Soil
Characteristics
• Soil is a thin layer on top of earth’s land surface.
• Soil is a natural resource and deeply affects every other part of the
ecosystem.
• Soil holds nutrient and water for plants and animals; water is filtered
and cleansed as it flow through soils; and soils affect the chemistry of
water and the amount of water that returns to the atmosphere to
form rain
Soil is composed of 3 main ingredients
• Minerals of different size
• Organic materials from the remains of dead plants and animals
• Open spaces (pores) that can be filled with air or water
Good soil composition contains:
Factors in soil development:
• Parent Material: The rock or minerals from which the soil derives. The
nature of the parent rock can either be native to the area or
transported to the area by wind, water, or glacier. Has a direct effect
on the ultimate soil profile.
• Climate: This is measured by precipitation and temperature. It results
in partial weathering of parent material, which forms the substrate
for soil.
• Living Organisms: These include nitrogen-fixing bacteria, Rhizobium,
fungi, insects, worms, snails etc. that help decompose litter and
recycle nutrients.
• Topography: This refers to the physical characteristics of the location
when the soil is formed. Topographic factors that affect a soil’s profile
include drainage, slope direction, elevation, and wind exposure.
Formation of mature soil
• With sufficient time, a mature soil profile reaches a state of
equilibrium. Feedback mechanisms involving both abiotic and biotic
factors work to preserve the mature soil profile. The relative
abundance of sand, silt and clay are called the soil texture.
• The category with the smallest particles is clay (<0.002 mm).
• The next largest is silt (0.002-0.05mm in diameter).
• Sand is the coarsest soil with particles 0.05-2.0 mm in diameter.
• Sand particles are too large to sick together, and sandy soils have
larger pores; which means they can hold more water. Clays easily
adhere to each other and there is little room between particles for
water; clay soils are extremely compact.
Soil Texture
Effect of pH on soil
• Another very important characteristic of soil types is soil acidity or
alkalinity. pH ranges from 0-14 and is a measurement of the
concentration of hydrogen ions. Most soils fall into the pH range of 48, meaning that soils range from being neutral to slightly acidic. Soil
pH is important because it affects the solubility of nutrients; and thus
in turn determines the extent to which these nutrients are available
for absorption by plant roots. If the soil is too basic or too acidic,
certain soil nutrients will not be able to be used by regional plants.
WHERE DOES SOIL COME FROM?
• Soil is a combination of organic materials and rock that has been
broken down by chemical and biological weathering. The types of
minerals found in the soil in a particular region will depend on the
type of base rock in that region.
• All weathering is placed into three broad categories:
• Physical weathering (mechanical weathering) – Any process that
breaks rock down into smaller pieces without changing the chemistry
of the rock. The forces responsible for it include wind and water.
• Chemical Weathering – Occurs as a result of chemical reactions
between water and other atmospheric gases, and the bedrock in a
region.
• Biological Weathering – Weathering that takes place as a result of the
activities of living organisms.
SOIL LAYERS/HORIZION
Horizon
Name
Description
O
Surface Litter
Upper most layer. Leaves, waste, live organisms and partial
decomposed organic debris including decomposing organisms.
The dark, crumbly material resulting from decomposition of
organic material is called humus. Maybe very thick in deciduous
forests and very thin in tundra or desert.
A
Topsoil
Made up of weathered rock and some organic matter that has
traveled down from the O layer. Some living organisms and
inorganic materials. Topsoil is typically very thick in grasslands.
Important role in plant growth. E Zone of Leaching Dissolved and
suspended materials move downward.
B
Subsoil
Tends to be yellowish in color due to the accumulation of iron,
aluminum, humic compounds, and clay leached down from the
A and E horizons. It can be rich in nutrients in areas where
rainwater leeched (washed) nutrients from the topsoil.
C
R
Weathered Parent Material Partially broken down organic minerals. Large pieces that have
not undergone much weathering.
Bedrock
Bedrock
SOIL COMPONENTS
COMPONENT
DESCRIPTION
Clay
Very fine particles. Compacts easily. Forms large, dense
clumps when wet. Low permeability to water; therefore,
upper layers become water logged.
Gravel
Coarse particles. Consists of rock fragments.
Loam
About equal mixtures of clay, sand, silt and humus. Rich in
nutrients. Holds water but does not become waterlogged.
Sand
Sedimentary material coarser than silt. Water flows through
too quickly for most crops. Good for crops and plants
requiring low amounts of water.
Silt
Sedimentary material consisting of very fine particles
between the size of sand and clay. Easily transported by
water.
Water
Retention and
Availability
CLAY
SAND
LOAM
Nutrient
Storage
Capability
Air Space
Primary
Production
CLAY
SAND
LOAM
Water
Retention and
Availability
Nutrient
Storage
Capability
Air Space
Primary
Production
Sticky and easily
waterlogged
High
Low
Medium/Low
Fast draining soil
that dries out
easily
Low
High
Low
Medium to High
Medium
Medium
Medium
Primary productivity of soil depends on:
•mineral content
•drainage
•water-holding capacity
•airspaces
•biota
•potential to hold organic materials
Soil Degradation
Soil degradation concerns the decline of soil quality or
quantity. Soil degradation results from changes to
biological, geological or chemical components of a soil.
Human activities such as overgrazing, deforestation,
unsustainable agriculture and irrigation cause processes
of degradation.
These include soil erosion (loss of soil to wind and
water), toxification and salinization (increase in salt
concentration).
Erosion
•Soil erosion is the movement of weathered rock or soil components from one place to
another.
•Erosion is caused by flowing water, wind and human activity (cultivating inappropriate land,
burning of native vegetation, deforestation, and construction).
•Soil erosion destroys the soil profile, decreases water-holding capacity of the soil, and
increases soil compaction.
•Because water cannot percolate through the soil, it runs off the land, taking more soil with it
(positive feedback loop).
•Because the soil cannot hold water crops grown in areas of soil erosion frequently suffer
from water shortages.
•In areas of low precipitation, erosion leads to significant droughts.
•Poor agriculture techniques that lead to soil erosion include monoculture, row cropping,
overgrazing, improper plowing the soil and removing crop wastes instead of plowing the
organic matter back into the soil.
Three types of soil erosion
1. Sheet erosion – soil moves off as a horizontal layer.
2. Rill erosion – fast-flowing water cuts small
channels in the soil.
3. Gully erosion – extreme case of rill erosion, where
over time, channels increase in size and depth.
Soil Conservation Measures
• Soil conditioners (for example, use of lime and organic materials)
• Wind reduction techniques (wind breaks, shelter belts, strip cultivation)
• Cultivation techniques (terracing, contour plowing)
• Efforts to stop plowing of marginal lands.
What does
sustainability
mean?
Sustainability
Meeting the needs of the
present without
compromising the ability of
future generations to meet
their needs.
Assignment
• Read the article on “French Fries”
• Fill in the table on the back side
• As a group, pick ONE item that you are measuring
around the school and fill in a chart as a group. This
can only be one quantifiable item. Be prepared to
present to the class.
• Read the other two articles and fill in a separate
chart for both.