Ch. 4 continued

Soil Properties

Soil ColorMunsell Color system:

Hue value/chroma color lightness/brightness

Ex: 10YR 5/6 isLighter than10YR 3/6; andbrighter than10YR 5/2

Causes or sources of color in soil

• Humus or decomposed organic matter black• Wet soils are darker than dry soils• Oxidized Fe oxides (Fe+3) are redder and brighter

than reduced Fe minerals (Fe+2) gleyed• Carbonates, gypsum, salts white masses or

nodules in soil and overall lighter values• Mn oxides black; usually associated with red

Fe oxides• Other minerals yellow, green, white, etc…

Color as indicator of environment

• Bright colors indicate oxidized (aerobic or oxygen-rich) environment

• Dull, grayed or bluish colors indicate reduced (anaerobic or water-logged) environment

• Pale yellow jarosite indicates oxidized (and now acidified) sulfidic soils

Soil Texture(proportion of different-sized

particles)

Master VariableBecause it affects so many other soil

properties and variables

Basic Soil PropertyBecause it is not readily

subject to change

Figure 4.1, page 96, Brady and Weil, 2004

“fine earth fraction (< 2-mm)” “coarse fraction (> 2-mm)”

Soil Separates

size • Sand and silt contain primary minerals such as quartz & feldspars• Clay is dominated by secondary minerals, formed from the

weathering of primary minerals

What does particle size tell us about mineralogy?

Sand and silt are largely chemically inert, while clay is chemically active (charged)

Mainly because of size, shape, and chemical characteristics of clay

Cations (+) stick to clay particles because of negative charges on the clay

These terms actually refer to texture, not weight

• A heavy soil is a clayey soil – called ‘heavy’ because of the soil's ability to retain

moisture and the difficulty in working the soil (and it’s heavy after rain or irrigation)

• A light soil is a sandy soil – called ‘light’ because it is usually easy to work in the

field and drains quickly• Actually, a clayey soil weighs less than a sandy soil

when dry

Heavy or Light Soils

8cm

8cm

8cm

Original Cube=

8cm x 8cm x 8cm

2 x 2 x 2

4 cm x 4cm x 4cm

Calculate: (1) total surface area of original cube (uncut) and

(2) total surface area after cutting it into smaller cube of the given sizes

2 x 2 x 2

Surface area for each face (8 x 8) = 64 cm2

A cube has six faces

Total surface area (6 faces 64 cm2 per face) = 6 x 64 = 384 cm2

Each cube is 2 cm on each side, the same mass of material would now be present as 64 smaller cubes

Surface area for one face of a small cube = 2 x 2 = 4 cm2

Surface area for each cube (6 faces, 4 cm2 per face) = 6 x 4 = 24 cm2

Total surface area (64 cubes, 24 cm2 per cube) = 64 x 24 = 1536 cm2

2 cm

2 cm

2 cm

Specific surface area

The surface area to volume ratio greatly increases as the particle size decreases and the shape changes from rounded to plate like

As the surface area increases, so does the ability to adsorb compounds and interact with the soil solution:

more chemical reactions, more biological interactions, more surface for water to cling to

Clay vs Silt vs Sand:

Colloidal Clay Silt Sand clay

Specific surface areaCECSorption capacityStickinessPlasticityCohesionBulk densityPorosityWater holding cap.

How particle size affects soil properties

Soil separates

Property Clay Silt Sand

SizeSpecific surface areaWater holding capacityStickiness/plasticityCation exchange capacityChemical sorptionPorosity amountPore sizeBulk DensityPuddlingTillage

<0.002 mmVery highHighHighHighHighHighSmallLowHighHard

0.05 - 0.002 mmModerateModerateLowModerateModerateModerateModerateMediumMediumMedium

2 - 0.05 mmVery lowLowVery lowLowLowLowLargeHighLowEasy

How particle size affects soil properties

Soil Texture

• Texture influences most other soil properties• Soil texture does not change in nature over a

short period of time• Adding organic matter may improve

characteristics of soils but not change texture • Large quantities of sand, silt, or clay must be

added and thoroughly mixed before the texture significantly changes

• Mixing media for potting soil, golf greens, etc. is generally the only time when textural modification is economically feasible