Soil Physical Properties
The physical properties of any soil are due to the interaction of three factors: (1) soil particle size and their ratio in the soil mixture; (2) structure of soil aggregates in the mixture; and (3) the type and level of organic matter in the soil. Soil texture influences soil drainage, moisture retention, nutrient retention, and the kinds of plants that can flourish in a given soil type.

Sand is the largest of the soil particles, and is usually composed of quartz or other silica minerals. Sand particles are large enough to be easily viewed by the unaided eye or by low-power microscopy (10-50x). Under magnification, sand particles may be jagged and irregular, or smooth and nearly round, depending upon the degree of weathering. Because of their sizes, sand particles are separated by voids (pore spaces) which easily permit air and water to flow. The higher the sand content of a soil, generally means that aeration will be high and that drainage will be quite excellent. Sandy soils, because they tend to be well-drained, often support drought-adapted vegetation. Drought-adapted vegetation may take on features associated with xeric (dry) conditions, including glaucous (blue-green) leaves, hairy leaves, succulence, thorns, stomatal pits and other "desert-plant" adaptations.

Silts are finer particles, and are often called "microsands." These are intermediate in size between sands and clays. Silt diameters' ranges from 0.002 to 0.05 mm. Silts can be observed under high-power magnification. They are typically irregular in morphology and are seldom flat or smooth. Mississippi has one of the most extensive deposits of aeolian (wind) deposited silts in North America. The Mississippi loess belt extends from below Memphis, Tennessee and continues in a crescent that delineates the eastern edges of the northwestern river floodplain (the "Delta") down to Natchez and southward into Louisiana. Loess silt is unusual in that the particles have rounded edges that provide unique physical and structural properties to the loess or brown loam soils.

Silt particles are notably uniform in size and shape. This makes silty soils very friable when moist. Moist silts form water films around the particles and this allows each particle to slide past another. High silt soils have very low load-bearing strength when moist, and they can collapse or shift under high precipitation, especially on slopes. Silts tend to form fragipans - compacted layers 30-60 cm deep. Fragipans restrict air and water movement, and dictate vegetation composition in some areas. For example, normally moist-soil and riverside sycamores are fairly common in the loess bluffs in elevated landscape positions.

Clay particles are quite small and have diameters of less than 0.002 mm.These require very high magnification (> 1000 x) to be viewed. Clay particles have diameters ranging from 25 times less than the smallest sand or largest silt to 1000 times less than the largest sand particles! While some people think that clays are the weathering byproduct of silt weathering, clays are silicate minerals that are synthetically formed by crystallization and precipitation of products of both mineral weathering and dissolution, and sedimentary processes. Clay particles are so tiny that they become tightly packed, such that clays typically have little or no pore space. Clay minerals are normally flat and as such, stack together like plates or pieces of paper. Because of the tightness of clays, they tend to be impervious to air and water movement. The pore spaces or voids between clay particles are microscopic, yet form tremendous surface areas that allow a large amount of water to bind to clay particles. So, clays are resistant to water flow, but, when wet, hold much water and retain that water quite tightly.

Clay mineralogy is the study of the kinds of clays and their physical and chemical behavior. We recognize three major clay groups. Kaolinitic clays are 6-sided flatish crystals formed during extreme weathering. We see soils with high kaolinite content in our ultisols in Mississippi. Smectites are also known as montmorillinites. Clay particles are complex, flat crystals that form in young soils (e.g., inceptisols, entisols, alfisols, vertisols). Smectites are made of bi-layers of crystal units that are loosely held together.

Most of the clays are nearly same-sized, yet they exhibit markedly different properties. For example, as smectites are wetted, the layers move farther apart (they swell). As they dry, they grow closer together (they shrink). Thus, smectites are termed "shrink-swell" clays. While illites are also 2-sheeted, potassium ions in the clay structure prevent "shrink-swell" phenomena. Kaolinite clays have only one layer and do not "shrink-swell."

Clays and clay loams are very common in slack-water soils in the flood plains of northwestern Mississippi (here, they're called "gumbos" and "buckshots"), and in the northern blackland prairies. Illites are only found in young soils. Shrink-swell clays are problematic in that they are difficult construct buildings on, and they tend to crack open as they dry in summer. Although nearly pure smectites occur in Mississippi, most clay soils are mixtures of the three types.

Soil Names Based Upon Texture
Soil names at the "common" level are based upon the relative amounts of sand, silt and clay found in the surface soil (O-A horizons or topsoil). Thus, we can talk about soils in the context of their textural mix - sands, fine sands, fine sandy loams, sandy loams, loams, silty loams, clay loams, clays, sandy clays, and so on. An "ideal" soil would have about 40% sand, less than 20% clay, and the balance as silt. This would probably be a friable soil that has good moisture retention coupled with good drainage.

Most are familiar with the soil textural expressions loam, clay, silt loam and so forth. Soil scientists use laboratory procedures to determine the exact proportions of the particle size classes in a soil. The results as percent sand, silt and clay are plotted on a texture triangle graph and the soil texture is determined.

Although laboratory texture determinations are best for exacting research, with practice, so-called ribbon tests can give reasonably good guesses. The ribbon test is performed by working in the hand a very moist clump of soil slightly larger than the diameter of a quarter. The soil should be kneaded until it has fairly uniform wetness throughout. Ribbons are formed by working the ball of soil in the palm up through the thumb and forefinger. Use the guidelines below to interpret the soil response.