The above flow diagram is a good starting point to get to the textural class. Relative stiffness of the soil together with ribbon length and knowledge of the textural triangle can help guide the determination of clay percentage.
The USDA textural triangle. Knowledge of the clay percent at some or all of the boundaries can be useful.
For the Students of Agronomy, Soils, and Environmental Sciences (SASES) Region IV Collegiate Soil Judging contest the following applies:
Estimates of the clay content as a weight percentage of the soil fines should be placed in the space provided. A scaled range for correct answers compared to values estimated by the judges will be used according to:
Actual % Allowed deviation
<20 +/- 2
20-40 +/- 3
>40 +/- 4
The textural class and % clay for each horizon will be determined by the judges and supported by laboratory data. Soil Texture Classes as defined in the United States Department of Agriculture Soil Survey Manual's Chapter 3 and their official abbreviations (supplied to contestants as an attachment to the SASES Region IV Official Judging Guide) will be used. Deviation from standard nomenclature will be incorrect (i.e., sandy silt, silty loam). Credit for sand, loamy sand, and sandy loam textures will NOT be given if sand modifiers are needed (i.e. very fine, fine, or coarse). Note: abbreviations need to match Attachment 2 as well.
Modification of the textural class will be required if the horizon contains more than 15% by volume coarse fragments (>2 mm), which includes carbonate and ironstone nodules. Sieves will be allowed during the contest. For the purpose of this contest, only the following terms will be used to describe coarse fragments:
Gravelly - fragments 2-75 mm diameter of any lithology and shape.
Cobbly - fragments of any shape and lithology that are > 75 mm diameter by their long axis.
If gravels and cobbles occur in the same horizon, the dominant condition should be described
Coarse fragment modifiers are required as follows:
Coarse fragment Modifier
<15% none needed
15-34% gravelly or cobbly
35-60% very gravelly or very cobbly
>60% extremely gravelly or extremely cobbly
One difference here is that while the soil seperates of the "fine earth fraction" --those less than 2 mm in diameter-- are reported on a mass fraction as percentage the "coarse fraction" --those greater than 2 mm in diameter-- are reported as a volume fraction of the volume of the soil horizon, or layer, being judged.
For example, if the horizon has a texture of clay loam with 40% by volume gravel-size fragments, the correct texture designation should be VGR CL (very gravelly clay loam).
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