Soils 205-General Soils

Lecture 4-Soil Texture

I.  Soil texture is a measure of the relative proportions of sand, silt, and clay in a soil sample.  Sand, silt, and clay are defined by particle diameter and, combined, are known as soil separates.

    a.  definition of sand, silt and clay; based on USDA classification scheme

    b.  Properties of the soil separates

            i.  sand

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        Sandy soils will hold less water than fine textured soils

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            ii.  silt       

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            iii.  clay

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        c.  Coarse vs. fine textured soils

II.  Determination of soil texture and textural classes.  Particle size analysis- determination of the % of the various sized soil particles.

    a.  In the field- Texture by the "feel" method

        sand- feels gritty

        silt- feels greasy

        sand- allows soil to be "ribboned"; feels sticky

    b.  In the laboratory: mechanical analysis using sedimentation

        - Sands are determined by sieving

        - silt and clay are determined based on settling time using  Stoke's Law

    c.  Once the percentages of sand, silt, and clay are known, a sample is placed into one of twelve    textural classes based on a classification scheme developed by the USDA.  The twelve classes can be seen in the Textural Triangle shown below.