Texture refers to the amount of sand, silt,and clay in a soil sample. The distribution of particle sizes determines the soil texture, which can be assessed in the field or by a particle-size analysis in the laboratory. A field analysis is carried out in the following way: a small soil sample is taken, water is added to the sample, it is kneaded between the fingers and thumb until the aggregates are broken down. The guidelines to determine the particle class are as following:

..Sand : Sand particles are large enough to grate against each other and they can be detected by sight. Sand shows no stickiness or plasticity when wet.

..Silt: Grains cannot be detected by feel, but their presence makes the soil feel smooth and soapy and only very slightly sticky.

..Clay: A characteristic of clay is the stickiness. If the soil sample can be rolled easily and the sample is sticky and plastic when wet (or hard and cloddy when dry) it indicates a high clay content. Note that a high organic matter content tend to smoothen the soil and can influence the feeling for clay. 

Table 9.5.1.1. Soil texture classes.

A variety of systems are used to define the size ranges of particles, where the ranges of sand, silt, and clay that define a particle class differs among countries. In the U.S. the soil texture is classified based on the U.S.D.A. system, which is used in this course. The classification of particle sizes are the following (units: mm):

..clay: < 0.002

..silt: 0.002 - 0.05

..fine sand: 0.05 - 0.1

..medium sand: 0.1 - 0.5

..coarse sand: 0.5 - 1.0

..very coarse sand: 1.0 - 2.0

..gravel: 2.0 - 762.0

..cobbles: > 762.0

Soil texture in the field is determined using a texture triangle (Figure 9.5.1.1.). For example, a particle size distribution of 33 % clay, 33 % silt, and 33 % sand would result in the soil texture class 'clay loam'.

 Figure 9.5.1.1. Trinagular diagram of soil textural classes (USDA triangle).

Particles greater than 2 mm are removed from a textural soil classification. The presence of larger particles is recognized by the use of modifiers added to the textural class (e.g. gravelly, cobbly, stony) (Table 9.5.1.2. and 9.5.1.3).

Table 9.5.1.2. Terms for rock fragments. 

 

Shape and size [mm]	Adjective
Spherical and cubelike: 2 - 75 2 - 5 5 - 20 20 - 75 75 - 250 250 - 600 > 600	xxx gravelly fine gravelly medium gravelly coarse gravelly cobbly stony bouldery
Flat: 2 - 150 150 - 380 380 - 600 > 600	channery flaggy stony bouldery

Table 9.5.1.3. Modifier for rock fragments.

 

Rock fragments by volume [%]	Adjectival modifier
< 15	no modifier
15 - 30	gravelly loam
30 - 60	very flaggy loam
> 60	extremely bouldery loam

The distinction between a mineral and an organic horizon is made by the organic carbon content. Layers which contain > 20 % organic carbon and are not water saturated for periods more than a few days are classed as organic soil material. If a layer is saturated for a longer period it is considered to be organic soil material if it has:

.. = 12 % organic carbon and no clay, or

..= 18 % organic carbon and >= 60 % clay, or

..12 - 18 % organic carbon and 0 - 60 % clay.

Figure 9.5.1.2. Relationship between soil texture and pore size.

Information provided by: http://www.soils.wisc.edu