Soils 205 Lecture 2- notes

Video session                                                 Pages in Text.

3 123-133

 

Soil Texture and Structure

Texture- the proportions of sand, silt, and clay particles.  The classification scheme, illustrated below, was devised by the USDA

 

Coarse textured vs. fine textured soils

 

Particle size analysis- determination of the % of various sized particles

1.  In the laboratory: mechanical analysis

- Sands are determined by sieving

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

 

 

Stoke's Law:

2.  Field determination- Texture by the "feel" method

sand- feels gritty
silt- feels greasy
clay- allows soil to be "ribboned", feels sticky

Textual class

Based on the results of particle size analysis, a soil is placed into one of 12 textural classes.  Boundaries between textural classes represent differences in behavior of the soil due to the texture.  The textural class is determined by using the textural triangle shown below.

 

Using the Textural Triangle- Find the textural class of a soil with 50% sand, 25% silt, and 25% clay

 

answer:  this soil is a sandy clay

 

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

Types of soil structure

1. Platy

s thin horizontal lenses, can decrease infiltration

s both surface and subsurface

s some are parent material influenced

2. Columnar and Prismatic

rounded tops flat tops
rounded columns square faces

s vertical pillars

s subsoil in arid and semiarid regions

s Columnar is best expressed in Na+ influenced soils

3. Blocky and Subangular Blocky

more rectangular,
flatter faces
more rounded

s usually well-defined blocks from 1 to 10 cm

s subsoils - usually higher in clay and humid

4. Spheroidal (granular & crumb)

              

s rounded aggregates, <1 cm in diameter, encourages infiltration

s stable to wetting

s surface soils - esp. grasslands and where earthworm are active

 

Structure type sequence

 

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