Environmental Geology – Spring 2010 – Lecture 7
Soils

Pedosphere – Soils comprise the very top 2 meters or so of the pedosphere which can extend to as much as 200 meters deep.

Brief review of weathering processes:

Mechanical weathering – physical disintegration of rock to form sediment –big pieces to little pieces.

Examples: tree roots splitting rock, glacial abrasion, frost wedging, human activity

Chemical weathering – chemical transformation of minerals in the rock.
Example: conversion of K – feldspar to kaolinite

Primary chemical weathering processes:

1.    Oxidation – Example: Iron Oxide (Hematite) >>>> Goethite: 4FeO + 2H2O + O2 >>>>> 4FeOOH
2.    Dissolution – calcite/limestone in water/carbonic acid: CaCO3 + H2CO3 >>>>> 2HCO3 + Ca
3.    Hydrolysis – K-Feldspar >>>> kaolinite: 4KAlSiO8 + 4H + 2H2O >>>>> 4K + Al4Si4O10(OH)8 + 8SiO2

Soil – Combination of mineral, organics, water, and air
In general, about 50% mineral and organics and about 50% voids which are filled with water and air.

Primary Soil Forming Factors (Five)

1.    Parent material – some rocks and minerals are more resistant to weathering than others.  As example, quartz does not weather as easily as K-Feldspar.
2.    Time – greater time = greater weathering and greater soil formation
3.    Climate – This is the most significant soil forming factor.  Chemical weathering drives soil formation.  Greatest chemical weathering occurs in areas that are hot and moist.  Water drives most of the chemical weathering processes, and leaches the soluble elements like Ca, K, Na, Mg, from the soil profile.  The less soluble minerals like Fe and Al are concentrated in the soil.
4.    Topography – The slope of the land controls soil formation in several ways.  It impacts microclimate including the availability of water and the temperature.  A north facing slope in the northern hemisphere is generally cooler than a south facing slope.  Cooler climate will lead to less soil development.  The top of the hill receives precipitation that falls form the sky, while the toe of the hill receives precipitation from the sky plus precipitation that runs off the slope.  Greater moisture at the toe of the slope results in greater soil formation.
5.    Plants and animals – aerate soil, increase water infiltration, stir soil.  It’s estimated that earth worms bring 2.5 kg/m2 to the surface per year.

Soil Profile
O = very top organic horizon
A = mineral + organic material that has been leached down from the O horizon
B = accumulation horizon (subsoil).  This is the zone where soluble minerals picked up by infiltrating water in the A horizon are deposited.
C = Partially weathered parent material

Soil Characteristics

Texture – looked at textural triangle
Sandy, Silty, Clayey

Color – determined by composition of parent material and the environment the soil is formed in.
Organic rich – dark browns, tans, yellows
Gray – wet most of the time, poorly drained

Soil Structure
Blocky, Platy, Granular, Prismatic

Soils are classified in a hierarchical system
10 soil orders, 47 suborders, 206 families and great groups, etc.  Each division provides more specific detail about the formation and properties of a particular soil.

Estimating Rates of Soil Erosion

Measure directly – too many variables to realistically do this at all sites.

Estimate from reservoir depletion
Know size of drainage area, volume of reservoir, time reservoir was shut in.  Given this you can calculate the sediment yield on a per hectare basis for the drainage basin.

Example:

Reservoir has an initial volume of 1000 m3; after 1 year the volume of the reservoir is reduced to 975 m3.  Sediment yield is 25 m3.  If the basin size is 5,000 km2 then the sediment yield is 200 m3/km2/year.

Sediment Yield = 5,000 km2/25 m3/year = 200 m3/km2/year

Natural Resource Conservation Service uses the Universal Soil Loss Equation

A = R x K x L x S x C x P

A = average annual soil loss
R = long term rainfall runoff factor – different soils in different areas under varying climatic conditions respond differently to precipitation events
K = soil erodibility – some soils such as silty soils are more readily eroded
L = hillslope length – longer slope causes greater water velocity, greater soil erosion
S = hillslope gradient – steeper gradient causes greater water velocity, greater soil erosion
C = Soil cover factor – vegetation binds soil and reduces erosion
P = erosion control practice factor – examples: contour farming, grassed waterways, buffers

Why is soil erosion a problem?
•    Chokes streams and reservoirs
•    Carries nutrients and other contaminants into water bodies.
•    Fills in harbors
•    Causes loss of fertility of original soil
•    Aesthetic problems (gullies are just not attractive)
•    Increases water treatment costs
•    Increases flood risk