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Soil texture
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Soil texture
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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

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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 held 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

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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. 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.[1] 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.[5]

Soil separates

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Particle size classifications used by different countries, diameters in μm

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.[6] 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.[7]

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

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Texture by feel

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Texture by feel flow chart

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.[9]

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.

Flow Chart to determine soil texture as used by the 4th edition of the WRB

Sieving

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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.

Schematic representation of sieve method

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.[10]

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

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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[11] 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.[12]

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.[12] 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.[12] 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:[12]

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.[13] 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.[14] 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.[14] 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.[14]

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.[15] 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

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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.[16] 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.[17]

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).[16]

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.[18] 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.[18]

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.[19] 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.[18]

Additional methods

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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.[20]

X-ray sedimentation

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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.

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Soil texture

View on Grokipedia
from Grokipedia
Soil texture is the relative proportion by weight of , , and clay particles in a soil, which fundamentally determines its physical properties and behavior. These particles are categorized by according to the USDA system: particles range from 0.05 to 2.0 mm in diameter, from 0.002 to 0.05 mm, and clay smaller than 0.002 mm. Soil texture excludes larger rock fragments and , focusing solely on the fine-earth fraction less than 2 mm. The classification of soil texture is typically determined using the USDA soil textural triangle, a graphical tool that delineates 12 major textural classes based on the percentages of , , and clay. Common classes include , loamy , sandy , , , , sandy clay , clay , silty clay , sandy clay, silty clay, and clay, with each class reflecting a specific balance of particle sizes. Texture can be assessed in the field through the "feel method," where soil is rubbed between fingers to gauge grittiness (), smoothness (), or stickiness (clay), or more precisely via laboratory particle-size analysis using or sieving techniques. Soil texture profoundly influences critical soil functions, including retention and drainage, , availability, and resistance. Coarse-textured soils like sands drain quickly but hold less and s, while fine-textured soils like clays retain more and cations but may compact and limit root penetration. These properties make soil texture a key factor in , , and ecosystem services, guiding decisions on suitability, needs, and conservation practices.

Fundamentals

Definition and Composition

Soil texture refers to the relative proportions of sand, silt, and clay particles in the mineral fraction of soil, specifically the fine-earth portion consisting of particles less than 2 mm in diameter, excluding organic matter and larger fragments. This composition determines fundamental physical characteristics of the soil, such as its behavior in relation to water, air, and nutrient movement, but it is distinct from soil structure, which describes the arrangement of these particles into aggregates or peds. The primary particle classes are defined by specific size ranges according to the (USDA) system, which is widely adopted in . Sand particles range from 0.05 to 2.0 mm, imparting a coarse, gritty feel; silt particles range from 0.002 to 0.05 mm, contributing smoothness and flour-like texture; and clay particles are smaller than 0.002 mm, providing stickiness and plasticity due to their fine size and high surface area. These size classes align closely with international standards, such as ISO 11277, which also delineates sand as 0.05–2 mm, silt as 0.002–0.05 mm, and clay as <0.002 mm for mineral soil materials. Soil texture is quantitatively expressed as the weight percentages of sand, silt, and clay, which sum to 100% of the fine-earth fraction. For example, a loam soil typically consists of about 40% sand, 40% silt, and 20% clay, balancing drainage and water retention properties. This mineral-dominated composition fundamentally influences broader soil behaviors, such as varying water-holding capacities across textures.

Historical Development

In the late 19th century, Vasily Dokuchaev, often regarded as the founder of modern soil science, advanced understanding by studying the chernozem soils of Russia and recognizing the role of mineral particles in soil formation and fertility, linking texture to broader pedogenic processes. The early 20th century marked significant progress in particle size analysis, with Swedish scientist Albert Atterberg proposing the first systematic soil texture classification in 1905 based on detailed studies of southern Swedish soils, defining particle size categories that influenced subsequent international standards. In the United States, the USDA began adopting texture-based approaches in the 1920s as part of expanding soil surveys, using particle distribution to differentiate soil types for agricultural purposes. A pivotal milestone occurred in 1924 with the formation of the International Society of Soil Science (now the International Union of Soil Sciences) in Rome, which standardized soil particle separates—such as sand, silt, and clay—facilitating global comparability in texture assessments. The USDA further refined its system in 1938 by publishing the soil texture triangle in the Yearbook of Agriculture, which graphically represented texture classes based on percentages of sand, silt, and clay, enabling more precise mapping. Post-World War II developments included refinements in soil characterization techniques, driven by increased needs for detailed agricultural mapping and conservation, incorporating more accurate particle analysis into national survey programs. Throughout this period, soil texture evaluation evolved from qualitative, feel-based assessments—relying on manual manipulation to gauge particle cohesion—toward quantitative methods, such as sieving and sedimentation, which provided measurable particle size distributions and reduced subjectivity in classifications.

Components and Properties

Soil Particle Classes

Soil particles, also known as soil separates, are classified primarily by size into three main classes: sand, silt, and clay, each with distinct origins and physical characteristics that contribute to soil texture. Sand particles, typically ranging from 0.05 to 2 mm in diameter, originate from the mechanical weathering of primary minerals such as quartz and feldspar in rocks like granite and sandstone, resulting in coarse, granular material that dominates in arid and coastal environments. Silt particles, between 0.002 and 0.05 mm, form through a combination of physical breakdown and deposition processes, often derived from glacial till or wind-blown loess deposits, as seen in regions like the Midwest United States and the Loess Plateau in China. Clay particles, finer than 0.002 mm, arise from the intensive chemical weathering of silicate minerals, such as feldspars and micas, under humid conditions, leading to secondary minerals like kaolinite, illite, and montmorillonite that are prevalent in tropical and temperate soils. The physical properties of these classes vary significantly, influencing their behavior in soil systems. Sand exhibits coarseness that promotes rapid drainage and low water retention, making it ideal for aeration but prone to drought stress in agricultural settings, with particles often rounded or angular due to abrasion during transport. Silt, with its fine yet non-plastic texture, offers moderate water-holding capacity but is highly susceptible to erosion by wind and water because of weak interparticle cohesion, leading to issues like dust storms in silty landscapes. Clay, in contrast, displays high plasticity when wet, allowing it to be molded, and possesses an expansive surface area—approximately 800 m² per gram for montmorillonite clays compared to just 0.01 m² per gram for sand—which facilitates strong adsorption of water, nutrients, and ions through cation exchange capacity (CEC) that can exceed 100 cmol/kg in smectitic clays. Microscopically, these particles exhibit behaviors rooted in their size and composition. Clay's colloidal nature, where particles behave as suspensions due to their small size and negative surface charge, promotes flocculation into aggregates when electrolytes are present, enhancing soil structure stability as described in classic soil colloid studies. Silt particles show intermediate cohesion, forming loose, flour-like aggregates that are easily dispersed, contributing to its role as a transitional class between sand and clay in texture dynamics. Sand, being inert and non-colloidal, lacks significant surface reactivity, resulting in minimal chemical interactions and primarily physical roles in soil porosity. These quantitative differences in surface area underscore the textural implications of individual particles before aggregation: clay's vast interface drives reactivity and nutrient retention, while sand's minimal area limits such functions, and silt provides a balance that affects overall soil workability without the extremes of the other classes.

Influence on Soil Behavior

Soil texture significantly influences the physical properties of soil, particularly permeability, porosity, and bulk density. Sandy soils exhibit high permeability due to their larger pore spaces, allowing rapid water and air movement, whereas clayey soils have low permeability owing to smaller pores and higher tortuosity, which restrict flow. Porosity, the volume of pore space in soil, varies inversely with bulk density and is generally higher in fine-textured soils like clays (around 40-55%) compared to coarse-textured sands (35-45%), owing to the abundance of micropores in finer particles and aggregates. Bulk density, typically ranging from 1.0 to 1.8 g/cm³, tends to decrease with finer textures (e.g., 1.0-1.4 g/cm³ for clays versus 1.4-1.8 g/cm³ for sands), as clay content promotes greater pore space, though compaction from management can elevate it across textures. Hydrologically, soil texture determines water holding capacity, infiltration rates, and field capacity. Clay soils possess the highest water holding capacity due to their abundance of micropores that retain water against gravity, followed by silt and then sand, which drains quickly. Infiltration rates are fastest in sandy textures (up to 10-20 cm/hour) and slowest in clays (less than 1 cm/hour), affecting runoff and erosion potential during precipitation. Field capacity, the water content after drainage, can be estimated as a function of texture percentages, with finer textures yielding higher values (e.g., 0.3-0.4 m³/m³ in loams versus 0.1-0.2 m³/m³ in sands). Chemically, texture affects nutrient retention and pH buffering primarily through clay content. Clays provide high cation exchange capacity (CEC), typically 10-150 cmol/kg, enabling strong adsorption of nutrients like potassium and calcium, which reduces leaching in fine-textured soils compared to sands with low CEC (under 5 cmol/kg). This CEC arises from negatively charged clay surfaces, enhancing fertility in clay-rich soils. Fine textures also improve pH buffering by increasing the soil's resistance to acidification or alkalization, as clays and associated organic matter neutralize added ions more effectively than coarse materials. Thermal properties of soil are modulated by texture, influencing heat transfer and temperature dynamics. Coarse-textured soils, such as sands, exhibit higher thermal conductivity (around 1-2 W/m·K when dry) due to better particle contact and lower water retention, facilitating quicker heat conduction from the surface to depth. In contrast, fine-textured soils have lower conductivity (0.2-1 W/m·K) because of trapped air and water, leading to more stable but slower-changing temperature regimes; for example, clay soils maintain cooler summer and warmer winter profiles than sands in temperate climates. These variations affect microbial activity and root growth by altering seasonal soil temperature patterns.

Classification Systems

Texture Classes and Triangle

The USDA soil texture classification system categorizes soils into 12 standard texture classes based on the relative proportions of , silt, and clay particles, which together comprise 100% of the mineral fraction finer than 2 mm. This system is widely used in soil surveys and agricultural assessments to standardize descriptions of soil physical properties. The classification relies on the USDA textural triangle, a ternary diagram that visually represents the proportions of sand, silt, and clay along its three axes, each ranging from 0% to 100%. Boundaries within the triangle delineate the 12 classes, such as sand (dominated by coarse particles), loam (balanced proportions), and clay (fine particles). The diagram facilitates intuitive interpretation by showing how small changes in particle percentages shift a soil from one class to another. The 12 USDA texture classes and their defining percentage ranges are as follows (note: some classes include specific conditions such as limits on silt + clay ratios to ensure mutually exclusive categories; percentages are by weight of the fine-earth fraction <2 mm, with the third component filling the remainder):
Texture ClassSand (%) (%)Clay (%)Additional Conditions
Sand85–1000–150–10silt + 1.5 × clay < 15%
Loamy Sand70–900–300–15silt + 1.5 × clay ≥ 15%, silt + 2 × clay < 30%
Sandy Loam43–850–500–20>52% sand or (silt + 2 × clay ≥ 30%)
23–5228–507–27-
Silt Loam0–5050–880–27≥50% silt; for 0–12% clay, 50–80% silt; for 12–27% clay, 50–88% silt
0–2080–1000–12-
Sandy Clay Loam45–800–2820–35-
Clay Loam20–4515–5327–40-
Silty Clay Loam0–2040–7327–40-
Sandy Clay45–650–2035–55-
Silty Clay0–2040–6040–60-
Clay0–450–4040–100-
These ranges are derived from laboratory and ensure consistent . To assign a texture class to a sample, one determines the percentages of , , and clay through , then plots these values on the textural by marking the intersection point from each axis. The class is identified by the region enclosing that point, following the predefined boundaries; for instance, points near the apex fall into the class, while those in the central balanced area indicate . Interpretation rules prioritize the dominant while considering transitional zones, ensuring consistent across surveys. This system simplifies communication in by providing a common vocabulary for texture, essential for mapping and in agricultural and environmental contexts. However, it has limitations, such as excluding , rock fragments, and certain mineral types (e.g., volcanic soils), which can alter effective texture and behavior.

International Variations

Soil texture classification systems vary internationally, reflecting differences in particle size definitions and class boundaries that arise from regional research traditions and standards. In the , the system adopted by the of defines as particles between 0.06 and 2 mm, as 0.002 to 0.06 mm, and clay as less than 0.002 mm. This finer lower limit for compared to the USDA's 0.05 mm results in adjusted boundaries for texture classes; for instance, clay loam is defined with 25–35% clay, 25–50% silt, and 15–50% , shifting some soils toward sandier designations. The Food and Agriculture Organization (FAO) and World Reference Base (WRB) for Soil Resources provide a harmonized international framework, aligning with ISO 11277:2009 standards where clay is ≤2 μm, silt is >2 to ≤63 μm, and sand is >63 to ≤2000 μm. This extends the silt range beyond the USDA's upper limit of 50 μm, influencing the 15 principal texture classes (plus qualifiers) used across 32 reference soil groups. European variations, such as the German DIN system (KA5), also follow a 63 μm silt-sand boundary but emphasize finer subdivisions within silt and clay, with clay broadly encompassing <2 μm while extending classifications to include up to 30 detailed texture types like "lehmiger Ton" (loamy clay). In Australia, the system narrows silt to 2-20 μm, with sand from 20 to 2000 μm and clay <2 μm, leading to classes that explicitly incorporate gravel (>2 mm) modifiers, such as "gravelly sandy clay loam," to account for coarser fractions prevalent in arid regions. These variations pose challenges for global comparisons, necessitating conversion tables and algorithms to reconcile discrepancies. For example, a soil sample with 40% , 40% , and 20% clay classified as "loam" under the USDA system may be reclassified as "sandy clay loam" in the UK system due to the shifted silt-sand boundary. Tools like models have been developed to map distributions between systems, though errors persist in borderline cases involving fine sands.

Determination Methods

Field-Based Techniques

Field-based techniques for determining soil texture involve qualitative assessments performed directly in the field, relying on manual manipulation of soil samples to estimate the proportions of , , and clay without requiring equipment. These methods are particularly useful in soil surveys, , and environmental assessments where rapid evaluations are needed. Common approaches include the texture by feel method and the ribbon test, which leverage tactile sensations to infer particle size distributions based on behaviors such as grittiness for , silkiness for , and stickiness for clay. The texture by feel method is a widely adopted field technique that allows operators to classify soil texture by assessing the relative contents of , , and clay through sensory evaluation. To perform it, collect a representative sample from the desired depth, remove coarse fragments and , and moisten it to a consistency similar to that of —neither too wet nor too dry. Squeeze the sample and rub it between the thumb and forefinger to evaluate grittiness, which indicates high content if pronounced (>50% feels distinctly gritty); smoothness and flexibility suggest silt dominance; and plasticity or stickiness points to clay if the sample molds easily without cracking. Prolonged rubbing can reveal cemented aggregates that mimic coarser textures but break down to expose finer particles. Studies show exact-match accuracy of about 66% for professional soil scientists and 27–41% for technicians when compared to results, improving with experience and including adjacent classes up to 91% for experts familiar with local variations. The ribbon test complements texture by feel by specifically gauging clay content through the 's plasticity. Begin by forming a moistened ball of about the size of a , then press it between the thumb and forefinger to extrude a thin ribbon of uniform thickness. The length and durability of the ribbon before it breaks indicate clay proportion: ribbons <25 mm suggest low clay (<15%, e.g., sandy or loamy textures), 25–50 mm moderate clay (15–25%, e.g., clay loam), and those exceeding 50 mm high clay (>25%, e.g., clay textures). This test is conducted at a level slightly above the 's plastic limit to ensure reliable plasticity assessment. It is often integrated into broader texture evaluations during field sampling. Simple field kits and tools, such as the jar test, provide a semi-quantitative alternative for rough texture estimation via observations. In the jar test, fill a clear, straight-sided jar one-third full with , add water to nearly full, include a dispersing agent like dish soap (1 teaspoon), and vigorously shake for several minutes. Allow for 24-48 hours: settles first within minutes (coarse layer at bottom), followed by after 1-2 hours (middle layer), and clay remains suspended longest (top layer). Measure layer depths to approximate percentages, then reference a texture for classification. This method requires minimal equipment—a and ruler—and is suitable for on-site use, though it demands patience for observation. Training for field operators, especially in soil surveys, emphasizes hands-on practice with known soil samples to build proficiency in these techniques. Programs by organizations like the USDA (NRCS) include calibration against laboratory data, familiarity with regional soil characteristics (e.g., and influences), and use of standardized guides to minimize variability. Experienced surveyors achieve higher consistency through repeated field exercises and reference to flowcharts for texture class assignment. These field methods offer key advantages, including speed—results in minutes to hours—and low cost, requiring no specialized tools beyond basic items like jars or one's hands, making them ideal for large-scale mapping or remote assessments. However, they are inherently subjective, with accuracy affected by operator experience, soil moisture levels, and interferences such as , which can mask true particle behaviors, or cemented aggregates that alter feel. Limitations also include reduced precision for fine distinctions between similar textures, necessitating periodic lab verification for critical applications.

Laboratory Sieving and Sedimentation

Laboratory sieving and sedimentation represent foundational quantitative techniques for determining soil texture in controlled settings, separating particles based on size through mechanical screening for coarser fractions and gravitational settling for finer ones. These methods, standardized by organizations like the USDA, enable precise measurement of sand, silt, and clay percentages, essential for texture classification. Sieving typically handles particles from gravel down to fine sand, while sedimentation targets silt and clay using principles of particle settling velocity. Sample preparation is critical to ensure accurate particle separation and minimize errors from aggregation or organic interference. Soil samples are first air-dried at 30–35°C to prevent structural alteration, then gently crushed and passed through a 2 mm to isolate the fine-earth , discarding coarser . For dispersion, 50 g of the oven-dried (at 105°C) is treated with a 0.5% () solution—approximately 25 mL—along with 200–250 mL deionized , allowing 30 minutes for initial breakdown of aggregates before mechanical agitation, such as high-speed mixing for 10 minutes at 10,000 rpm. This chemical prevents by coating particles with negative charges, though incomplete dispersion can lead to aggregation errors, overestimating coarser fractions by up to 5–10% in clay-rich soils. (H₂O₂) may be added (e.g., 15 mL of 30%) to remove if present, followed by drying and re-weighing. Dry sieving is employed for initial separation of coarse particles, particularly in non-cohesive soils, using a stack of sieves with progressively smaller mesh sizes, such as 2 mm ( to ), 1 mm, 0.5 mm, 0.25 mm, 0.105 mm (), and 0.05 mm (). A 50–100 g air-dried sample is placed on the top and mechanically shaken for 10–15 minutes using a rotary shaker at 200 oscillations per minute to avoid particle breakage. Retained material on each is weighed after brushing residues, and the percentage of is calculated as: %sand=(∑mass of sand fractionstotal oven-dry mass)×100\% \text{sand} = \left( \frac{\sum \text{mass of sand fractions}}{\text{total oven-dry mass}} \right) \times 100 This method provides reproducible separation for particles >0.05 mm, with precision typically within 1% for replicate analyses, though it underperforms in wet climates where aggregates form. Wet sieving follows for finer discrimination, where the dispersed sample is poured through a 0.05–0.053 mm sieve after agitation; the sand retained is oven-dried at 105°C and weighed, while the filtrate proceeds to sedimentation. Wet sieving enhances accuracy for aggregated soils by breaking clods during water immersion and shaking, yielding sand fractions with <2% variability. The hydrometer method quantifies silt and clay through sedimentation, relying on Stokes' law, which describes the terminal settling velocity vv of spherical particles in a fluid: v=2r2(ρp−ρf)g9ηv = \frac{2r^2 (\rho_p - \rho_f) g}{9 \eta} where rr is particle radius, ρp\rho_p and ρf\rho_f are densities of particle (typically 2.65 g/cm³) and fluid (1.0 g/cm³), gg is gravitational acceleration (980 cm/s²), and η\eta is fluid viscosity (adjusted for temperature, e.g., 0.01 poise at 20°C). After wet sieving, the <0.05 mm suspension (in 1 L cylinder with dispersant) is vigorously mixed and allowed to settle; an ASTM 152H hydrometer measures suspension density at 40 seconds (for silt + clay) and 2 hours (for clay), corrected for temperature (±0.36 g/L per °C deviation from 20°C) and blank readings. Percentages are derived as: %clay=(corrected reading at 2 h−blank at 2 hoven-dry sample mass)×100\% \text{clay} = \left( \frac{\text{corrected reading at 2 h} - \text{blank at 2 h}}{\text{oven-dry sample mass}} \right) \times 100 %silt + clay=(corrected reading at 40 s−blank at 40 soven-dry sample mass)×100\% \text{silt + clay} = \left( \frac{\text{corrected reading at 40 s} - \text{blank at 40 s}}{\text{oven-dry sample mass}} \right) \times 100 %silt=%silt + clay−%clay\% \text{silt} = \% \text{silt + clay} - \% \text{clay} %sand=100−(%silt+%clay)\% \text{sand} = 100 - (\% \text{silt} + \% \text{clay}) This approach assumes non-interacting spherical particles, with errors from organic residues or poor dispersion potentially inflating clay estimates by 3–5%. Overall precision for the hydrometer method is 2–5%, suitable for textural class assignment but less exact than pipette methods for research. Sieving reproducibility remains high, often <1% coefficient of variation, supporting reliable texture profiles when combined with sedimentation.

Optical and Advanced Methods

Optical methods for soil texture analysis leverage light and imaging to determine distributions with high precision and efficiency, often surpassing traditional techniques in speed while requiring careful calibration for accuracy. diffraction, a prominent optical technique, operates on the principle of light where a beam interacts with dispersed particles in a liquid medium, producing diffraction patterns analyzed via Mie theory to derive size distributions. This method assumes spherical particles and uses data to model for non-spherical particles, enabling rapid measurements across a broad range from 0.02 to 2000 μm. Instruments such as the Malvern Mastersizer series exemplify this approach, processing samples in under 10 minutes and providing volumetric distributions suitable for , , and clay fractions in texture . X-ray sedimentation combines principles of gravitational with absorptiometry to quantify fine soil particles by measuring the of X-rays as particles settle in a suspension column. In this method, soil samples are dispersed and allowed to settle, with an beam detecting changes in suspension density over time; for finer fractions like clay, a accelerates to enhance resolution below 0.002 mm. Devices such as the Sedigraph series automate this process, offering high-resolution distributions from 300 μm down to 0.1 μm and closely matching traditional results when sample preparation follows standards like AASHTO T88. This technique excels in analyzing cohesive soils where optical methods may falter due to . Scanning microscopy (SEM) provides direct visual assessment of particle morphology and size through high-resolution imaging, often paired with digital image analysis software to generate curves. SEM scans a focused beam over gold-coated or carbon-sputtered samples in a , producing secondary images that reveal surface textures and shapes at magnifications up to 100,000x, allowing of individual particles from nanometers to micrometers. Quantitative analysis via software like extracts size data from these images, validating distributions from other methods and elucidating microstructural influences on texture, such as aggregation in clay-rich soils. While SEM is labor-intensive and limited to small sample areas, it remains invaluable for confirmatory studies in research. Emerging techniques like (NIRS) offer indirect prediction of soil texture by correlating spectral reflectance patterns in the 400–2500 nm range with fractions through models. NIRS scans soil samples non-destructively, capturing and bands related to composition and that influence texture; multivariate regressions, such as modified partial least squares, build predictive equations from reference data, achieving accurate estimates of , , and clay contents with errors below 5% in calibrated datasets. Since the early , NIRS has automated texture analysis in soil laboratories, enabling high-throughput processing of hundreds of samples daily and integration with portable field devices for on-site predictions.

Applications and Implications

Agricultural and Environmental Uses

Soil texture significantly influences selection and agricultural practices by affecting retention, availability, and development. Coarse-textured soils, such as sands, are well-suited for crops like potatoes and carrots, which require excellent drainage and to prevent rot and promote easy tuber or expansion. Loamy soils, with their balanced proportions of sand, silt, and clay, are ideal for growing peppers such as Capsicum chinense, which thrive in well-drained yet moisture-retentive conditions, as supported by agricultural guides from Sri Lanka indicating light loamy or sandy loam soils are suitable for chili cultivation. Conversely, fine-textured clay soils support irrigation-intensive crops like , thanks to their superior water-holding capacity that maintains saturated conditions essential for paddy cultivation. These preferences stem from texture's control over soil hydrology, with sandy soils draining rapidly and clays retaining longer. Fertility management strategies are adapted to soil texture to optimize retention and . In sandy soils, which have low and poor holding, incorporating such as at rates of 2-4 inches per bed enhances water and retention, fostering microbial activity and reducing leaching losses. For clay soils, which often suffer from compaction and poor drainage, liming applications based on soil tests raise and improve , while can further aid calcium supply without altering texture significantly. Fertilizer rates are also texture-dependent; sands necessitate more frequent, split applications to match low retention, whereas clays require conservative dosing to avoid excess accumulation and environmental runoff. Soil texture is integral to and conservation planning, with silt-dominated soils exhibiting the highest erodibility due to their fine particles' susceptibility to detachment and surface sealing. For instance, soils, composed largely of , yield elevated loss rates under rainfall, often exceeding those of sands or clays. The USDA's Revised Universal Soil Loss Equation (RUSLE2) incorporates texture via the erodibility factor (), assigning values up to 0.65 for high- soils to predict annual loss accurately and guide practices like contour farming or cover cropping in vulnerable areas. In environmental management, soil texture governs dynamics and carbon storage. Fine-textured clays excel at sorbing like through interactions and high surface area, binding 50-90% of adsorbed metals non-exchangeably to limit leaching into . This sorption capacity is enhanced in neutral to alkaline conditions, making clay-rich soils effective natural barriers against contamination. Additionally, fine textures promote greater , with soil organic carbon stocks 2-4 times higher than in coarse sands due to stronger organo-mineral associations and reduced rates. Fine soils thus support higher permanganate-oxidizable carbon levels, aiding long-term atmospheric CO₂ mitigation.

Engineering and Management Considerations

Soil texture plays a pivotal role in by influencing key properties such as , , and plasticity, which directly affect and load-bearing capacity. Coarse-textured soils like sands derive higher primarily from frictional resistance between particles, enabling them to support heavier loads with less deformation compared to fine-textured soils. In contrast, fine-textured clays exhibit greater due to their cohesive nature and ability to undergo consolidation under sustained loading, leading to potential settlement issues in foundations or embankments. The , including the liquid limit (LL) and plastic limit (PL), quantify plasticity through the plasticity index (PI = LL - PL), which increases with higher clay content and correlates with behaviors like shrink-swell potential and reduced at higher contents. In construction applications, soil texture guides and to optimize performance and mitigate risks. For road bases, sandy textures are preferred for their superior drainage, which prevents weakening from water accumulation and enhances long-term pavement durability under traffic loads. Foundations on expansive clays must be avoided or specially designed, as these soils' volume changes with moisture fluctuations can cause differential settlement and structural cracking. Compaction standards, evaluated via the Proctor test, vary by texture; sands achieve maximum dry density at lower moisture contents due to their non-cohesive nature, while clays require higher optimum moisture for effective compaction, influencing embankment construction specifications. Land management practices incorporate soil texture for and planning to ensure and functionality. Fine-textured soils, such as clays, are utilized in waste containment systems like landfill liners owing to their low permeability, which restricts migration and complies with regulatory requirements for hydraulic barriers. Conversely, coarse-textured support septic systems effectively through high infiltration rates, typically requiring medium sand or finer textures with hydraulic loading rates of 1.0–1.25 gallons per day per square foot to facilitate dispersal without contamination. In regions with heavy clays, strategies emphasize enhanced drainage installations, such as curtain drains or underdrains, to manage perched water and prevent saturation-related failures. Case studies highlight both risks and advantages of soil texture in engineering contexts. In , expansive clays have led to widespread foundation failures in buildings, with shrinkage and swelling causing wall cracks and floor heaves, as documented in regions like Bowie County where differential movements distorted structures without mitigation. On the positive side, permeable pavements installed over sandy soils in stormwater management projects, such as those in , have demonstrated benefits including 85–90% runoff reduction and improved through pollutant , with case studies showing sustained infiltration even in variable climates when underlain by high-conductivity sands.

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