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SOIL PHYSICAL PROPERTIES
Texture and structure are two terms used to describe the physical properties of soil. Though they are sometimes used interchangeably to describe the same physical characteristics of soil, they are quite different. SOIL PHYSICAL PROPERTIES
Soil Texture
Soil texture is used to describe the relative amounts of sand, silt, and clay that make up the mineral portion of a given soil. This is one aspect of the soil that cannot be altered economically over large areas. Texture in small areas may be changed for specialty crops, such as adding sand to a bed in clay soil, but soil management has no effect on texture. SOIL PHYSICAL PROPERTIES
Soils that are formed in place from parent rock material generally reflect the content of the rocks from which they were formed. Alluvial soils covering flood plains are subject to the addition of new material during floods, but the material deposited is usually the same texture. Glacial soils were formed by past forces and are texturally stable now. SOIL PHYSICAL PROPERTIES
The four common names used to classify soils give an indication of the size of the mineral particles that make it up:
- Sandy
- Silty
- Clayey
- Loamy
Three of these, sandy, silty and clayey contain large amounts of or only those for which they are named. Loamy describes a class of soils that contain a certain percentage of each particle size.
Factors Affecting The Physical Properties of Soil
Sandy
Sandy describes soils that contain at least 70% sand-size particles and up to 15% clay particles. These soils have a course to moderately coarse texture. Individual sand grains can be seen and sandy soil crumbles when squeezed.
Silty
Silty describes soils that contain from 40-100% silt, with sand and/or clay making up the rest of those containing less than 100% silt. These soils have a medium texture. Only the larger of the individual grains can be seen. Silt feels and acts like sand when dry and like clay when wet. Dry silt feels grainy like sand and crumbles easily. Wet silt feels smooth and slippery like clay and tends to smear and stick together.
Clayey
Clayey describes soil that contains at least 35% of clay particles with the rest made up in different combinations of sand and silt. These soils have a fine texture. Individual clay particles cannot be seen without magnification. Clay with any moisture content feels slick and clayey soils don’t crumble easily. SOIL PHYSICAL PROPERTIES
Loamy
Loamy describes soils with varying amounts of the three soil-size particles. These soils have many subdivisions and make up the most complicated soil textural class.
All the different types of loam soils, from Clay Loam to Silt Loam to Loamy Sand, contain from 10-55% clay, from 60-88% silt, and from 0-85% sand. However, the Loam Soil classification itself has a relatively low percentage of clay with sand and silt making up the rest: 7-27% clay, 28-50% silt, and 22-52% sand.
SOIL PHYSICAL PROPERTIES
Soil Textural Classes
| Sand | Dry – Loose and single grained; feels gritty. Moist will form very easily crumbled ball. Sand: 85-100%, Silt: 0-15%, Clay: 0-10% |
| Loamy Sand | Dry – Silt and clay may mask sand; feels loose, gritty. Moist – feels gritty; forms easily – crumbled ball; stains fingers slightly. Sand: 70-90%, Silt: 0-3-%, Clay: 0-15% |
| Sandy Loam | Dry – Clods easily broke; Sand can be seen and felt. Moist – moderately gritty; forms a ball that can stand careful handling; definitely stains fingers. Sand: 43-85%, Silt: 0-50%, Clay: 0-20% |
| Loam | Dry – Clods are moderately difficult to break; somewhat gritty. Moist – Neither very gritty nor very smooth; forms a ball; stains fingers. Sand: 23-52%, Silt: 28-50%, Clay: 7-27% |
| Silt Loam | Dry – Clods are difficult to break; when pulverized feel smooth, soft, and floury, showing fingerprints. Moist – Has smooth or slick buttery feel; stains fingers. Sand: 0-50%, Silt: 50-88%, Clay: 0-27% |
| Clay Loam | Dry – Clods are very difficult to break with fingers. Moist – Has a slightly gritty feel; stains fingers; ribbons fairly well. Sand: 20-45%, Silt: 15-53%, Clay: 27-40% |
| Silty Clay Loam | Same as above, but very smooth. Sand: 0-20%, Silt: 40-73%, Clay: 27-40% |
| Sandy Clay Loam | Same as for Clay Loam. Sand: 45-80%, Silt: 0-28%, Clay: 20-35% |
| Clay | Dry – Clods cannot be broken with fingers without extreme pressure. Moist – Quite plastic and usually sticky when wet; stains fingers. (A silty clay feels smooth, a sandy clay feels gritty.) Sand: 0-45%, Silt: 0-40%, Clay: 40-100% |
Characteristics of the Various Soil Classes
| Characteristics | Sand | Silt | Clay |
| Looseness | Good | Fair | Poor |
| Air Space | Good | Fair to Good | Poor |
| Drainage | Good | Fair to Good | Poor |
| The tendency to Form Clods | Poor | Fair | Good |
| Ease of Working | Good | Fair to Good | Poor |
| Moisture Holding Ability | Poor | Fair to Good | Good |
| Fertility | Poor | Fair to Good | Fair to Good |
Soil Structure
Soil structure is defined as the combination or arrangement of primary soil particles into secondary groupings or arrangements called aggregates or peds. There are small pores or voids within the individual peds (micropores) and there are large pores between the individual peds (macropores). This is the place where almost all of the soil water and the air is contained.
If we combine mineral particles, hummus, and water, then sterilize them, they will remain in the same arrangement. Structure in the soil is a result of activity by the living organisms and plant roots in the soil and their interactions.
There are a number of different types of soil structures. The most common type found in the A Horizon (topsoil) is Spheroidal. Spheroidal structures contain rounded peds or aggregates and are usually loosely associated. This type of structure is common in well-managed pastures and annual crop fields.
The spheroidal structure is broken down into two subtypes: Granular and Crumb. Both of these are characterized by relatively high organic matter content (5% or greater). Granular is a moderately porous spheroidal structure. When a granular structure is very porous, the term crumb structure is applied. SOIL PHYSICAL PROPERTIES
The other types of soil structure are:
- Plate-like (platy)– peds are arranged in relatively thin horizontal plates, usually a product of soil-forming processes or parent material.
- Prism-like- vertically oriented or columnar peds, found in poorly drained soils in humid areas and in subsoils in arid and semiarid regions.
- Block-like (blocky)– common in subsoils, particularly in humid regions.
SOIL PHYSICAL PROPERTIES
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