Soil texture

Last updated April 02, 2024

Soil texture is a classification instrument used both in the field and laboratory to determine soil classes based on their physical texture. Soil texture can be determined using qualitative methods such as texture by feel, and quantitative methods such as the hydrometer method based on Stokes' law. Soil texture has agricultural applications such as determining crop suitability and to predict the response of the soil to environmental and management conditions such as drought or calcium (lime) requirements. Soil texture focuses on the particles that are less than two millimeters in diameter which include sand, silt, and clay. The USDA soil taxonomy and WRB soil classification systems use 12 textural classes whereas the UK-ADAS system uses 11.^[1] These classifications are based on the percentages of sand, silt, and clay in the soil.

History

The first classification, the International system, was first proposed by Albert Atterberg in 1905 and was based on his studies in southern Sweden. Atterberg chose 20 μm for the upper limit of silt fraction because particles smaller than that size were not visible to the naked eye, the suspension could be coagulated by salts, capillary rise within 24 hours was most rapid in this fraction, and the pores between compacted particles were so small as to prevent the entry of root hairs.^[2] Commission One of the International Society of Soil Science (ISSS) recommended its use at the first International Congress of Soil Science in Washington in 1927.^[3] Australia adopted this system, and its equal logarithmic intervals are an attractive feature worth maintaining.^[4] The United States Department of Agriculture (USDA) adopted its own system in 1938, and the Food and Agriculture Organization (FAO) used the USDA system in the FAO-UNESCO world soil map and recommended its use.

Classification

Soil texture triangle, showing the 12 major textural classes, and particle size scales as defined by the USDA SoilTextureTriangle.svg — Soil texture triangle, showing the 12 major textural classes, and particle size scales as defined by the USDA

In the United States, twelve major soil texture classifications are defined by the United States Department of Agriculture.^[1] The twelve classifications are sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay.^[5] Soil textures are classified by the fractions of each soil separate (sand, silt, and clay) present in a soil. Classifications are typically named for the primary constituent particle size or a combination of the most abundant particles sizes, e.g. "sandy clay" or "silty clay". A fourth term, loam, is used to describe equal properties of sand, silt, and clay in a soil sample, and lends to the naming of even more classifications, e.g. "clay loam" or "silt loam".

Determining soil texture is often aided with the use of a soil texture triangle plot.^[5] An example of a soil triangle is found on the right side of the page. One side of the triangle represents percent sand, the second side represents percent clay, and the third side represents percent silt. If the percentages of sand, clay, and silt in the soil sample are known, then the triangle can be used to determine the soil texture classification. For example, if a soil is 70 percent sand and 10 percent clay then the soil is classified as a sandy loam. The same method can be used starting on any side of the soil triangle. If the texture by feel method was used to determine the soil type, the triangle can also provide a rough estimate on the percentages of sand, silt, and clay in the soil.

Chemical and physical properties of a soil are related to texture. Particle size and distribution will affect a soil's capacity for holding water and nutrients. Fine textured soils generally have a higher capacity for water retention, whereas sandy soils contain large pore spaces that allow leaching.^[6]

Soil separates

Soil separates are specific ranges of particle sizes. The smallest particles are clay particles and are classified as having diameters of less than 0.002 mm. Clay particles are plate-shaped instead of spherical, allowing for an increased specific surface area.^[7] The next smallest particles are silt particles and have diameters between 0.002 mm and 0.05 mm (in USDA soil taxonomy). The largest particles are sand particles and are larger than 0.05 mm in diameter. Furthermore, large sand particles can be described as coarse, intermediate as medium, and the smaller as fine. Other countries have their own particle size classifications.

Name of soil separate	Diameter limits (mm) (USDA classification)	Diameter limits (mm) (WRB classification)
Clay	less than 0.002	less than 0.002
Silt	0.002 – 0.05	0.002 – 0.063
Very fine sand	0.05 – 0.10	0.063 – 0.125
Fine sand	0.10 – 0.25	0.125 – 0.20
Medium sand	0.25 – 0.50	0.20 – 0.63
Coarse sand	0.50 – 1.00	0.63 – 1.25
Very coarse sand	1.00 – 2.00	1.25 – 2.00

Methodology

Texture by feel

Hand analysis is a simple and effective means to rapidly assess and classify a soil's physical condition. Correctly executed, the procedure allows for rapid and frequent assessment of soil characteristics with little or no equipment. It is thus a useful tool for identifying spatial variation both within and between fields as well as identifying progressive changes and boundaries between soil map units (soil series). Texture by feel is a qualitative method, as it does not provide exact values of sand, silt, and clay. Although qualitative, the texture by feel flowchart can be an accurate way for a scientist or interested individual to analyze the relative proportions of sand, silt, and clay.^[8]

The texture by feel method involves taking a small sample of soil and making a ribbon. A ribbon can be made by taking a ball of soil and pushing the soil between the thumb and forefinger and squeezing it upward into a ribbon. Allow the ribbon to emerge and extend over the forefinger, breaking from its own weight. Measuring the length of the ribbon can help determine the amount of clay in the sample. After making a ribbon, excessively wet a small pinch of soil in the palm of the hand and rub in with the forefinger to determine the amount of sand in the sample. Soils that have a high percentage of sand, such as sandy loam or sandy clay, have a gritty texture.^[1] Soils that have a high percentage of silt, such as silty loam or silty clay, feel smooth.^[1] Soils that have a high percentage of clay, such as clay loam, have a sticky feel. Although the texture by feel method takes practice, it is a useful way to determine soil texture, especially in the field.

The international soil classification system World Reference Base for Soil Resources (WRB) uses an alternative method to determine texture by feel, offering another flow chart.

Sieving

Sieving is a long-established but still widely used soil analysis technique. In sieving, a known weight of sample material passes through finer sieves. The amount collected on each sieve is weighted to determine the percentage weight in each size fraction.

The method is used to determine the grain size distribution of soils that are greater than 75 µm in diameter, as sieving has a strong disadvantage in the lower measurement border. In fact, in case of finer fraction at high content of clay and silt (below 60 µm), the dispersion becomes challenging because of the high cohesiveness of particles, stickiness of powder to the sieve, and electrostatic charges. Moreover, in the sieving particles pass with the smallest side through the mesh opening, which means that the plate-shaped clay and silt particles might be sieved as well. In all this generally leads to a massive underestimation of the fine fraction.^[9]

In order to measure silt and clay (with a particle size below 60 µm), a second, independent sizing method (most often hydrometer or pipette technique) is used on the sample taken from the bottom sieve. Particle size distribution obtained from sieve analysis should be combined with the data from a sedimentation analysis to establish a complete particle size distribution of the sample.

Hydrometer method

Sedimentation analysis (e.g. pipette method, hydrometer) is commonly used in the soil industry or in geology to classify sediments.The hydrometer method was developed in 1927^[10] and is still widely used today.

The hydrometer method of determining soil texture is a quantitative measurement providing estimates of the percent sand, clay, and silt in the soil based on Stokes' law, which expresses the relationship between the settling velocity and particle size.^[11]

According to this law the particles settle down because of the weight and gravity action. However, there are two additional forces acting in the opposite direction of particles's motion which determines the equilibrium condition at which the particle falls at a constant velocity called terminal velocity.

The hydrometer method requires the use of sodium hexametaphosphate, which acts as a dispersing agent to separate soil aggregates. The soil is mixed with the sodium hexametaphosphate solution on an orbital shaker overnight. The solution is transferred to one liter graduated cylinders and filled with water. The soil solution is mixed with a metal plunger to disperse the soil particles.^[11] The soil particles separate based on size and sink to the bottom. Sand particles sink to the bottom of the cylinder first. Silt particles sink to the bottom of the cylinder after the sand. Clay particles separate out above the silt layer.

Measurements are taken using a soil hydrometer. A soil hydrometer measures the relative density of liquids (density of a liquid compared to the density of water). The hydrometer is lowered into the cylinder containing the soil mixture at different times, forty-five seconds to measure sand content, one and a half hours to measure silt content and between six and twenty-four hours (depending on the protocol used) to measure clay. The number on the hydrometer that is visible (above the soil solution) is recorded.^[11] A blank (containing only water and the dispersing agent) is used to calibrate the hydrometer. The values recorded from the readings are used to calculate the percent clay, silt and sand. The blank is subtracted from each of the three readings. The calculations are as follows:^[11]

Percent silt = (dried mass of soil – sand hydrometer reading – blank reading) / (dried mass of soil) *100

Percent clay = (clay hydrometer reading – blank reading) / (dried mass of soil) *100

Percent sand = 100 – (percent clay + percent silt)

The Stokes' diameter determined via sedimentation method is the diameter of a sphere having the same settling velocity and same density as the particle.^[12] This is the reason why the sedimentation analysis applies well when assuming that particles are spherical, have similar densities, have negligible interactions and are small enough to ensure that the fluid flow stays laminar.^[13] Deviations from Stokes' equation are to be expected in case of irregularly shaped particles, such as clay particles which are mostly platy or tubular. The stable position during settling of particles with such shapes is with the maximum cross-sectional area being perpendicular to the direction of motion.^[13] For this reason, the drag resistance of particles increases and the settling velocity decreases. The particle diameter is directly proportional to the settling velocity. Therefore, with lower velocity, the calculated diameter also decreases determining an overestimation of the fine size fraction.^[13]

Sedimentation analysis shows anyways limits for particles smaller than 0.2 micron because such small particles undergo Brownian motion in the suspension and do not settle anymore as per the Stokes' law.^[14] Sedimentation analysis can be operated continuously with a high degree of accuracy and repeatability. The particle size distribution of soil containing a significant number of finer particles (silt and clay) cannot be performed by sieve analysis solely, therefore sedimentation analysis is used to determine the lower range of the particle size distribution.

Laser Diffraction

Laser diffraction is a measurement technique for determining the particle size distribution of samples, either dispersed in a liquid or as a dry powder. The technique is based on light waves getting bent when encountering particles in a sample.^[15] The measured equivalent spherical diameter is the diameter of a sphere having on the cross-sectional area the same diffraction pattern as the investigated particle.^[16]

The angle of diffraction depends on the particle size, hence the pattern of diffraction depends on the relative amounts of different particle sizes present in that sample. This diffraction pattern is then detected and analyzed by means of Mie and Fraunhofer diffraction models. The outcome of the measurement is a particle size distribution (PSD).^[15]

By means of laser diffraction not only the particle size distribution and the corresponding volume weighted D-values can be determined but also the percentage of particles in the main size classes used for the soil classification.

Compared to other techniques laser diffraction is a fast and cost-effective method to measure particle size and quickly analyze soil samples. A big advantage is the built-in dispersion (e.g. dispersion by air pressure or ultrasound dispersion) unit of laser diffraction instruments. Therefore, dry samples can be measured without external sample preparation steps, which are required for sieving and sedimentation analysis. Moreover, since the sample can be dispersed properly, there is no need to combine two different measurement techniques to obtain the full range of the particle size distribution, including the silt and clay content.

Both Fraunhofer and Mie laser diffraction theories assume that particles are spherically shaped. This results in a small measurement error, since small particles in soil samples, such as clay and silt in particular, are elongated and anisotropic.^[17] The particle diameter in the laser diffraction method is determined in relation to their potential volume, which is calculated on the basis of an optical diffraction image at the edges of the particle cross-section. The volume of clay particles is the diameter of the plate’s cross-section, which is treated in the calculations as the diameter of the sphere. Therefore, their dimensions are usually overestimated in comparison to those measured via sedimentation analysis.^[17]

The error associated with the assumption of the sphericity of particles depends furthermore on the degree of anisotropy. The optical properties of anisotropic particles, such as refractive index and absorption index, change according to their orientation relative to the laser beam which is also variable. Therefore, at different particles orientations different cross-sections will be measured and different diffraction patterns produced.

For clays with sizes close to the wavelength of a laser beam, Mie theory would be desirable. This requires precise knowledge of the complex refractive index of the particles’ material, including their absorption coefficient.^[18] Because these parameters are often difficult to retrieve, especially the light absorption coefficients for various particles and soil grains, Fraunhofer theory, which only takes into account the light diffraction phenomena at the edge of the particles, is often recommended for natural soils.^[17]

Additional methods

There are several additional quantitative methods to determine soil texture. Some examples of these methods are the pipette method, the x-ray sedimentation, the particulate organic matter (POM) method, the rapid method.^[19]

X-ray sedimentation

The x-ray sedimentation technique is a hybrid technique which combines sedimentation and x-ray absorption. The particle size is calculated from the terminal settling velocities of particles by applying Stokes' law. The adsorption of the x-radiation is used to determine the relative mass concentration for each size class by applying the Beer-Lambert-Bouguer law.

Related Research Articles

A hydrometer or lactometer is an instrument used for measuring density or relative density of liquids based on the concept of buoyancy. They are typically calibrated and graduated with one or more scales such as specific gravity.

Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil or as sediment mixed in suspension with water. Silt usually has a floury feel when dry, and lacks plasticity when wet. Silt can also be felt by the tongue as granular when placed on the front teeth.

Loam is soil composed mostly of sand, silt, and a smaller amount of clay. By weight, its mineral composition is about 40–40–20% concentration of sand–silt–clay, respectively. These proportions can vary to a degree, however, and result in different types of loam soils: sandy loam, silty loam, clay loam, sandy clay loam, silty clay loam, and loam.

Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as geophysical engineering, coastal engineering, agricultural engineering, hydrology and soil physics.

<span class="mw-page-title-main">Mudrock</span> Type of sedimentary rock

Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 1⁄16 mm and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature.

The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter is described below :

<span class="mw-page-title-main">Soil morphology</span> Description of soil horizons

Soil morphology is the branch of soil science dedicated to the technical description of soil, particularly physical properties including texture, color, structure, and consistence. Morphological evaluations of soil are typically performed in the field on a soil profile containing multiple horizons.

Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. In the early 1870s a consulting chemist to the Royal Agricultural Society in England, named J. Thomas Way, performed many experiments on how soils exchange ions, and is considered the father of soil chemistry. Other scientists who contributed to this branch of ecology include Edmund Ruffin, and Linus Pauling.

<span class="mw-page-title-main">Geotechnical investigation</span> Work done to obtain information on the physical properties of soil earthworks and foundations

Geotechnical investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions; this type of investigation is called a site investigation. Geotechnical investigations are also used to measure the thermal resistance of soils or backfill materials required for underground transmission lines, oil and gas pipelines, radioactive waste disposal, and solar thermal storage facilities. A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves soil sampling and laboratory tests of the soil samples retrieved.

A sieve analysis is a practice or procedure used in civil engineering and chemical engineering to assess the particle size distribution of a granular material by allowing the material to pass through a series of sieves of progressively smaller mesh size and weighing the amount of material that is stopped by each sieve as a fraction of the whole mass.

In soil science, soil gradation is a classification of a coarse-grained soil that ranks the soil based on the different particle sizes contained in the soil. Soil gradation is an important aspect of soil mechanics and geotechnical engineering because it is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity. In a design, the gradation of the in situ soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil, if it is not high in clay quality.

The equivalent spherical diameter of an irregularly shaped object is the diameter of a sphere of equivalent geometric, optical, electrical, aerodynamic or hydrodynamic behavior to that of the particle under investigation.

In granulometry, the particle-size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size. Significant energy is usually required to disintegrate soil, etc. particles into the PSD that is then called a grain size distribution.

Particle size is a notion introduced for comparing dimensions of solid particles, liquid particles (droplets), or gaseous particles (bubbles). The notion of particle size applies to particles in colloids, in ecology, in granular material, and to particles that form a granular material.

Particle size analysis, particle size measurement, or simply particle sizing, is the collective name of the technical procedures, or laboratory techniques which determines the size range, and/or the average, or mean size of the particles in a powder or liquid sample.

$<span class="mw-page-title-main">Laser diffraction analysis</span> Technology for measuring geometrical dimensions of particle$

Laser diffraction analysis, also known as laser diffraction spectroscopy, is a technology that utilizes diffraction patterns of a laser beam passed through any object ranging from nanometers to millimeters in size to quickly measure geometrical dimensions of a particle. This particle size analysis process does not depend on volumetric flow rate, the amount of particles that passes through a surface over time.

Grain size is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.

The Olympic soil series is a type of deep, dark reddish brown moderately fine-textured soil which has developed on mafic rock such as basalt. The series covers large areas in southwestern Washington and western Oregon, and usually supports forests of Douglas-fir, red alder, western redcedar, western hemlock, and bigleaf maple.

Soil aggregate stability is a measure of the ability of soil aggregates—soil particles that bind together—to resist breaking apart when exposed to external forces such as water erosion and wind erosion, shrinking and swelling processes, and tillage. Soil aggregate stability is a measure of soil structure and can be affected by soil management.

References

1 2 3 4 Soil Science Division Staff. 2017. Soil survey sand. C. Ditzler, K. Scheffe, and H.C. Monger (eds.). USDA Handbook 18. Government Printing Office, Washington, D.C.
↑ Atterberg A (1905) Die rationalle Klassifikation der Sande und Kiese. Chemiker Zeitung 29, 195–198.
↑ Davis ROE, Bennett HH (1927) "Grouping of soils on the basis of mechanical analysis." United States Department of Agriculture Departmental Circulation No. 419.
↑ Marshall TJ (1947) "Mechanical composition of soil in relation to field descriptions of texture." Council for Scientific and Industrial Research, Bulletin No. 224, Melbourne.
1 2 Soil Survey Division Staff (1993). Soil survey manual. United States Department of Agriculture. pp. 63–65. Archived from the original on 19 January 2022. Retrieved 30 August 2014.
↑ Lindbo; Hayes; Adewunmi (2012). Know Soil Know Life: Physical Properties of Soil and Soil Formation. Soil Science Society of America. p. 17. ISBN 9780891189541.
↑ Foth, Henry D. (1990). Fundamentals of Soil Science 8th Edition . Canada: John Wiley & Sons. p. 23. ISBN 0-471-52279-1.
↑ Thien, Steven. "Determining Soil Texture by the "Feel Method"" (PDF). NDHealth.gov.
↑ "Standard Test Method for Particle-Size Analysis of Soils". www.astm.org. Retrieved 2022-07-20.
↑ Bouyoucos G. 1951. A recalibration of the hydrometer method for making mechanical analysis of soils. American Society of Agronomy.
1 2 3 4 Bouyoucos, George. 1936. Directions for making mechanical analysis of soils by the hydrometer method. Soil Science. Vol 42 Issue 3: pp 225–230
↑ Henk G. Merkus; Gabriel M. H. Meesters (2014). Particulate products : tailoring properties for optimal performance. Cham: Springer. ISBN 978-3-319-00714-4. OCLC 864591828.
1 2 3 Ferro, Vito; Mirabile, Stefano (2009-06-30). "Comparing Particle Size Distribution Analysis by Sedimentation and Laser Diffraction Method". Journal of Agricultural Engineering. 40 (2): 35. doi: 10.4081/jae.2009.2.35 . hdl: 10447/40752 . ISSN 2239-6268. S2CID 67844152.
↑ Ranjan, Gopal (2007). Basic and applied soil mechanics. [Place of publication not identified]: [publisher not identified]. ISBN 978-81-224-1223-9. OCLC 171112208.
1 2 "Laser diffraction for particle sizing :: Anton Paar Wiki". Anton Paar. Retrieved 2022-07-20.
↑ "Particle size analysis methods: Dynamic light scattering vs. laser diffraction :: Anton Paar Wiki". Anton Paar (in German). Retrieved 2022-07-20.
1 2 3 Gorączko, Aleksandra; Topoliński, Szymon (2020-01-31). "Particle Size Distribution of Natural Clayey Soils: A Discussion on the Use of Laser Diffraction Analysis (LDA)". Geosciences. 10 (2): 55. Bibcode:2020Geosc..10...55G. doi: 10.3390/geosciences10020055 . ISSN 2076-3263.
↑ Ryżak, Magdalena; Bieganowski, Andrzej (August 2011). "Methodological aspects of determining soil particle-size distribution using the laser diffraction method". Journal of Plant Nutrition and Soil Science. 174 (4): 624–633. Bibcode:2011JPNSS.174..624R. doi:10.1002/jpln.201000255. ISSN 1436-8730.
↑ Kettler, T., Doran, J., Gilbert, T., 2001. Simplified method for soil particle-size determination to accompany soil-quality analyses. Soil Sci. Soc. Am. J. 65:849–853