Soil Physical Properties
AGS 105 Soils
Soil color affects the absorption of solar radiation, but the main reason for studying soil color is that color provides clues to the nature of other soil properties and conditions.
The principal causes of soil colors are:
Soil colors are described using the Munsell system that includes the hue, value, and chroma.
Dry soil is generally lighter (higher in value) than moist soil.
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Coarse fragments (bigger than 2 mm) are not considered to be part of the fine earth fraction to which the term texture applies. Organic matter is disregarded in determining soil texture. Soil texture describes the distribution of particles into different size classes, the main divisions being sand (2 mm to 0.05 mm), silt (0.05 mm to 0.002 mm), and clay (smaller than 0.002 mm). The size of soil particles (i.e., the texture) greatly influences the surface area present in a given mass of soil. The surface of soil particles is the site of water adsorption, gas adsorption, mineral weathering, cation exchange, and much microbial colonization. Therefore, many processes and properties are very much affected by surface area and soil texture. Mineralogically, sand and silt fractions usually consist mainly of quartz and other primary minerals. The clay fraction, by contrast, contains mostly secondary minerals, often silicate clays. The texture of a soil can be described by designating a textural class, such as sandy loam, silty clay, or loam. These can be determined from a textural triangle chart if the percentage of two of the three size fractions is known. The percentages of sand, silt, and clay can be determined by the "feel" methods or by various sedimentation methods.
The relationship between the surface area of a given mass of material and the size of its particles. In the single large cube (a) each face has 64 cm2 of surface area. The cube has six faces, so the cube has a total of 384 cm2 surface area. If the same cube of material was cut into smaller cubes (b) so that each cube was only 2 cm on each side, then the same mass of material would now be present as 64 smaller cubes. Each face of each small cube would have 4 cm2 of surface area, giving 24 cm2 of surface area for each cube. The total mass would therefore have 1536 cm2 of surface area. This is four times as much surface area as the single large cube. Since clay particles are very, very small and usually platelike in shape, their surface area is thousands of times greater than that of the same mass of sand particles.
The major soil textural classes are defined by the percentages of sand, silt, and clay according to the heavy boundary lines shown on the textural triangle. If these percentages have been determined for a soil sample by particle size analysis, then the triangle can be used to determine the soil textural class name that applies to that soil sample.
To use the graph, first find the appropriate clay percentage along the left side of the triangle, then draw a line from that location across the graph going parallel to the base of the triangle. Next find the sand percentage along the base of the triangle, then draw a line inward going parallel to the triangle side labeled "Percent silt." The small arrows indicate the proper direction in which to draw the lines. The name of the compartment in which these two lines intersect indicates the textural class of the soil sample. Percentages for any two of the three soil separates is all that is required. Because the percentages for sand, silt, and clay add up to 100%, the third percentage can easily be calculated if the other two are known. If all three percentages are used, the three lines will all intersect at the same point.
Consider, as an example, a soil that has been determined to contain 15% sand, 15% clay, and 70% silt. This example is indicated by the light dashed lines that intersect in the compartment labeled "Silt loam." What is the textural class of another soil sample that has 33% sand, 33% silt, and 33% clay? The lines (not shown) for this second example would intersect in the center of the "Clay loam" compartment.
The term structure relates to the grouping of primary soil particles into aggregates or peds. Soil structure may greatly modify the influence of soil texture on water and gas movement, heat transfer, aeration, and tilth.
Soil structure is categorized by its type (shape of the peds), size, and strength of expression.
The main types of structure are:
Structureless conditions include massive clays and single grain sands.
The formation of soil structure is promoted by chemical (flocculation) processes, physical processes (e.g., freezing, drying), and, especially, biological processes (e.g., binding of particles by microbial glues and fungal hyphae, as well as by fine plant roots). Soil aggregates have a hierarchical organization in that macro aggregates (about 1-5 mm) are comprised of clusters of micro aggregates (about 0.1 to 1mm), which in turn consist of sub-micro aggregates (about 0.01 to 0.1 mm) which are finally comprised of primary mineral and organic particles held together by iron oxides and clay-humus domains. Biological factors most affect the larger aggregates.
When the mass (weight) per unit volume of soil, or density of soil, is expressed with regard to the volume of only the solids, the property is called the particle density, and ranges from less than 1 (for organic soils) to over 3 (for some mineral soils) Mg/m3. For most mineral soils, 2.65 Mg/m3, the density of quartz, is assumed to be the particle density.
When the volume of the entire (usually undisturbed) soil sample (solids plus pores) is considered, the resulting property is called the bulk density.
Soil bulk density is influenced by both texture (sands are generally more dense) and structure (granular structure lowers bulk density). Bulk density is indicative of the pore space in a soil and the ease with which roots may penetrate a soil.
The uniformity of grain size and the type of packing arrangement significantly affect the bulk density of sandy materials. Materials consisting of all similar-sized grains are termed well sorted (or poorly graded). Those with a variety of grain sizes are well graded (or poorly sorted). In either case, compaction of the particles into a tight packing arrangement markedly increases the bulk density of the material and decreases its porosity. Note that the size distribution of sand and gravel particles can be described as either graded or sorted, the two terms having essentially opposite meanings. Geologists usually speak of rivers having sorted the sand grains by size as deposits were laid down. Engineers usually are concerned as to whether or not sand consists of a gradation of sizes (i.e., is well graded or not).
Pore space in mineral soils is typically about 50% of the soil volume. The total pore space can be divided into macropores, mostly between peds, and micro pores, mostly within peds. Under moisture conditions favorable for plant growth, most of the macro pores are air-filled and most of the micropores are water-filled. Minimum tillage and good organic matter management practices in arable soils promote macropore space approaching that of virgin soils. The influence of bulk density on root penetration in compacted soils is related to soil strength (penetration resistance) as modified by soil texture and water content.
Percent Pore Space = 100 - (100 x (Bulk Density/Particle Density)). Mineral surface soils typically have bulk densities between 1.1 and 1.5 Mg/m3, while that of compact subsoils may be 2.0 or higher. Bulk density greater than 1.6 greatly impairs the growth of most plant roots.
Various types of soil pores. (a) Many soil pores occur as packing pores, spaces left between primary soil particles. The size and shape of these spaces is largely dependent on the size and shape of the primary sand, silt, and clay particles and their packing arrangement. (b) In soils with structural peds, the spaces between the peds form interped pores. These may be rather planar in shape, as with the cracks between prismatic peds, or they may be more irregular, like those between loosely packed granular aggregates. (c) Biopores are formed by organisms such as earthworms, insects, and plant roots. Most of these are long, sometimes branched channels, but some are round cavities left by insect nests and the like.
Compaction, from foot or vehicular traffic, acts by breaking down soil aggregates. Beating rain and excess tillage also destroy soil structure. Aggregate stability is encouraged by fungal hyphae, especially those that are mycorrhizal. Microbial polysaccharides act as glue to hold particles together in aggregates. Humus also plays a role in stabilizing aggregates. Multivalent cations encourage both flocculation and clay-organic matter complexes. Hard crusts that form when surface structure is broken down cause serious problems by reducing both water infiltration and seedling emergence. Certain synthetic soil conditioners hold promise in stabilizing surface structure.
Soil management practices that promote good soil structure include:
Tilth refers to the physical condition of soil in relation to plant growth. Soil tilth can change rapidly in response to management practices and conditions. Conventional tillage usually involves both primary tillage (plowing) and secondary tillage (harrowing) to create a seedbed, bury residues, and kill weeds. Negative long-term effects of conventional tillage include the formation of dense plow pans and the accelerated loss of soil organic matter and hence structural stability. Conservation tillage generally keeps the soil surface covered, manipulates the soil less thoroughly and promotes stable macropores.
Larger aggregates are often composed of an agglomeration of smaller aggregates. This illustration shows four levels in this hierarchy of soil aggregates. The different factors important for aggregation at each level are indicated. (a) A macroaggregate composed of many microaggregates bound together mainly by a kind of sticky network formed from fungal hyphae and fine roots. (b) A microaggregate consisting mainly of fine sand grains and smaller clumps of silt grains, clay, and organic debris bound together by root hairs, fungal hyphae, and microbial gums. (c) A very small submicroaggregate consisting of fine silt particles encrusted with organic debris and tiny bits of plant and microbial debris (called particulate organic matter) encrusted with even smaller packets of clay, humus, and Fe or Al oxides. (d) Clusters of parallel and random clay platelets interacting with Fe or Al oxides and organic polymers at the smallest scale. These organoclay clusters or domains bind to the surfaces of humus particles and the smallest of mineral grains.
Soil consistence describes the resistance of a soil to deformation at various moisture contents. Consistence may be hard, loose, or soft when dry and sticky and plastic when moist. Friable soil crumbles easily. Some soil horizons are cemented or indurated. Consistency is an engineering term used to describe the resistance of soil to penetration by an object such as a thumbnail.
Soil strength helps predict the susceptibility of a soil to sudden failure or rupture under a stress; it is a measure of the coherence among soil particles and is closely related to the content and type of clay.
A walk along a beach such as this one in Oregon illustrates the concept of soil strength for sandy materials. The dry sand (lower right) has little strength and your feet easily mire into it as you walk along. There is nothing to hold the individual sand particles together to permit them to serve as a firm base. As you move toward the ocean where the soil has been thoroughly wetted by the incoming waves, but where there is no standing water (lower center), you find firm footing, indicating considerably higher soil strength. Thin water films act as bridges between sand particles, holding them together and thereby resisting penetration by the feet. If you stand in shallow water along the edge of the ocean (lower left), once again your feet penetrate the surface sand, indicating that soil strength has been reduced. Each sand particle is completely surrounded by water, which acts more as a lubricant than as a binding force. If you were to drive an automobile over the same areas, only the wetted but not submerged sand would provide a firm base. Soil strength is a property of great concern to engineers, but also must be reckoned with by plant roots as they try to penetrate soils with high bulk densities.
In order to avoid uneven settlement, engineers may strive to completely compact a soil before loading it with a building or highway. The optimum water content for such compaction can be found by a Proctor Test. The plastic limit and liquid limit are moisture contents at which a given soil just begins to act to be malleable or flowable. The difference between these moisture contents, the plasticity index, is used by engineers to classify soil material with regard to its suitability for such construction purposes as roadbeds. Certain clay minerals engender a high degree of stickiness and plasticity to soils, as well as swell/shrink properties that can threaten the stability of construction foundations.