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Soil
General Soil Information
Definition of Soil
Soil – relatively thin surface
layer of the Earth’s crust
consisting of mineral and
organic matter that is
affected by agents such as
weather, wind, water, and
organisms.
Dynamic membrane that
covers earth’s surface
Connects biological and
geological world
Ecosystem Services Provided by Soil
Importance of Soil
Organisms, mainly microorganisms, inhabit the soil &
depend on it for shelter, food & water.
Plants anchor themselves into the soil, and get their
nutrients and water.
Terrestrial plants could not survive without soil, therefore,
humans could not exist without soil either.
SOIL: A RENEWABLE RESOURCE
Soil is a slowly renewed resource that provides
most of the nutrients needed for plant growth and
also helps purify water.
Soil formation begins when bedrock is broken
down by physical, chemical and biological
processes called weathering.
Mature soils, or soils that have developed over a
long time are arranged in a series of horizontal
layers called soil horizons.
Major Components of Soil
Pore Space (50%)
This may contain air
and/or water
Soil Space (50%)
• Organic Matter
• Mineral Matter
Soil Formation & Parent Material
The rock that has slowly broken down into
smaller particles by biological, chemical, and
physical weathering.
To form 2.5 cm (1 in.) it may take from 200-1000
years.
Soil Properties: Texture
The percentages
(by weight) of
different sized
particles of sand,
silt and clay that it
contains.
For Example: a
soil has 24% clay,
41% silt, and 35%
sand. The soil is
classified as:
Medium
loam
Texture
Largest → Smallest
Sand → Silt → Clay
>2mm in diameter = gravel/stones
(not actually considered soil
because it doesn’t have direct
value to plants.
0.05 to 2mm = sand (the largest
soil particles) can be seen easily
with the eye.
0.002 to 0.05mm = silt – about the
size of flour and barely visible with
the eye.
<.002mm = clay (has the greatest
surface value) – only seen under
and electronic microscope.
Texture: The Ribbon Test
To tell the difference in
soil, take the soil, moisten
it, and rub it between your
fingers and thumb.
Gritty -has a lot of sand
Sticky- high clay content
and you should be able to
roll it into a clump
Silt- smooth, like flour.
Physical Properties of Soils
Texture: the amount of sand
silt, and clay
Structure: How soil particles
are organized and clumped
together. (Sand, silt, clay)
Friability: How easily the soil
can be crumbled
Porosity: A measure of the
volume of soil and the average
distances between the spaces
Permeability
The rate at which
water and air moves
from upper to lower
soil layers. It is
distances between
those spaces.
Why does soil need
air spaces?
Aerobic microorganisms
need oxygen for cellular
respiration!
Can increase the water holding capacity of a soil by adding clay
Shrink-Swell Potential
Some soils, like clays, swell when H2O gets in
them, then they dry and crack. This is bad for
house foundations, etc.
Important for builders to know
Soil Nutrients
C HOPKN’S CaFe Mg B Mn Cu Zn Cl Mo
Macronutrients: larger in atomic structure.
Plants need them in large amounts
Ex. Nitrogen, Phosphorus & Potassium.
Micronutrients: smaller in atomic structure.
Plants need them in small amounts. Ex.
Selenium, Zinc & Iron
pH
The pH of most soils ranges
from 4.0 to 8.0.
Plants are affected by pH
because of the solubility of
nutrient minerals.
At a pH of 4.5, N, P, K, S, Ca, Mg are not very available to plants
At a pH of 4.5, Fe, Mn, and Al are very available to plants and they are toxic to
plants in high amounts
Easy and inexpensive way to make an acidic soil more basic is to add crushed
limestone (to fields) or egg shells to small gardens or house plants
Inexpensive way to make Azalea beds more acidic is to add coffee grounds or
pine needles or pine bark to the beds
Climate and Soils
Probably the most important factor
soils produced from the same parent
material under different climates will
have different properties
Climate governs the rate and type of soil
formation
Main determinant of vegetation
distribution.
Soil climate has two major components;
moisture (precipitation) and temperature,
influencing evaporation.
When precipitation exceeds evaporation,
leaching of the soil will occur.
Temperature determines the rate of
reactions:
Chemical and biological decay
Influences weathering of soil
Soil Horizons
A horizon
B horizon
C horizon
Soil Horizons O: Organic Horizons
O Horizon
Organic Layers of Decaying
Plant and Animal Tissue
(detritus)
Aids Soil Structural
Development
Helps to Retain Moisture
Enriches Soil with nutrients as
the leaf litter decomposes
Desert soils do not have O
horizon; no leaf litter
Dark in Color Because of Humus
Material - 1,000,000 bacteria per
cm3
Soil Horizons: A Horizon or “ Topsoil”
A Horizon
It is dark and rich in accumulated
organic matter and humus.
It has a granular texture and is
somewhat nutrient-poor due to
the loss of many nutrient
minerals to deeper layers and by
leaching.
Humus: A dark-brown or black
organic substance made up of
decayed plant or animal matter.
Grasslands and forests have deep A
horizons (lots of humus which
increases water holding capacity
Soil Horizons: E Horizons
Mineral Horizon Near
Surface
E Horizon
Movement of Silicate Clay,
Iron, and Aluminum from the
A Horizon through Eluviation
Eluviation: movement of
materials suspended through
a soil
Horizon does not mean a
water table is present, but
the horizon can be
associated with high water
table
Soil Horizons: B Horizons
Zone of Maximum Accumulation
The light-colored subsoil
beneath the A-horizon
often a zone of illuviation
where nutrient minerals have
leached out of the topsoil and
litter accumulate
B Horizon
Typically rich in iron and
aluminum compounds and clay.
Parent Material (C-horizon)
Contains
weathered pieces
of rock & borders
the un-weathered
solid parent
material. Most
roots do not go
down this deep and
it is often saturated
with groundwater.
Layers in Mature Soils
Infiltration: the downward movement of
water through soil.
Leaching: dissolving of minerals and organic
matter in upper layers carrying them to lower
layers.
The soil type determines the degree of
infiltration and leaching.
Fertilizers and Labels
Organic Fertilizers – animal
manure, crop residues, bone
meal, and compost
Inorganic Fertilizers – manmade from chemical
compounds
Benefits – exact
compositions are known;
they are soluble & thus
immediately available to the
plant
Costs – quickly leach away;
this pollutes the water;
doesn’t help the water
holding capacity of the soil
like organic fertilizers do
N –P–K
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