Sand particles are the largest of the three soil separates, ranging in size from 0.05 mm to 2.0 mm in diameter. Sand particles can be further divided into five categories, ranging from very fine sand to very coarse sand. Sand particles are visible to the naked eye, and you can feel the individual grains of sand with your fingers. They are often composed of the mineral quartz, SiO2, although other minerals may be present. The dominance of quartz means that sands contain little in the way of plant nutrients.
Silt particles are between sand and clay in size, and range between 0.002 mm and 0.05 mm. Although they are similar to sand particles in shape and in mineral composition, silt particles cannot be seen with the naked eye. Unlike sand, which feels rough and gritty, silt particles feel smooth like flour or powder. Silt is composed of weatherable minerals, and its smaller size (and increased surface area) allows weathering at rapid enough rates to release significant amounts of plant nutrients.
Clay particles are the smallest of the three separates, less than 0.02 mm in diameter. The have very large surface areas, and carry a negative charge. Because of these two characteristics, clay holds a tremendous amount of water and plant nutrients. Clay soils can be very hard when they are dry, and sticky and plastic (moldable, like the clay in art class) when wet. Unlike grains of sand or silt, clays tend to be shaped like tiny flakes or flat plates. There are numerous pores between clay particles but they are so small that air and water cannot move very fast. There are different kinds of clay minerals, and these minerals give different properties to the soil. Shrinking and swelling, plasticity, water-holding capacity and soil strength are affected by the kind of clay mineral.
If you are having difficulty picturing the relative size of sand, silt and clay to one another, imagine this picture. You are standing outside a large sports stadium (e.g., Faurot Field at the University of Missouri, Busch Stadium in St. Louis, or Arrowhead Stadium in Kansas City), and someone parks a car next to the stadium. Now, walk up to the car and place a postage stamp on the car window. The stadium represents a sand-sized particle, the car represents a silt-sized particle, and the stamp, a clay-sized particle.
Pore size can be greatly affected by texture, which is defined as the "relative proportions of sand, silt and clay." Textures that are coarse (or dominated by sand) have larger diameter pores than finer textures with high amounts of silt or clay. Sand particles are too big to be able to fit close to one another. These larger voids can be occupied by smaller silts and clays in a soil.
Many properties important to plants can be linked to texture and the amount of surface area it provides. (Again, smaller particles have more surface area in relation to their diameter than large particles.) If a soil has more surface area, it can attract and hold more water and plant nutrients. Cohesion, or the tendency to "stick together," is stronger in clayey soils than in sands. Put some sand in the palm of your hand and shake it gently back and forth; will the sand grains stay together, or do they separate?
Many soils have particles larger than sand called coarse fragments. Size classes range from pebbles (or gravel, 2 mm to 75 mm), cobbles (75 mm to 250 mm), stones (250 mm to 600 mm), and finally boulders (=600 mm). Coarse fragments can affect water movement in the soil, or prevent root penetration, and they also occupy volume in the soil that could have been occupied by soil-sized particles.
The major soil textural classes (there are 12 of them) are defined by the percentage of sand, silt and clay, and they are shown on the Textural Triangle by the heavy lines. Soil texture may be determined in a laboratory by particle size analysis or in the field with a "texture by feel" estimate.
To use the Textural Triangle, first find the appropriate clay percentage on the left-hand side of the graph; draw a line across parallel to the base of the triangle. Next, find the sand percentage along the base of the triangle; draw a line upward parallel to the right side of the triangle (the one labeled "Percent silt"). The area where these two lines intersect will be the textural class of the soil. You need to know only two of the three percentages (sand, silt and clay) to find the textural class because all three percentages must total 100%.
Let’s work with two examples for practice, involving the concept of "loam" texture. Often, people think that loam means equal amounts of sand, silt and clay. Find the soil textural class if there is 33% sand, 33% silt, and 34% clay. Is it a loam? Now find the percentages of sand, silt and clay if you are in about the center of the "loam" area on the Textural Triangle. (If you place your pointer about where the "a" in "loam" is, you will be in about the center of the "loam" area.) This will give you percentages of around 40% sand, 42% silt and 18% clay—not equal amounts! Soil scientists think of a loam soil as one where each of the soil separates has nearly equal influence.
Look at each side of the Textural Triangle. Notice how much sand is required to get the word "sand" in the textural class name. Do you get about 45 – 50%? Now look at the side labeled "Percent silt." How much silt is needed to get the word silt in the textural class name? Look at the third side of the triangle, for percent clay. You need a much smaller amount of clay to get the word clay in the textural class name. You should now understand that the smaller the particle size (sand to silt to clay), the greater the influence the particle has on texture.
Textural class names can be defined as a grouping of various ranges of textural separates such that each grouping possesses unique management properties. You may be confused by the similarity of some of the names. Remember that the last word is the most important one. (Read the schematic below from right to left.)
| if another modifier | modifier | central theme | ||
| next in influence | ← | next in influence | ← | last term |
| Sandy | clay | loam |
Soil scientists use two methods to determine soil texture. One method, called "particle size analysis," is a laboratory technique; this method is used when it is important to know the precise percentages of sand, silt and clay. The other method is called "texture by feel" and it is commonly used as when it is not possible or necessary to determine exact percentages. (Believe it or not, a good soil scientist can estimate the clay percentage within 3% of an actual laboratory-measured value!)
The laboratory technique is based on Stoke’s Law, the principle that larger, heavier particles (such as sand) will settle out of water faster than smaller particles (such as clay). An instrument called a hydrometer is used to measure the density (grams per liter; g/L) of a soil-water mixture, that is, how many soil particles are in suspension. A chemical is mixed with the silt/clay solution (Calgon™ may be used) to help break apart clay aggregates. After carefully stirring the suspension with a special rod, the hydrometer is slowly lowered to float in the suspension. After 40 seconds, a reading is taken; this reading will tell how much sand is in the soil. Two hours later a second reading is taken to determine how much clay is present. Values obtained are entered into equations to calculate percents of sand and clay; silt is determined by subtracting sand plus clay from 100%.
This method has been modified to analyze a large number of samples at one time. Because sands fall so quickly and they may carry smaller particles of silt or clay downward, it is common to remove the sand by sieving before putting the soil into the sedimentation cylinder (it is then dried and weighed). A pipette is used to remove some of the soil suspension at timed intervals; the aliquots removed are dried in an oven and weighed to give grams of soil per liter of solution (g/L) reading.
Find out how you can determine soil texture at home.
Texturing the soil "by feel" is a way to estimate soil texture. To try this technique, mix about a tablespoon of soil (or less, if you have small hands) with a little water; use your hand and fingers to knead into a putty-like consistency, if necessary adding water a little at a time. If you find you have too much water, add a little more soil. When the water is evenly mixed throughout your soil, rub the sample with your thumb and fingers. Is it gritty? Or is it smooth and powdery? Is it sticky? These questions will help you decide if your sample has a lot of sand, silt, or clay in it. Then squeeze the sample between your thumb and index finger to make a ribbon. The length of the ribbon will tell you how much clay is in your soil. You can estimate the percent sand by adding a lot of water to a pea-sized lump of your sample from the ribboning. Break up the soil with your index finger, and then hold your hand out flat (palm up, of course!). Do you see the sand grains sticking up out of the water? Estimate the percentage of sand compared to the pea-sized lump you started with. With the sand percentage, plus the clay estimate from ribboning, you can find the textural class from the Textural Triangle.
There are many different types of clay minerals, each with unique chemical and behavioral properties which arise from the structure of the clay minerals. But nearly all clays contain just two basic components which occur in different arrangements. These two basic building blocks of all clay minerals are the silica tetrahedron and the aluminum octahedron.
When scientists talk about 1:1 ("one to one") or 2:1 clays, they refer to the ratio of silica tetrahedron sheets to aluminum octahedron sheets. A 1:1 clay has one of each sheet (think of an oatmeal cookie with icing on top); 2:1 clays have two tetrahedral sheets on either side of an aluminum octahedron sheet, (like an Oreo cookie). These tetrahedral and octahedral sheets are variously arranged and modified during mineral formation to create several types of clay minerals.
Kaolinite is one of the 1:1 clay minerals (Barak and Nater, 2003). It does not shrink when dry or swell when wet, which makes it well-suited for uses such as construction of roads and buildings, for septic adsorption fields, and pottery. X-ray diffraction analysis has shown that sheets of kaolinite are arranged like pages in a book; this affects the amount of surface area available for holding water or cations like Ca2+ or K+. Imagine a closed book with many pages; each page has surface area on the front and back, which might be a lot of surface area. But if the book is closed the pages are so tightly packed the surface area is not available. The only practical surface for water or cations to attach (or "adsorb") to would be the edges of the book, plus the front and back covers. Because of this arrangement, kaolinite has less external surface area than other clay minerals, no internal surface area, and less capacity for holding water and cations.
The 2:1 clay minerals look much different. Using X-ray diffraction analysis, montmorillonite (one of the smectite clay minerals) looks like a sponge (Barak and Nater, 2003). Observe in the figure above the differences in spacing between the 2:1 sheets. The larger interlayer spaces have the capacity to hold water molecules and a variety of cations (some of which, like Na+, cause the clay to disperse) with important consequences for plant growth. Also, with larger interlayer spaces comes a greater tendency for shrink/swell behavior (not all 2:1 clays are expanding). If a clay swells when wet, it is poorly suited for building site development or for septic leach fields. However, these clays are excellent for sewage lagoons or wildlife ponds; if they remain wet they "seal" and hold water.
| Characteristic | Kaolinite | Montmorillonite |
|---|---|---|
| Layer type | 1:1 | 2:1 |
| Typical chemical formula † | [Si4] Al4O10(OH)8 | Mx[Si8]Al3.2Fe0.2Mg0.6O20(OH)4 |
| Particle size (µm) ‡ | 0.5 – 5.0 | 0.01 – 1.0 |
| Specific Surface area (m2/g) †† | 7 – 30 | 600 - 800 |
| Shrink/swell potential | non-expansive | highly expansive |
| Interlayer space | none (very small) | very large |
| Cation Exchange Capacity (cmolc/kg soil) †† | 2 - 15 | 80 - 150 |
| Data obtained from: † Sposito, 2008; ‡ Brady and Weil, 2008; †† Sparks, 2003. | ||
Text blurb describing the setup and process of the timelapse.
Soil texture is one of the most important single properties of soil. It influences water movement and retention. It determines the amount of surface area, affecting chemical reactivity and nutrient-holding capacity. And texture is a factor in the erosion potential of the soil.
There are twelve soil textural classes comprised of various proportions of sand, silt and clay—the three soil separates. Organic matter is not included as a soil separate and therefore can not change the texture of a soil. If the percentages of sand and clay, with or without silt, are known, the Textural Triangle can be used to find the textural class name, for example loam, or sandy clay.
There are two main classes of clay minerals, described by the ratio of primary building blocks of tetrahedrons and octahedrons. Each type of clay has different properties and behaviors. First are the 1:1 clay minerals like kaolinite. A relatively large-sized clay mineral, kaolinite is a non-expanding clay well-suited to construction activities, septic leach fields, or ceramics (whether industrial or hobby). The second type is the 2:1 clay minerals. Many of these are expanding clays which shrink when dry and swell when wet. These types of clay have very large surface to volume ratios and have the capacity to hold large quantities of water and cations.
Add a sample of sandy soil to a plastic bottle filled with water. Do the same with a silty soil and a clayey soil. Time how long it takes for the particles to settle.
One practical application of this experiment concerns soil erosion. As particles are transported by running water, the velocity of the water keeps those particles in suspension. As the velocity slows, larger particles will fall out of suspension before smaller particles. You can use the time it takes each particles size class to settle to understand that texture influence erosion potential: the longer it takes to settle, the farther the particle can be transported by water.
An alternative to this experiment is to put a crushed or ground soil sample into a plastic container. Shake the container thoroughly (a lid would be helpful!) and allow the soil to settle. Take a ruler and measure the thickness of sand, silt and clay layers. From this you can estimate percentages of each soil separate.
Use the Textural Triangle to determine percentages of sand, silt and clay in several soil textural classes. Discuss how the amount of sand might affect pore size and water drainage. How would the amount of clay impact water-holding capacity?
Using materials as described in above construct your own silica tetrahedrons and aluminum octahedrons. Substitute aluminum for silica in the tetrahedrons and magnesium for aluminum in the octahedrons. Will magnesium (you can use large “shooter” marbles to represent magnesium) fit in the tetrahedrons? Can you make a 1:1 and a 2:1 clay mineral by combining tetrahedral and octahedral units (hint: they share the “apical” oxygen)?