SOIL (chapter 5)
I.
Why is soil important?
A. Contains the minerals and nutrients plants need to grow
B. Stores, absorbs, and filters water
C. How long does it take to form a few centimeters of soil?
1. It takes 200-400 years to form 1 cm of soil
II.
Characteristics of Soil
A. Soil is the lose covering of rock particles and decaying organic matter(humus)
B. Soil is part of the regolith that supports the growth of plants.
C. Regolith is the layer of rock and mineral fragments that covers most of Earth’s land surface.
D. Results from both physical and chemical weathering
E. Soil composition
1. Soil has four major components:
a. Mineral matter, or broken-down rock
b. Humus, which is the decayed remains of organisms
c. Water
d. Air
F. Soil texture
1. Texture refers to the proportions of different particle sizes.
a. Sand (large size)
b. Silt (soil & mud that sink to bottom of rivers or lakes)
c. Clay (small size)
2. Loam (a mixture of all three sizes) is best suited for plant life.
3. Soils are composed of particles that can be sand, silt or clay sized.
4. Based on the soils % composition of each type of particle that determines the soils texture
G. Soil formation
1. The most important factors in soil formation are (1) parent material, (2) time, (3) climate, (4)
organisms and (5) slope.
2. Parent material
a. Residual soil = parent material is the bedrock
b. Transported soil = parent material has been carried from elsewhere and deposited
c. Parent material is the source of the mineral matter in soils
d. Parent material is important because:
i.
It affects the rate of weathering
ii. Chemical makeup affects the soils fertility
3. Time
a. The longer a soil has been forming the thicker it is.
b. As a soil gets older other characteristics affect the soil more than the parent material
4. Climate
a. Has the greatest effect on soil formation
b. Variations in temperature and precipitation affect the rate, depth and type of weathering
5. Organisms
a. Organisms influence the soil's physical and chemical properties.
b. Organisms put organic matter into soil
c. Plants are the main source of organic matter
d. Plants release nutrients when they decay.
6. Slope
a. Angle of the slope
i. Steep slopes have higher erosion. Little water can soak in, and soils on steep
slopes are often poorly developed (thin).
ii. Best slope for soil formation is a gentle angle
b. Orientation (= the direction the slope is facing) influences soil formation.
i.
Soil temperature
ii. Moisture
H. The soil profile
1. Soil varies in composition, texture, structure, and color at different depths.
2.
Soil horizons are zones or layers of soil.
3. A soil profile is a vertical section through all the soil horizons.
a. The A horizon is commonly know as topsoil.
b. The B horizon is subsoil and contains clay particles washed out from the A horizon
c. The C horizon is between B horizon and unaltered parent material.
I.
Soil Types
1. Three common types of soil are (1) pedalfer, (2) pedocal, and (3) laterite.
a. Pedalfer
i.
Best developed under forest vegetation
ii. Accumulation of iron oxides and aluminum-rich clays in the B horizon
b. Pedocal
i.
Accumulates calcium carbonate
ii. Found in drier grasslands
c. Laterite
i. Found in hot, wet, tropical climates
ii. Intense chemical weathering
J. Soil erosion
1. Water erodes soil
2. Rates of erosion
a. Human activities that remove natural vegetation -- such as farming, logging, and
construction -- greatly accelerate (= speed up) erosion.
3. Sediment deposition
a. Sediments fill water reservoirs
b. Sediments are contaminated by pesticides and fertilizers.
4. Ways to control erosion
a. Planting windbreaks = rows of trees
b. Terracing hillsides
c. Plowing along contours of hills
d. Rotating crops
5.