A. Erosion is the movement of soil, especially topsoil, by wind or water from one
place to another.
1. It takes 200-1000 years to make 2.5 cm of soil. To not lose more soil than is
created, erosion rates need to average less than 5 tons/year.
2. The rate of erosion on undisturbed land is less than 1/4/ton/acre/year.
3. Erosion rates on cultivated land.
a. Farmland: 7.6 tons/acre/year in the United States and Europe or 13.7-17.8
tons/acre/year in Asia, Africa, and South America.
b. Annual erosion rates for agricultural land worldwide are 18 to 100 times faster
than renewal rates.
B. Not all erosion is bad. The dust spewed into the air does settle. This dust is the source
of minerals and nutrients for some ocean ecosystems (can also be the cause of
overnutrification), and dust can stimulate cloud formation. The dust also contains soil
organisms that are deposited onto islands.
IV. What Leads to Bare Soil and Erosion?
V. What Happens after Soil Erosion?
A. Loss of soil fertility.
B. Sedimentation - eroding soils fills reservoirs, streams, estuaries, and bays.
C. Desertification - the process whereby the water holding capacity of soil is greatly
diminished and a desert is formed.
SOIL AND THE SOIL ECOSYSTEMS
I. Why do we Care about Soil Erosion?
A. Erosion adversely affects soil therefore adversely affects plants, and plants are the
base of the food chain.
B. Soil and Plants
1. Soil provides three essential factors
a. Water and water-holding capacity
b. Mineral nutrients and nutrient-holding capacity
2. Two additional conditions soil provides for plants
a. Relative acidity
b. Salt content
Fig. 8.3 Soil production involves a dynamic interaction among mineral particles, detritus, and members of the detritus food web.
C. Soil Characteristics
1. Soil profiles
2. Soil classes
3. Soil textures
II. The Soil System - What is the Soil Ecosystem?
1. Soil Textures
4. Soil Organisms
Fig. 8.4 This figure shows major horizons from the surface to the parent material in an idealized soil profile.
Fig. 8.5 Relative proportions of sand, clay and silt are represented on each axis. Major soil classes are indicated on the triangle. Percent of sand, clay or silt is represented on each axis.
B. What happens when the soil system is working well?
1. Topsoil buildup - humus
2. Water and nutrient holding capacity
4. Soil workability
C. What happens when the soil system is not working well? - mineralization.
Fig. 8.7 In addition to the amount and frequency of precipitation, the size of this reservoir depends on the soil's ability to allow water to infiltrate, hold water, and minimize direct evaporation.
Fig. 8.8 A host of organisms, major examples of which are shown here, feed on detritus and burrow through the soil forming a humus-rich topsoil with a loose clumpy structure.
Fig. 8.13 Topsoil must be recognized as a dynamic balance between detritus additions and humus forming processes, and the breakdown and loss of detritus and humus. If additions of detritus are not sufficient there will be a gradual deterioration of the soil.
Fig. 8.15 As wind erosion removes the finer particles, the larger grains and stones are concentrated on the surface.
Fig. 8.17 Deforestation, overgrazing and overcultivation result in the degradation of soils in every region of the world.
Fig. 8.21 Throughout the world, overcultivation, overgrazing and deforestation are causing soil degradation in vast areas.