to which water moves without severe restriction. Concentrations or patches of minerals (including carbonates, oxides and sulfates) that accumulate as a result of soil processes may indicate contrasts or other changes in the soil significant to moisture conditions.
Wastewater is best treated by unsaturated soil material. Where large proportions of rock fragments are present (the coarse fragment fraction, greater than 2 mm in size), less soil is available for contact with and treatment of wastewater contaminants. Coarse fragments are essentially inert with respect to biological activity, so that the volume of treatment medium is restricted if the content of coarse fragments is high. Coarse fragments in the soil are described in terms of size, shape, abundance and form.
Boundaries are marked breaks between soil horizons. The shape and contrast of transitions between horizons indicates the degree of development of distinct horizon features. Abrupt breaks or discontinuities in soil properties may indicate marked changes in soil drainage conditions. Relatively young soils, those formed from recently deposited materials or at filled and disturbed sites, may exhibit less soil horizon development.