Particle Size Distribution of Soils

1. What is Soil Texture?

Let us give a clear definition. Granulometric (or mechanical) composition of soil is the relative percentage content of particles (or, as they are also called, mechanical elements) of various sizes (Foth, 1990). Simply put, it is what the solid part of the soil is made of—"bricks" of different sizes.

In any soil we find particles of very different sizes: from huge boulders to the finest colloidal particles that can only be seen under an electron microscope. However, in agronomy, we are primarily interested in the fine earth—the part of the soil that passes through a 2 mm sieve (Mukha et al., 2003). It is in this material, in particles smaller than 2 mm, that all the life and fertility of the soil are concentrated. Larger fragments (gravel, stones) are called the skeletal part of the soil—they are certainly important for drainage, but their influence on the soil's ability to retain moisture and nutrients is minimal.

Soil texture is not just an abstract number. It is one of the most fundamental and conservative characteristics of soil. Unlike moisture content or structure, it is virtually impossible to change it in the field. To turn a sandy soil into a clayey one, you would essentially have to replace the entire soil mass. Therefore, information about the particle size composition is the baseline from which we start when assessing any land plot (Weil & Brady, 2017).

How does this work in practice?

Take a dry soil sample in your hands. If it feels rough like sandpaper, you immediately understand that it is a sandy soil. If it resembles flour, smooth and non‑sticky—it is a loam or sandy loam. If it is sticky and plastic, and you can roll it into a "sausage"—it is clay. This is the field method of determining texture, the so‑called "organoleptic method" or the "feel" method. An experienced soil scientist can determine the composition with high accuracy simply by kneading a moist sample between their fingers (Foth, 1990).

But for science, this is not enough. We must give precise quantitative estimates. That is why we distinguish different particle‑size fractions and measure their proportions. And this brings us to the central question of our lecture: Why does particle size matter so much?

To answer this, we must consider the physical phenomena associated with particle size and shape. The first of these is the unique geometry of the surface.

2. Size Fractions

We now come to the question of how the particles of fine earth are customarily divided into groups, or fractions. Although this division may look like a simple classification, it is deeply physically grounded. Each fraction is not just a size range; it is a qualitatively different level of material organization with properties unique to it.

In most countries of the world, including Russia and the USA, the following basic scheme is adopted (Foth, 1990; Weil & Brady, 2017; Mukha et al., 2003):

• Sand: Particles with diameters from 2.0 to 0.05 mm.
• Silt: Particles with diameters from 0.05 to 0.002 mm.
• Clay: Particles with diameters less than 0.002 mm (i.e., < 2 microns).

As you can see, the boundaries between fractions are drawn at 2 mm, 0.05 mm, and 0.002 mm. These numbers are not arbitrary.

The 2 mm boundary separates fine earth from the skeleton. This is because particles larger than 2 mm hardly participate in the processes that determine fertility: they do not retain moisture, are not a source of nutrients, and do not affect the stickiness or plasticity of the soil.

The 0.05 mm (50 microns) boundary is approximately the limit of resolution of the human eye. Particles larger than this we can see and feel their roughness. Silt particles are no longer distinguishable with the naked eye, and to the touch they give a sensation of smoothness, similar to flour.

The 0.002 mm (2 microns) boundary is perhaps the most important and physically justified limit. This is where the world of colloidal particles begins. Clay particles are so small that their behaviour no longer obeys the laws of classical mechanics and begins to be governed by surface forces, which we will discuss later (Weil & Brady, 2017). They do not settle out of water according to Stokes' law like sand; they can remain suspended for a very long time.

Note the key feature of this scale: it is logarithmic. The particle size range from 2 mm to 0.002 mm spans three orders of magnitude. This means that in one gram of clay particles, the surface area can be thousands of times larger than in one gram of sand. This is the main secret of soil texture.

How are fractions determined?

In the laboratory, this is done using a method based on Stokes' law. The essence is simple: in still water, large particles settle faster than small ones (Foth, 1990; Marshall et al., 1996). The soil is dispersed (separated into individual particles) in water, and at certain time intervals, the density of the suspension at a certain depth is measured with a hydrometer (the hydrometer method). Knowing the time and depth, Stokes' formula is used to calculate the maximum particle size that could have settled by that moment. Sand settles in 40 seconds, while clay requires hours to settle (Foth, 1990).

In practice, in the field, the "feel" method mentioned earlier is more often used. But remember: this method gives only an approximate estimate. Only laboratory analysis provides accurate numbers.

Visualising the scale

To appreciate the difference in size, imagine that we enlarge a medium‑sized sand grain (0.5 mm) to the size of a tennis ball (about 4 cm in diameter). At this scale, a silt particle would be the size of a poppy seed, and a clay particle would be barely visible dust. Such a colossal difference in size is the reason why sand, silt, and clay behave differently.

Now that we know what fractions are, we can move on to the main question: Why does size matter? This question will be at the centre of our next section, where we will discuss the concepts of specific surface area and surface energy.

3. Why Size Matters

We come to the central question of our lecture. In the previous sections, we learned that soil particles are divided into fractions, and that the size range of these fractions spans three orders of magnitude. Now we must answer the key question: why does this size have such a huge impact on soil properties?

The answer lies in three fundamental physical concepts that are inextricably linked: specific surface area, contact area, and surface energy. Let us examine them in turn.

Specific surface area

Imagine a 1 cm cube of ice. Its total surface area is 6 cm². Now imagine that we divide this cube into 1000 cubes with an edge of 1 mm. The total surface area of these small cubes would be 60 cm²—10 times larger, although the mass of the substance remains the same (Weil & Brady, 2017). If we continue crushing to colloidal sizes, the surface area will increase to tens and hundreds of square metres per gram!

Specific surface area is the total surface area of all particles contained in a unit mass (or volume) of soil. It is this characteristic, not just the "amount of clay," that is key to understanding physical properties.

Let us give quantitative estimates (White, 2006; Shukla, 2023):

• Coarse sand (particle diameter ~1 mm) has a specific surface area of about 0.01 m²/g.
• Fine sand (~0.1 mm) — about 0.1 m²/g.
• Silt (~0.01 mm) — about 1 m²/g.
• Kaolinite (clay mineral) — 5–40 m²/g.
• Montmorillonite (swelling clay) — up to 750 m²/g or more!

The difference between sand and clay is thousands of times! This means that one gram of clay particles has a surface area comparable to a football field, whereas one gram of sand has the area of a postage stamp. This colossal difference underlies all the phenomena we will discuss.

Contact area

The increase in specific surface area has another important consequence: it multiplies the contact area between the solid phase of the soil and the liquid or gaseous phase (Foth, 1990). The greater the contact area, the more intense are all processes occurring at the phase interface:

• adsorption of water and gases;
• dissolution and ion exchange;
• interaction with root hairs and microorganisms;
• chemical reactions, including weathering.

In a sandy soil, the contact zone is small, so processes proceed slowly. In a clayey soil, the contact area is huge, so it reacts to environmental changes quickly and strongly.

Surface energy

Why does increasing the surface area so strongly affect the activity of a substance? The reason is that atoms and molecules on the surface of a solid are in a special, non‑equilibrium state. Inside a crystal, each atom is surrounded by neighbours, and its chemical bonds are fully saturated. On the surface, however, some bonds are "broken," and the atoms experience unsaturation. This unsaturation creates excess energy—surface energy—which the system tends to reduce (White, 2006).

The simplest way to reduce surface energy is adsorption: attracting and holding molecules from the surrounding environment (water, gases, dissolved ions) onto the surface. In essence, the surface "heals" its wounds by capturing foreign particles. The larger the surface, the more "wound surfaces" it has, and the greater its adsorption potential.

This principle is universal. It explains why:

• clay particles attract and hold thick water films around themselves;
• in clayey soils, the air is more humid;
• clay particles can hold nutrient ions on their surface, making them available to roots;
• clayey soils have high heat capacity and warm up slowly in spring.

Thus, particle size determines specific surface area, and specific surface area and its associated surface energy determine almost all physical and many chemical properties of soil. This is the key understanding that should serve as your foundation.

Now, armed with this knowledge, let us look at how particle size affects the most important soil processes: interaction with water, air, heat, and dissolved substances. We will address these in the following sections.

4. Particle Size and Water

We now come to one of the most important practical aspects of our topic—how soil particle size determines the behaviour of water in the soil. This is perhaps the most direct and tangible manifestation of the physical principles we discussed in the previous section.

Why does sand quickly transmit water, while clay retains it?

The answer lies in the size and number of pores that form between particles.

Imagine two containers: one filled with large balls (analogous to sandy soil), the other with microscopic balls or plates (analogous to clayey soil). In the first case, large gaps—macropores—remain between the balls. Water flows easily and quickly through them under gravity. In the second case, the gaps between particles are extremely small—these are micropores and even nanopores.

Here the phenomenon of capillarity comes into play. You have probably observed how water rises in a thin glass tube dipped into a vessel. The thinner the tube, the higher the water rises. This effect is due to surface tension forces and wetting of the walls. In soil, the capillaries are the pores between particles. The finer the particles, the thinner the pores, and the higher and more strongly water is held in them (Marshall et al., 1996; Shukla, 2023).

Quantitatively, this is described by the capillary rise equation, which in simplified form looks like:

where h is the height of water rise, r is the radius of the capillary (pore). From this formula it is clear that the height of rise is inversely proportional to the pore radius: the thinner the capillary, the higher the water rises. For a pore diameter of 0.01 mm, water can rise to about 3 metres, and for a pore diameter of 0.001 mm, already 30 metres! (Marshall et al., 1996).

Thus, in clayey soil, where fine pores predominate, water is held much more strongly than in sandy soil, where pores are large.

Forms of soil moisture

Depending on the strength of retention, several forms of water in soil are distinguished (Weil & Brady, 2017; Shukla, 2023):

1. Gravitational water—this is the water that flows freely downward under the influence of gravity. It is characteristic of large pores and is present in the soil only immediately after rain or irrigation. Sandy soils, where macropores dominate, quickly lose this water.

2. Capillary water—held in fine pores (capillaries) by surface tension forces. This water is the main source of moisture for plants. In loamy and clayey soils, there is much capillary water, and it is available to roots.

3. Film and hygroscopic water—these are the thinnest molecular layers of water held on the surface of solid particles by adsorption forces. This water is unavailable to plant roots because it is held too tightly. In clayey soils, with their enormous specific surface area, the amount of such water can be significant.

Available water for plants

For the agronomist, two indicators are critically important:

Field capacity (FC)—the maximum amount of water the soil can hold against gravity. This is the upper limit of water available to plants. In clayey soils, FC is higher than in sandy soils, due to the larger volume of capillary pores (Foth, 1990; Scheffer, 2018).

Wilting point (WP)—the soil moisture at which plants can no longer extract water and begin to wilt. This is the lower limit of available water. In clayey soils, WP is also higher, since a significant part of the water is held as unavailable films on particle surfaces.

The difference between FC and WP is the available water capacity—the water that plants can use (Weil & Brady, 2017; Foth, 1990). This is where the practical significance of texture becomes evident.

Table 4.2 (compiled from Foth, 1990; Weil & Brady, 2017)

As can be seen from the table, loamy soils are often the most favourable for plants: they retain water reasonably well, but are not waterlogged, and most of the moisture is in a form accessible to roots. Sandy soils, on the contrary, easily let water through but dry out quickly ("light" soils). Clayey soils retain much water, but it is difficult for roots to extract, especially in dry periods.

Why do sandy soils warm up quickly, while clayey soils warm up slowly?

This is related not only to the water regime but also to thermophysical properties, which we will consider in one of the following sections.

Summary of this section:

Particle size determines pore size, and pore size determines how water is held and moves in the soil. Fine particles (clay) create a system of fine capillaries that hold water against gravity but make it less available to plants. Coarse particles (sand) create large pores through which water quickly drains away, leading to frequent droughts. Loam occupies an optimal intermediate position. Thus, soil texture is the "conductor" of the soil water regime.

Now that we have dealt with water, let us see how particle size affects the second vital component of soil—its air regime. This is the subject of the next section.

5. Particle Size and Air

We move on to another crucial aspect that determines the "health" of the soil and its ability to support plant life—the air regime. If water is the blood of the soil, then air is its breath. And, as you have already guessed, this process is directly dependent on whether the soil particles are fine or coarse.

Why is air so important for soil and plants?

Plant roots, like all living cells, respire. They consume oxygen (O₂) in the process of respiration, which releases energy needed for growth and nutrient uptake, and they emit carbon dioxide (CO₂) (Foth, 1990). If there is insufficient oxygen in the soil, root respiration slows down, roots begin to suffocate, and the plant may die. This process is called hypoxia or oxygen starvation.

Soil microorganisms also need oxygen. It is aerobic bacteria and fungi that decompose organic residues, converting them into plant‑available nutrients. In the absence of oxygen, this process slows down, and organic matter accumulates in undecomposed form, while decomposition proceeds along anaerobic pathways, producing toxic products for plants (methane, hydrogen sulfide, reduced compounds of iron and manganese) (Foth, 1990; Weil & Brady, 2017).

How does particle size determine the air regime?

The answer to this question is a mirror image of what we discussed in the section on water. Air in the soil occupies the same pores that are not filled with water. Thus, soil aeration (its ability to supply roots with oxygen) is determined by the size and number of pores.

In sandy soils, as we recall, large macropores predominate. Even after heavy rain or irrigation, water quickly drains from these pores under gravity, making room for air. Therefore, sandy soils are always well aerated: air circulates freely in the large channels, providing roots with sufficient oxygen. The flip side of this coin is low water‑holding capacity.

In clayey soils, the situation is fundamentally different. Here, fine micropores and nanopores dominate. Water in them is held very strongly by capillary forces, so in clayey soils the pores are often filled with water, leaving very little free space for air. Gas exchange in such soil is impeded. After heavy rain or irrigation, clayey soil can remain waterlogged for several days, creating conditions for root oxygen starvation. This state is called waterlogging or flooding (Marshall et al., 1996; Weil & Brady, 2017).

How does this manifest in practice?

Imagine two fields after a downpour: one on sandy soil, the other on clayey. Water on the sandy field will infiltrate within a few hours, and you can work on it without problems. On the clayey field, water may stand in puddles for several days, forming mud and hindering any activity, while plants begin to yellow and languish from lack of oxygen.

Different plants respond differently to oxygen deficiency. Corn, for example, is very sensitive to waterlogging. Just a few days in flooded soil can significantly reduce its yield. Sorghum, on the other hand, has high tolerance to oxygen deficiency and can withstand flooding without visible damage for quite a long time (Eash et al., 2016). This is why in conditions of excessive moisture, preference is often given to crops with high hypoxia tolerance.

Capillary water and dissolved gases

It is important to understand that even in "dry" soil, the air in the pores always contains water vapour. The humidity of air in soil pores is usually close to 100% (Eash et al., 2016). In addition, gases, including oxygen and carbon dioxide, are dissolved in the soil water. Their concentrations depend on temperature, pressure, and biological activity.

When the soil becomes waterlogged, access of oxygen from the atmosphere is sharply reduced. This leads to rapid depletion of O₂ reserves dissolved in the soil water. At the same time, CO₂ produced by root and microbial respiration accumulates. In severe cases, this can lead to plant death (Foth, 1990).

Gas diffusion in soil

Air enters the soil and leaves it mainly by diffusion—a process in which gas molecules move from a region of high concentration to a region of low concentration. Oxygen diffuses from the atmosphere into the soil, where its concentration is reduced by root respiration. Carbon dioxide diffuses in the opposite direction.

The rate of gas diffusion in soil is about 10,000 times slower than in free air (Weil & Brady, 2017). This is because gas molecules have to travel through narrow, tortuous, and often partially water‑filled channels (tortuosity of the pore space). In clayey soils, tortuosity is maximal and the air pathways are most blocked by water, so the rate of gas exchange is minimal.

Summary of this section:

Thus, the air regime of the soil, like the water regime, is strictly controlled by the particle size composition. Sandy soils are well aerated but poor in moisture. Clayey soils, on the contrary, are rich in moisture but suffer from a lack of air, especially during periods of waterlogging. Loamy soils often find an optimal balance between moisture and air. For plants, both drought and oxygen starvation of roots are equally harmful, so knowledge of soil texture allows the agronomist to anticipate these problems and choose the right management strategy (drainage, tillage, crop selection).

Now that we have dealt with water and air, let us consider the third most important factor determined by particle size—the thermal regime of the soil. This is the subject of the next section.

6. Particle Size and Heat

We move on to the third critical aspect that determines the "well‑being" of the soil and its ability to create comfortable conditions for plants—the thermal regime. How the soil warms up and cools down affects sowing dates, seed germination rate, microbial activity, and ultimately the length of the growing season. And here, as you have guessed, particle size again plays a decisive role.

Why do some soils warm up quickly in spring while others warm up slowly?

Every gardener knows: sandy soils are "warm"—they warm up quickly after winter and allow earlier sowing. Clayey soils are "cold"—they remain wet and cold for a long time, and sowing must be delayed. What is the reason?

The answer lies in two key thermophysical characteristics: heat capacity and thermal conductivity.

Heat capacity is the amount of heat required to raise the temperature of a unit volume (or mass) of a substance by 1 °C. The heat capacities of different soil components differ greatly (Foth, 1990; Weil & Brady, 2017):

• Mineral particles (sand, clay): ~0.2 cal/(g·°C)—this means that only 0.2 calories are needed to heat 1 g of mineral matter by 1 degree.
• Water: ~1.0 cal/(g·°C)—5 times more! Water has one of the highest heat capacities among natural substances.
• Air: ~0.0003 cal/(g·°C)—negligible heat capacity.
• Organic matter: ~0.5 cal/(g·°C)—intermediate.

Thus, water is the main heat accumulator in soil. It is the water content that determines how much energy is needed to warm the soil.

How does particle size affect heat capacity?

Sandy soils, as we know, have low water‑holding capacity. Therefore, they contain little water. Their heat capacity is determined mainly by mineral particles (about 0.2 cal/(g·°C)). Little energy is needed to heat them, so they warm up quickly. Moreover, in dry sandy soils, air takes up much space, but it hardly participates in the heat balance due to its negligible heat capacity.

Clayey soils, on the contrary, contain much water (capillary and film water). Therefore, their heat capacity is close to that of water (about 0.5–0.8 cal/(g·°C) depending on moisture content). To heat a wet clayey soil, significantly more thermal energy is required. Therefore, in spring it remains cold for a long time (Scheffer, 2018; Foth, 1990).

Thermal conductivity is the ability of a substance to conduct heat. Heat in the soil propagates from warmer layers to colder ones. The speed of this process depends on thermal conductivity.

The thermal conductivities of soil components also differ (Foth, 1990; Marshall et al., 1996; Weil & Brady, 2017):

• Quartz (sand): high thermal conductivity (about 0.0088 cal/(cm·s·°C))—quartz conducts heat well.
• Clay minerals: medium thermal conductivity (about 0.0029 cal/(cm·s·°C)).
• Water: low thermal conductivity (about 0.0014 cal/(cm·s·°C)).
• Air: very low thermal conductivity (about 0.00006 cal/(cm·s·°C))—air is a poor conductor of heat.

It would seem that sand, composed of quartz, should conduct heat well and quickly transfer it to depth. However, in dry sand, contacts between particles are point‑like, and heat is poorly transmitted through air gaps. Therefore, the thermal conductivity of dry sand is low. When wetted, water fills the pores, increasing the contact area, and the thermal conductivity of sand increases.

In clayey soils, contacts between particles are more developed, but most of the pores are filled with water, which conducts heat less well than minerals. Therefore, the thermal conductivity of wet clay can be even lower than that of wet sand (Scheffer, 2018).

Practical implications for agronomy

1. Sowing dates: On sandy soils, you can sow earlier because they warm up faster. On clayey soils, sowing is delayed to avoid seed rot in cold waterlogged soil (Foth, 1990; Weil & Brady, 2017).

2. Spring frosts: On clear spring nights, the soil radiates heat intensively, and its temperature can drop below freezing. In dry sandy soils, this happens faster, and plants may suffer from frost. Wet clayey soils cool down more slowly because water releases the heat accumulated during the day, protecting plants (Marshall et al., 1996).

3. Length of growing season: The faster the soil warms up, the earlier active plant growth begins. This is especially important in regions with short summers. Sandy soils have an advantage here. However, in hot climates, rapid warming can lead to root overheating, and then clayey soils, which heat up more slowly, become more favourable (Weil & Brady, 2017).

4. Effect of organic matter: Adding organic amendments (peat, manure, compost) increases heat capacity and reduces thermal conductivity, making the soil more "inert"—warming and cooling more slowly. This can be useful for protection against sharp temperature fluctuations (Foth, 1990).

Summary of this section:

The thermal regime of the soil, like the water‑air regime, is determined by particle size through its influence on water‑holding capacity and packing density. Sandy soils (little water, few contacts) warm up and cool down quickly ("hot" and "cold" depending on the season). Clayey soils (much water, developed surface) warm up and cool down slowly ("cold" in spring but more stable in summer). Loamy soils occupy an intermediate position, which often makes them the most favourable for most crops.

Now that we have covered water, air, and heat, let us move on to another crucial property related to particle size—sorption capacity. This is the subject of the next section.

7. Particle Size and Sorption

We now turn to another fundamental aspect that directly links soil physics to its fertility—sorption. This term comes from the Latin sorbere—"to absorb". In soil science, sorption refers to the ability of the solid phase of the soil to absorb and retain molecules and ions from the soil solution and soil air on its surface (White, 2006; Weil & Brady, 2017).

We already know that clay particles have a colossal specific surface area and, consequently, high surface energy. These two factors make clayey soils powerful natural sorbents. Sand, with its negligible surface area, almost lacks this ability.

Why is sorption so important for plants?

Sorption is the mechanism by which the soil retains:

1. Nutrient elements (cations: K⁺, Ca²⁺, Mg²⁺, NH₄⁺, etc.) in a form available to roots, preventing their leaching with water (Foth, 1990).

2. Water (H₂O molecules), creating a moisture reserve that plants can use during dry periods (Marshall et al., 1996).

3. Pollutants (heavy metals, pesticides), limiting their spread in the environment (Weil & Brady, 2017).

How does sorption work at the physical level?

It is important to distinguish two main mechanisms, which often act together:

1. Physical adsorption (physical sorption)—retention of molecules or ions on the surface of a solid due to weak intermolecular forces (van der Waals forces). This is a reversible process: molecules can be easily adsorbed and desorbed when temperature or concentration changes. Physical adsorption is characteristic of the retention of water, gases, and non‑polar organic molecules (White, 2006).

2. Chemisorption (chemical sorption)—retention of ions by stronger electrostatic forces (attraction of opposite charges). Clay particles generally carry a negative charge on their surface (due to isomorphous substitutions in the crystal lattice). Therefore, they attract and retain positively charged ions—cations (Foth, 1990). This process is called cation exchange, and it is of great importance for plant nutrition.

It is important to understand: in the framework of our lecture, we will not delve into the chemical aspects of cation exchange, as this is a subject of a separate course (Agrochemistry). However, we must clearly grasp the physical reason for this phenomenon: the enormous surface of clay particles creates a huge number of active sites capable of retaining ions.

What is adsorption capacity?

The ability of a soil to retain cations is called cation exchange capacity (CEC) and is measured in milliequivalents per 100 g of soil (meq/100 g) or in cmol(⁺)/kg. This indicator strongly correlates with the content of clay particles and the type of clay minerals (Foth, 1990; Weil & Brady, 2017).

Table 7.1 (compiled from Weil & Brady, 2017; White, 2006)

As can be seen from the table, as specific surface area increases (and thus clay content), CEC increases sharply. Clayey soil can retain tens to hundreds of times more nutrient cations than sandy soil.

Practical implications for agronomy:

1. Fertilisers: In sandy soils, fertilisers are easily leached, so they must be applied frequently but in small doses. In clayey soils, fertilisers are retained firmly and can be applied less frequently but in higher doses (Foth, 1990).

2. Buffering: Clayey soils have high buffering capacity—they can resist sudden changes in pH and salt concentration. This means they are less sensitive to errors in fertiliser application or liming. Sandy soils, on the other hand, are easily "over‑acidified" or "salinised" (Weil & Brady, 2017).

3. Fertility: Soils with high clay content are generally more fertile because they retain nutrients better. However, this advantage can be negated by poor water‑air properties (waterlogging, lack of oxygen). Therefore, in practice, the most valuable are loamy soils, which combine good sorption capacity with acceptable water‑air regime (Marshall et al., 1996).

4. Self‑purification ability: Clayey soils can retain and neutralise many pollutants (heavy metals, pesticides), making them an important natural filter. Sandy soils, on the contrary, easily allow pollutants to pass into groundwater (Weil & Brady, 2017).

Summary of this section:

Thus, we have reached the logical conclusion of our analysis. Particle size determines specific surface area. Specific surface area determines the sorption capacity of the soil. The higher the sorption capacity, the better the soil retains nutrients, water, and pollutants. Sandy soils are poor because they cannot retain nutrients and water. Clayey soils are rich but often suffer from waterlogging and poor aeration. Loamy soils are the "golden mean", combining the advantages of both fractions.

Now that we have analysed all the key physical aspects of the influence of texture on soil properties, we can move on to a practical tool that allows us to visually classify soils according to this characteristic—the textural triangle. This is the subject of the final section of our lecture.

8. The Textural Triangle

We come to the final section of our lecture, where we will get acquainted with the main tool for classifying soils by particle size composition—the textural triangle (or fertility triangle). This simple but extremely effective graphical method allows us to visually determine which of the twelve main textural classes a particular soil belongs to, knowing only three numbers: the percentage of sand, silt, and clay (Foth, 1990; Weil & Brady, 2017).

How to read the textural triangle?

The textural triangle is an equilateral triangle, each side of which is divided into 100 divisions (percentages). The vertices of the triangle correspond to 100% content of one of the three fractions:

• Left vertex (0% sand, 0% silt, 100% clay)—the point of pure clay material.
• Lower right vertex (100% sand, 0% silt, 0% clay)—the point of pure sand.
• Upper vertex (0% sand, 100% silt, 0% clay)—the point of pure silt.

Inside the triangle, lines are drawn that divide it into fields corresponding to different textural classes (e.g., "sand", "loamy sand", "loam", "clay", etc.). Each field has its own boundaries for sand, silt, and clay content.

To determine the textural class of a soil, three simple steps are required:

1. Determine the percentage of sand, silt, and clay. These data are obtained from laboratory analysis or field "feel" determination (with subsequent calibration).

2. Find on the left side of the triangle the value corresponding to the clay content (in percent). Draw a line from this point parallel to the base of the triangle (i.e., to the right and slightly upward, toward the "Silt" vertex).

3. Find on the base of the triangle (bottom side) the value corresponding to the sand content (in percent). Draw a line from this point parallel to the right side of the triangle (i.e., upward and slightly left, toward the "Clay" vertex).

The point of intersection of these two lines is the position of your soil on the triangle. The textural class into which this point falls is the answer.

Example:

Suppose analysis shows: 60% sand, 20% silt, and 20% clay.

• On the clay scale (left side), find 20%. Draw a line parallel to the base.
• On the sand scale (bottom side), find 60%. Draw a line parallel to the right side.
• The intersection point lies within the field "loamy sand". So our soil is loamy sand (Foth, 1990).

Important: The sum of the percentages of sand, silt, and clay must always equal 100%. Therefore, if you know two values, the third is automatically calculated. This simplifies the procedure: you only need to draw two lines (for clay and for sand), and the intersection gives the desired point.

Main textural classes (according to USDA classification):

The USDA triangle distinguishes 12 main classes (Foth, 1990; Weil & Brady, 2017):

1. Sand

2. Loamy Sand

3. Sandy Loam

4. Loam

5. Silt Loam

6. Silt

7. Sandy Clay Loam

8. Clay Loam

9. Silty Clay Loam

10. Sandy Clay

11. Silty Clay

12. Clay

Each of these classes has a certain set of physical properties (permeability, water‑holding capacity, aeration, heat capacity, sorption capacity), which we have discussed in detail in the previous sections.

Why is it important to know how to read the triangle?

1. Practical classification: The triangle allows you to quickly and unambiguously classify any soil by its particle size composition. This facilitates information exchange between agronomists, soil scientists, farmers, and other specialists.

2. Predicting properties: Knowing the textural class, you can predict many physical and chemical properties of the soil (water regime, air regime, thermal regime, fertility, susceptibility to erosion, etc.). This helps in making correct agronomic decisions (crop selection, tillage systems, irrigation, fertilisation).

3. Basis for further research: The textural triangle serves as a starting point for more detailed study of the soil. It indicates which properties require closer attention and which research methods are most suitable for that soil type.

Conclusion to the lecture

Dear listeners! Today we have taken a journey into the world of soil particle size composition. We have discovered that it is not just a set of numbers, but a fundamental characteristic that determines almost all physical and many chemical properties of the soil. We have examined why particle size is so decisive, and how it affects the interaction of soil with water, air, heat, and dissolved substances. We have learned to use the textural triangle—the key tool for soil classification.

The main takeaway you should gain from this lecture:

The size of soil particles is not just a "trifle." It is the "architecture" of the soil that sets its fundamental properties. Knowing this architecture, we can predict and manage soil behaviour, creating optimal conditions for plant growth.

References

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8. Shukla, M.K. (2023). ‘Characteristics of Soils of the Vadose Zone’, in Soil Physics. An Introduction. Boca Raton, FL: CRC Press, pp. 23-54.
9. Skopp, J.M. (2012). ‘Physical Properties of Primary Particles’, in Huang, P.Ming., Li, Y., Sumner, M.E. (ed.) Handbook of Soil Sciences Properties and Processes. Boca Raton, FL: CRC Press, pp. 1-1:1-10.
10. Weil, R.R., Brady, N.C. (2017). ‘Soil Architecture and Physical Properties’, in The Nature and Properties of Soils. Essex, UK: Pearson Education, pp. 148-205.
11. White, R.E. (2006). ‘The Mineral Component of the Soil’, in Principles and Practice of Soil Science. The Soil as a Natural Resource. Malden, MA: Blackwell Publishing, pp. 11-33.
12. Муха, В.Д., Картамышев, Н.И., Муха, Д.В. (2003). ‘Минеральная часть твердой фазы почвы [Mineral Component of the Soil Solid Phase]’, in Агропочвоведение [Agropedology]. Москва: КолосС, pp. 40-58.
13. Муха, В.Д., Картамышев, Н.И., Муха, Д.В. (2003). ‘Почва как многофазная полидисперсная система [Soil as a Multiphase Polydisperse System]’, in Агропочвоведение [Agropedology]. Москва: КолосС, pp. 29-40.