Size Separates

© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Separates are the individual particles that together with organics, salts, and evaporates comprise a soil. We size them by placing them together in groupings that have a range in size with an upper and lower effective spherical diameter. The choice for the range in size is dictated by properties that one ‘size range’ exhibits that is different than other size ranges. Often these can be grouped for particular purposes. The groupings once formalized are termed ‘classes.’ The term fraction is used to connote part of a whole.

First we group the separates of the “whole” soil into two classes, the coarse fraction and the fine-earth fraction. The coarse fraction consists of the particles greater than 2 mm in diameter and the fine-earth fraction consist of particles equal to and less than 2 mm.

In the United States it is still common to see length measurements in the United States Customary System. This system is similar to the units of measure used in England when the U.S. became and independent country. The U.S. government passed the Metric Conversion Act of 1975 making the metric system the preferred system. Metric units are well adopted in science and medicine in the U.S. but still it is common in popular customary usage to have inch, foot, or yard used as a measure of length. It is useful to know that 1 mm = 0.0393701 inch and that 1 inch = 25.4 mm.

Coarse Fraction > 2 mm

Fine-Earth Fraction ≤ 2 mm

Both the coarse fraction and fine-earth fraction are further divided, again on the basis of size. The fine-earth fraction is divided first into three size separates: sand, silt, and clay. 

Coarse Fraction Groupings

Separates that are in the coarse fraction are important to consider because they occupy space in the soil but contribute little or no porosity (water and/or air storage) and little chemical reactivity (sorption and/or nutrient storage). Thus, coarse fragments reduce the soil volume available to hold water and reduce the ability of a given volume of soil to retain nutrients.

Coarse fragments also make cultivation more difficult. Frequently, course fraction and fine fraction are not capitalized when writing, therefore, recognizing that they are technical terms conveying an organized grouping of soil separates is critical to understanding the intention and deductions of the communications employing them.

Table -- Coarse fragment separates names and size classes.
Name / Class Size
Gravel 2-75 mm
Cobble 75-250 mm
Stone 250-600 mm
Boulder > 600 mm

Fine Earth Fraction Groupings

Table -- The United States Department of Agriculture (USDA) fine earth fraction mineral material (< 2 mm) three class size separates grouping names and size limits.
USDA Particle Size Groupings of Size Separates Size (mm)
Clay, total < 0.002
Silt, total 0.002 - 0.05
Sand, total 0.05 - 2.00

Each of these main groupings of size separates within the fine earth fraction has upper and lower bounds on the effective diameter of the size separates that fit in that category.

Note that the silt fraction is further separated into fine and coarse. This has particular importance to the mechanical properties of the silt size.

In an introductory soils course students are responsible for the size limits of the coarse fraction, fine earth fraction and for the size limits of main groupings of clay, silt, and sand.

The clay and sand are also subdivided. Memorizing the sand fractions is particularly useful since these sizes often show up in names of soil map units. Soil map units convey information about patterns of spatial variability across the landscape. Additionally, in more advanced courses students are asked to be responsible for the size limits of the fine silt and coarse silt. The clay is fractionated into fine, medium, and coarse clay. The upper size limit of the fine clay must be known to be successful at categorizing soil bodies in the (US) Soil Taxonomy.

Students in an introductory course are typically responsible for the size of the whole of the sand, silt, and the whole of the clay in the fine earth fraction (≤ 2 µm), the sub-fractions of sand, and the upper limit of the fine clay.

The divisions of the sand size fraction are important for both mechanical —cohesion— and water content influence. You may be asked for the sand fraction size, 2.00 to 0.05 mm, and you may be asked for the sand separates name and sizes as follows:

Sand Separate Diameter (mm)
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 course sand 1.00 - 2.00

Each of the sand separate size sub-categories has upper and lower bounds on the effective diameter of the size separates that fit in that category. These sub-categories of sand size separates show up often in soil map unit names, are important in engineering interpretations, and for erosion predictions. Knowing these is valuable for study and interpretation of soils functions.

Other particle size classification systems exist. When using size separates data for samples that have been characterized and recorded in the USDA textural classes the very fine sand is part of the sand, but when estimations are made for engineering and erosion purposes the very fine sand is grouped with silt.

When clay size separates act as individuals they are subject to translocation within the soil. When separating soil bodies on the landscape taxonomic systems recognize this process and separate soils based on the relative proportions of the fine clay. The sizes of the coarse clay, medium clay, and fine clay are listed below. The important value to know is the upper bounds of the fine clay: 0.00008 mm.

Clay Separate Diameter (mm) Diameter (µm)
coarse clay 0.002 mm to 0.0002 mm 2 µm to 0.02 µm
medium clay 0.0002 to 0.00008 mm 0.02 µm to 0.08 µm
fine clay < 0.00008 mm < 0.08 µm

A micrometer (µm) is 1/1,000 of a millimeter (mm).

An important point about clay

Clay has three meanings.  Be careful, because the context the term ‘clay’ is used in dictates the meaning

  1. Clay is a size separate fraction.
  2. Clay is a textural class name.
  3. Clay is a common name for a class of minerals.

Similarly, silt and sand also are size separate fractions and textural classes. The multiple meanings are a challenge to the casual user and the introductory soils course learner is expected to master this nuance. When in doubt write it out.

Know the size ranges for the coarse fraction, fine earth fraction, whole sand, very course sand, coarse sand, medium sand, fine sand, very fine sand, whole silt, whole clay, and fine clay.

Another section will define textural class which uses this knowledge.