Soil texture is a qualitative classification tool used in both the field and laboratory to determine classes for agricultural soils based on their physical texture. The classes are distinguished in the field by the 'textural feel' which can be further clarified by separating the relative proportions of sand, silt and clay using grading sieves: The Particle Size Distribution (PSD). The class is then used to determine crop suitability and to approximate the soils responses to environmental and management conditions such as drought or calcium (lime) requirements. A qualitative rather than a quantitative tool it is a fast, simple and effective means to assess the soils physical characteristics. Although the U.S.D.A. system uses 12 classes whilst the U.K.-ADAS uses just 11 the systems are mutually compatible as shown in the combined soil textural triangle below.
Contents |
Hand analysis, whilst an arbitrary technique, is an extremely 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 an extremely useful tool for identifying spatial variation both within and between plots (fields) as well as identifying progressive changes and boundaries between soil classes and orders.
The method involves taking a small sample of soil, sufficient to roll into a ball of approx 2.5cm diameter, from just below the surface. Using a small drop of water or 'spit' the sample is then moisten to the sticky point (the point at which it begins to adhere to the finger). The ball is then molded to determine its workability and its class according to the steps in the chart opposite.
Soil separates are specific ranges of particle sizes. In the United States, the smallest particles are clay particles and are classified by the USDA as having diameters of less than 0.002 mm. The next smallest particles are silt particles and have diameters between 0.002 mm and 0.05 mm. 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.
| Name of soil separate | Diameter limits (mm) (USDA classification) |
| Clay | less than 0.002 |
| Silt | 0.002–0.05 |
| Very fine sand | 0.05–0.10 |
| Fine sand | 0.10–0.25 |
| Medium sand | 0.25–0.50 |
| Coarse sand | 0.50–1.00 |
| Very coarse sand | 1.00–2.00 |
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 a roughly equal concentration of sand, silt, and clay, and lends to the naming of even more classifications, e.g. "clay loam" or "silt loam."
In the United States, twelve major soil texture classifications are defined by the USDA.
Determining the soil textures is often aided with the use of a soil texture triangle.
The first classification, the International system, was first proposed by Atterberg (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. Commission One of the International Society of Soil Science (ISSS) recommended its use at the first International Congress of Soil Science in Washington in 1927. Australia adopted this system and according to Marshall (1947) its equal logarithmic intervals are an attractive feature worth maintaining. The USDA adopted its own system in 1938, and the FAO used the USDA system in the FAO-UNESCO world soil map and recommended its use.
Rowell D (1994) Soil Science; Methods and Application, Longman Scientific & Technical, 1994 350 pages [1]