|Overview of soil physical properties and associated activities affecting soil physical properties and processes. Click on links to go to a specific section.|
|Property||Effect on stormwater bmp effectiveness||Processes affected||Desired value||Management strategies|
|Texture||High||Infiltration, water and air storage and movement||
||Add clay, silt, sand, or organic matter to amend undesirable soil properties (, , )|
|Structure||Moderate-high||Infiltration, plant rooting||Well-aggregated soil is generally preferred||Minimize activity in bmp during construction; alleviate compaction; utilize deep-rooting vegetation ()|
|Density||High||Plant rooting, infiltration||1.1 (clays) to 1.5 (sandy loams) g/cm3||Minimize activity in bmp during construction; alleviate compaction; amend soil (e.g. compost) (, )|
|Porosity||Moderate-high||Water and air transport||0.42-0.48 cm3/cm3||Minimize activity in bmp during construction; alleviate compaction; amend soil (e.g. compost) (, )|
|Consistence||Low||Typically not an important consideration in stormwater applications||Typically not an important consideration in stormwater applications||Typically not an important consideration in stormwater applications|
|Temperature||Low-moderate||Plant growth and biologic activity||70-80 oC for germination, 65-75 oC after germination||Add soil cover if temperature regulation is desired|
|Color||Low||Typically not an important consideration in stormwater applications||Typically not an important consideration in stormwater applications||Typically not an important consideration in stormwater applications|
|Aggregate stability||Moderate||Infiltration, erosion||Tests are generally qualitative; With sieve test, well aggregated soils retain 50 percent or more of the initial soil mass after sieving||Minimize tillage, amend with organic matter, provide soil cover|
|1 This table provides a general discussion. For detailed information on specific topics, see the links in this table.|
Physical properties of soil include color, texture, structure, porosity, density, consistence, aggregate stability, and temperature. These properties affect processes such as infiltration, erosion, nutrient cycling, and biologic activity. These properties also affect suitability of soil for different uses, such as stormwater infiltration, subgrade for roads, and strength for building.
This page provides an overview of soil physical properties, processes they affect, effects of human activities, discussion of stormwater applications, and links to related topics, including information on sampling, testing, and soil health assessments.
Soil physical properties include texture, structure, density, porosity, consistence, temperature, and color.
Soil texture (such as loam, sandy loam or clay) refers to the proportion of sand, silt and clay sized particles that make up the mineral fraction of the soil. Soil texture is determined with one of the following methods.
Other methods are used to determine texture, but these employ qualitative approaches. They include the feel method, ball and ribbon methods, and ball throwing method. These are described here. These methods may be satisfactory for some applications, such as determining if a soil may be suitable for infiltration, but they must be conducted by an experienced person, such as a professional soil scientist.
The following particle size distribution, based on sieve analysis, is commonly used to define soil particles.
Soil texture describes the distribution of these different size particles in a soil. There are twelve soils based on the distribution of sand, silt and clay. The adjacent image displays the soil texture triangle, which assigns soil type based on the distribution of sand, silt, and clay.
Texture affects many soil processes, including infiltration, drainage (water and air distribution), erosion, chemical processes, and biologic processes. These are discussed generally below.
This page provides a nice overview and discussion of soil structure.
Soil structure describes the arrangement and organization of soil particles in the soil, and the tendency of individual soil particles to bind together in aggregates. Aggregation affects water and air transport, which affects the movement of solutes and pollutants and affects biologic activity, including plant growth.
The development of soil structure is influenced by
Soil structure is typically divided into one of the following groups, as illustrated in the adjacent image.
Soil structure affects water and air movement in soil and is therefore important to soil biota. Platy and massive soils have restricted air and water movement, while granular and aggregated soils have enhanced air and water transport.
Soil structure can be altered by human activity, including the following.
Soil density is related to the mineral and organic composition of a soil and to soil structure. The standard measure of soil density is bulk density, defined as the proportion of the weight of a soil relative to its volume. It is expressed as a unit of weight per volume, and is commonly measured in units of grams per cubic centimeter (g/cm3). Bulk density is an indicator of the amount of pore space available within individual soil horizons, as it is inversely proportional to pore space:
Pore space = 1 – bulk density/particle density
where particle density is the weight/volume of the solid material. Typical particle densities are 2.65 g/cm3 for minerals and 1.3 g/cm3 for organic matter.
For example, at a bulk density of 1.60 g/cm3, pore space equals 0.40 or 40%. At a bulk density of 1.06 g/cm3, pore space equals 0.60 or 60%.
Because of the lower particle density of organic matter compared to minerals, soils with higher concentrations of organic matter have lower bulk densities. Typical bulk densities of different soils are illustrated in the adjacent table.
Comparison of bulk densities for undisturbed soils and common urban conditions. (Source: Schueler, T. 2000. The Compaction of Urban Soils: Technical Note #107 from Watershed Protection Techniques. 3(2): 661-665. Center for Watershed Protection, Ellicott City, MD.)
For information on alleviating soil compaction, see Alleviating compaction from construction activities
Link to this table
|Undisturbed soil type or urban condition||Surface bulk density (grams / cubic centimeter|
|Peat||0.2 to 0.3|
|Sandy soil||1.1 to 1.3|
|Silt||1.3 to 1.4|
|Silt loams||1.2 to 1.5|
|Organic silts / clays||1.0 to 1.2|
|Glacial till||1.6 to 2.0|
|Urban lawns||1.5 to 1.9|
|Crushed rock parking lot||1.5 to 2.0|
|Urban fill soils||1.8 to 2.0|
|Athletic fields||1.8 to 2.0|
|Rights of way and building pads (85% compaction)||1.5 to 1.8|
|Rights of way and building pads (95% compaction)||1.6 to 2.1|
Bulk density affects water and air transport in soils. Soils with high densities resist water and air transport. Soils with high density may also impede root growth.
Several human activities affect bulk density. Soils at construction sites are generally compacted as a result of excavation, mixing, stockpiling, equipment storage, and equipment traffic. In addition, exposed subsoil is susceptible to compaction. Clay soils and wet soils are more susceptible to compaction. Even at sites where selective grading is employed, compaction occurs as a result of construction equipment, stockpiling and vehicle traffic (Randrup, 1998; Lichter and Lindsay, 1994).
When soil is compacted, porosity decreases and bulk density increases. Typical increases in bulk density are shown in the adjacent table, with other compacting activities included for comparison. As a result, permeability of air and water in soil decreases, soil water-holding capacity is reduced, and root growth is impeded. On a watershed scale, soil compaction leads to increased runoff and erosion.
Increase in soil bulk density as a result of different land uses or activities.
Link to this table
|Land use or activity||Increase in bulk density (grams / cubic centimeter||Source (link to Reference list)|
|Grazing||0.12 to 0.20||Smith, 1999|
|Crops||0.25 to 0.35||Smith, 1999|
|Construction, mass grading||0.34 to 0.35||Randrup, 1998; Lichter and Lindsey, 1994|
|Construction, no grading||0.20||Lichter and Lindsey, 1994|
|Construction traffic||0.17 to 0.40||Lichter and Lindsey, 1994; Smith, 1999; Friedman, 1998|
|Athletic fields||0.38 to 0.54||Smith, 1999|
|Urban lawn and turf||0.30 to 0.40||Various sources|
The effects of compaction are difficult to overcome and may persist for decades. Natural processes such as freeze-thaw cycles, animal burrowing, and root growth only slowly diminish compaction. These natural processes are typically limited to the upper foot or two of soil. Even when bulk densities decrease, the original soil structure may not be achieved (Randrup, 1997; Schueler and Holland, 2000).
The following table summarizes results for different activities designed to alleviate compaction. The results suggest compost amendment is an effective method for alleviating compaction, while tillage is considerably less effective. Note however, this is an area of on-going research and some recent studies suggest properly conducted tillage can be effective at reducing compaction. For an example, link here.
Reported Activities that Restore or Decrease Soil Bulk Density
Link to this table
|Land use or activity||Decrease in bulk density (gms/cc)||Source|
|Tilling of soil||0.00 to 0.02||Randrup, 1918. Patterson and Bates, 1994|
|Spedialized soil loosening||0.05 to 0.15||Rolf, 1998|
|Selective grading||0.00||Randrup, 1998 and Lichter and Linsy, 1994|
|Soil amendments||0.17||Patterson and Bates, 1994|
|Compost amendments||0.25 to 0.35||Kolsti et al. 1995|
|Time||0.20||Legg et al, 1996|
|Reforestration||0.25 to 0.35||Article 36|
Pore space is that part of the bulk volume of soil that is not occupied by either mineral or organic matter but is open space occupied by either gases or water. As discussed above, soil porosity is inversely related to bulk density. In a productive, medium-textured soil the total pore space is typically about 50% of the soil volume. Ranges for soil porosity are shown in the adjacent table. Although porosity does not vary widely across soil textures, values for porosity minus field capacity vary widely, with greater values for coarse-textured soils (e.g. sands). Porosity minus field capacity represents pore space available after water drains from a soil by gravity and is thus considered to be the upper bound of plant available water, with wilting point considered the lower bound.
Although porosity is related to density, pore size is an important factor affecting soil processes. Soils with similar porosity may have different distributions of pore sizes. The smallest pores (<0.1 μm diameter) hold water too tightly for use by plant roots. Plant-available water is held in pores 0.1–75 μm in diameter. Macropores (>75 μm diameter) are generally air-filled when the soil is at field capacity, but they can rapidly transport water and solutes to deeper depths in the soil profile. Clay soils have smaller pores, but more total pore space than sands. Soils with extensive biologic activity have greater macroporosity (e.g. plant roots, animal burrows).
The pore size distribution affects the ability of plants and other organisms to access water and oxygen; large, continuous pores allow rapid transmission of air, water and dissolved nutrients through soil, and small pores store water between rainfall or irrigation events. Pore size variation also promotes biologic activity by compartmentalizing pore space, which reduces competition between organisms, including microbes.
The same human activities affecting soil structure and density affect porosity. Porosity can be enhanced in compacted soils by addition of organic material and through tillage. Macroporosity can be promoted with deeper rooting vegetation.
Recommended values for porosity, field capacity and wilting point for different soils.1
Link to this table.
|Soil||Hydrologic soil group||Porosity 2 (volume/volume)||Field capacity (volume/volume)||Wilting point (volume/volume)||Porosity minus field capacity (volume/volume)3||Field capacity minus wilting point (volume/volume)4|
|Sand||A (GM, SW, or SP)||0.43||0.17||0.025 to 0.09||0.26||0.11|
|Loamy sand||A (GM, SW, or SP)||0.44||0.09||0.04||0.35||0.05|
|Sandy loam||A (GM, SW, or SP)||0.45||0.14||0.05||0.31||0.09|
|Loam||B (ML or OL)||0.47||0.25 to 0.32||0.09 to 0.15||0.19||0.16|
|Silt loam||B (ML or OL)||0.50||0.28||0.11||0.22||0.17|
|Sandy clay loam||C||0.4||0.07|
|Silty clay loam||D||0.47 to 0.51||0.30 to 0.37||0.17 to 0.22||0.16||0.14|
1Sources of information include Saxton and Rawls (2006), Cornell University, USDA-NIFA, Minnesota Stormwater Manual
2Soil saturation is assumed to be equal to the porosity.
3This value may be used to represent the volume of water that will drain from a bioretention media.
4This value may be used to estimate the amount of water available for evapotranspiration
Soil consistence refers to the ease with which an individual ped can be crushed by the fingers. Soil consistence, and its description, depends on soil moisture content. Terms commonly used to describe consistence include the following.
Soil consistency is useful in estimating the ability of soil to support structures, such as buildings and roads. The measurement is not widely used for stormwater applications.
Soil temperature is affected by climate, water content of a soil, soil color, soil cover (e.g. presence or absence of mulch), depth in the soil profile, and air and water flow within a soil. Minnesota soils are generally slow to warm in spring due to climate, but the following conditions affect temperature.
Soil temperatures are an important consideration in vegetated systems. Human activities that affect temperature include the following.
Soil color is largely determined by the organic matter content, drainage conditions, degree of oxidation, and in some cases, presence of specific minerals. Soil color is not a widely used factor in stormwater applications.
Aggregate stability refers to the ability of soil aggregates to resist disintegration when disruptive forces associated with tillage and water or wind erosion are applied. Stable soil aggregates, in the presence of water, are important for water and air transport, root growth, habitat for soil biota, minimizing soil erodibility, protecting soil organic matter, and nutrient cycling.
Natural soil physical processes typically occur slowly, with the exception of soil temperature (fluctuates daily and seasonally) and color (changes in response to soil moisture). Soil porosity, density, and structure changes in response to biologic activity, as soil biota increases aggregation, creates macropores, and alters organic matter concentrations. These changes are limited to the upper biologically active area within the soil profile. Soil formation is a very slow process, and under natural conditions soil density and structure change slowly.
Humans can greatly accelerate soil physical processes. The major human effects on physical processes are described below.