Soils 205- General Soils

Lecture 5- Structure, density, and porosity

I.  Structure- The arrangement of soil particles.  Structure can be highly influence by management where texture is generally not.

Scanning electron micrograph of an aggregate from a sandy A horizon

     a.  importance of soil structure

     b.  Types of soil structure

          i.  platy

        - thin horizontal lenses, can decrease infiltration

        - found in both surface and subsurface

        - can be inherited from the parent material

          ii.  Prism-like

        prismatic columnar

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          iii.  Block-like

angular blocky    subangular blocky

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          iv.  granular

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     c.  sequence of soil structure

II.  Density- weight/volume; there are two types of density that we need to consider in soil:

particle density and bulk density

    a.  Particle density = weight of solids/volume of solids

    Particle density is essentially equal to specific gravity.  It is difficult to measure and often times

    we assume a value of 2.65 g/cm³ or 2.65 Mg/m³ (the particle density of quartz).

    b.  Bulk density = oven dry weight of soil/total volume

    Since bulk density is calculated based on the total volume (solids + pores), and a typical soil

    is 50% pore space, the bulk density should be about 1/2 the particle density (~1.33 g/cm³).

III. Factors influencing bulk density- Soils with more pore space have lower bulk density values-

Any factor that influence the pore space affects bulk density!

    a.  texture- the finer the texture, the lower the bulk density