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Soils 2


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Five components of mineral soil and typical percentage
Mineral - 50%
Organic material - 1-5%
Air - 20-30%
Water - 20-30%
Organisms - <1%
Volume/capacity for air and water in soil includes total void volume and the size and distribution of pores.
Types of minerals in soil by size
Sand - .05-2 mm
Silt - .002-.05mm
Clay - <.002mm
Soil texture
Proportion of sand, silt, and clay in a soil.
40% Sand, 40% Silt, 20% Clay
Primary minerals
Minerals that formed in the rock, inherited by the soil, e.g. quartz. Provides nutrients.
Secondary Minerals
Minerals formed in the soil. Usually clay sized, absorb nutrients and water.
Forms of Organic Matter
1. Recognizable detritus/litter
2. Humus (not recognizable)- waste from organisms
3. Colloidal material
Functions of organic matter
Holds nutrients in the soil, binds particles together (aggregates), source of energy/food for organisms, holds water.
Functions of soil water
Holds water for roots and animals
Acts as a solvent/medium transfering nutrients from soil particles to roots
Moves material through soil and out of soil - microscopically and macroscopically through layers.
Traits and functions of soil air
Has higher level of CO2 and lower 02 because of respiration by soil organisms. C02 and 02 vary frequently. Relatively humidity is usually 100%. Soil aeration key to organism health.
Traits and functions of soil organisms
Responsible for detrital breakdown which releases and ties up nutrients and forms humus (new O.M.).
Mixes soil.
Fights and contributes to disease.
Changes porosity through aggregation by organism exudates and humus. Make pores as move through soil.
Soil profile
Vertical cross section of a soil, that shows all the "layers" or horizons that have been altered by soil forming processes (pedogenic process)
Distinct layers in the soil. Distinguished by color, texture (usually change in clay content), chemical composition.
Master Horizons
O Horizon
Organic matter
Frequently on top
Common in forest soils, wetlands, not in prairie.
Usually lost by tilling and in urban areas
A Horizon
Mineral horizon with organic matter (humified)
Brown or black in color
Usually at the surface of mineral material
E Horizon
Mineral Horizon where the dominant feature is loss (of Fe, Al, O.M., clay)
Gray or whitish in color
Leached or eluviated horizon
B Horizon
Mineral horizon with either or both:
1. An illuvial accumulation of Fe, Al, O.M., clay - orange, yellow, light brown, or
2. Substantial alteration of original mineral material - shows pedagenic process - eliminated some rock structure, aggregated particles together, changed color to orange, yellow,grey, light brown
C Horizon
Mineral horizon that hasn't changed much. Lacks the properties of O,A,E,B. Limited biological impact.
R Layer
Bedrock - not a horizon
Transition Horizons
Where changing from one master horizon to the next and has characteritics of both. E.g:
AB, BC, CB, AE - a smooth transition
A/B B/C - an interfingering transition
Whatever letter is first is dominant
Subordinate horizons
Used to modify master horizon designations to indicate specific distinct properties within a master horizon - > lowercase letters
Review but don't memorize common subordinate horizons
Oi, Oe, Oa
Mineral subordinates - p,t,k, s, h, g, w
B horizons are always modified with a subordinate
Soil colors
Brown/black - organic matter
Red/orange/yellow - iron oxides/hydroxides
White - carbonates
Grey - either bare mineral grains, or anerobic conditions
Munsell color system
Standard system for accurate color description. Compare soil to standard coloer chips and describe hue, chroma, and value with a system of numbers and letters.
Rounded, irregularly shaped or blocky
Not plastic, sticky, or moldable
Large particles with large pores
Drains quickly (does not hold water), highly aerated
Can get coatigns of oxides, clay, O.M. and aggregate
Primary mineral, inherited
Very little nutrients
Small particles, not visible to the eye
Slightly moldable
Smaller pores than sand
Some water retention and less aeration than sand.
Low nutrition
Usually plate or rounded
Very small particles with small pores
Holds water well, poorly aerated
Large surface area
Shrink - swell properties
Secondary minerals and source of nutrients (Ca, MG, Fe, K)
Liquid and plastic limits in soil
Plastic limit is where there is enough moisture to mold the soil. From that point until the liquid limit, soil will support some weight. At liquid limit, water content is so high the soil can't hold together and liquifies.
What do plants need from soil
Temperature regulation
Protection from toxics
Meaning of "clay"
Clay sized particles - <.002 mm
Clay minerals - platey secondary minerals
Soil strength
Capacity of soil to withstand a stress

Determined by particle size (texture), water content, minerals (clay in particular) and organic matter content
Soil structure - def
arrangement of soil particles in groups or clusters - called aggregates or peds. Varies with horizon and soil type.
Types of soil structures
Platey - Found in any horizon. Inherited from rock or from compaction

Prismlike - Found in arid to semi arid B horizons

Blocky - Found in forest B horizons

Spherical or granular - Found grassland A horizons

Massive - all particles stuck together

Single grained - no particles stuck together
Causes of soil structural development
Don't really know - the following contribute:

Climatic conditions like wet/dry cycles and freeze/thaw

Clay content

Oxides, O.M. and carbonates

Organisms, plant roots, exudates
Particle density
Density of solids in a soil - nothing to do with pores. Although changes with mineral type, small enough that we can use 2.65 g/cm3 as average density of mineral soils. Organic soils are probably less then 1 g/cm3
Bulk Density Db
Mass of solids in a soil per unit volume - includes pores.
Arrangement of particles can change bulk density. Db effects root growth. Density greater than 1.6 g/cm3 stops root growth. "Good" range is .08 to 1.2 g/cm3
Factors that effect bulk density
O.M. - decreases Db
Loams - lower Db
Volcanic ash - lower Db
Compacted clay - greater Db
Sand - greater Db
Humus - decrease Db
Plants, roots, organisms - decrease Db
Plowing - decreases temporarily and ultimately increases because of loss of organic matter and compaction from equipment
Formula for % porosity
100 - (Db/Dp x 100)
Formula for Bulk Density
Ws/V in g/cm3

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