Soil Notes
Definition
• 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.
Composition – 4 Distinct Parts
•
•
•
•
Mineral particles (45% of “typical” soil)
Organic matter (about 5%)
Water (about 25%)
Air (about 25%)
What is in soil?
• Organic
– Decomposing material
• Leaf litter
• Dead plants and animals
• Provide nutrients!
– Living organisms
•
•
•
•
Bacteria
Soil-invertebrates
DECOMPOSERS
All require oxygen
• Inorganic
– Minerals
• Macronutrients
– primary – N, P,K
– Secondary – Ca, Mg, S
• Micronutrients
– B,Cu, Fe, Cl, Mn, Mo, Zn
– Rock
• Pores
– Store O2 and nitrogen gas
– May fill with water and roots
Importance
• Basis of all life
• 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.
• A few inches of topsoil – all the difference
between life and death
Civilizations collapse
• The nation that destroys its soil destroys itself.
Franklin D. Roosevelt
• Easter Island
• Norse people in Greenland – collapsed in 1400’s: all
starved/froze to death
• Sumerians – disappeared 2000 BC: great civilization
on Tigris & Euphrates River – long term irrigation led
to salt build up
• Iceland –10th century settlers: cut down most trees, ½
of soil eroded into sea. Realized land/soil was
vulnerable & slow to recover – took corrective action
– Turned to fish & greenhouses
– 95% of energy from geothermal & hydroelectric
Dust Bowl
• US environmental disaster in 1930’s
• Large area in KS, CO, TX, OK had to be
abandoned
• Poor cultivation practices & drought
• Severe soil erosion
– Plowed prairie grass
– Left bare between crops
– Overgrazed
• 1935 Soil Erosion Act
– Est Soil Conservation Service
– Gives tech assistance to farmers &
ranchers to prevent soil erosion
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.
– Hundreds to thousands of years to build up layers
– Depending on climate:
• 0.4 inch of topsoil takes 15 – hundreds of years to form
Shaping the Planet
• The main forces that shape
the surface of the Earth
are:
Weathering and
erosion
Deposition
The deposition of sand particles at the mouth of
rivers cause deltas such as here at the Nile delta.
Particles carried by the wind have eroded these
sandstone formations, called Hoodoos.
Erosion by moving water has caused the
formation of the Grand Canyon
Weathering and Erosion
• Weathering is the breaking down of rocks
on the surface by local conditions with no
movement.
• Erosion is the break up and removal of
those rocks. The movement deposits them
elsewhere.
• Forms of weathering and erosion:
Chemical: rocks are dissolved by the acids
or other reactive chemicals, including
leaching. Rusting is an example when water
reacts with iron and oxygen.
Physical: rocks are broken down into
smaller and smaller pieces by mechanical
forces such as wind or water moving
particles against each other.
Physical Weathering
• Ex. erosion (wind, water, ice, etc.)
Chemical Weathering
• A plant’s roots or animal cells undergo
cell respiration and the CO2 produced
diffuses into soil, reacts with H2O &
forms carbonic acid (H2CO3). This eats
parts of the rock away.
Deposition
• Deposition, the process by which material is added or deposited on
to land, also occurs by both chemical and physical means:
Chemical deposition: chemicals (often pure elements or compounds)
precipitate out of solution and form crystalline structures.
Physical deposition: sediments are laid down by wind or water in large
areas and may form strata, showing the different types of sediment
and mode of deposition.
Silica deposits
Limestone stalactites
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 Formation
Stage 1
Stage 2
Stage 3
Stage 4
O horizon
O horizon
Disintegrating
parent rock
A horizon
Weathered
parent rock
(C horizon)
O horizon
A horizon
B horizon
Bedrock
C horizon
Bedrock
C horizon
Bedrock
Bedrock
Soil Horizons
Mature soils, or
soils that have
developed over a
long time are
arranged in a
series of
horizontal layers
called soil
horizons.
O-Horizon
• The uppermost layer; it is rich
in organic material.
• Plant litter accumulates and
gradually decays. May or may
not include the leaf litter layer
• Organic detritus (bits of leaves,
twigs, etc) on top of a layer of
partially decomposed organic
matter (called humus)
• Soil is brown or black
underneath litter
• In desert soils the O-horizon is
completely absent, but in
certain organically rich soils it
may be the dominant layer.
A-Horizon
• Topsoil
• Consists of partially decomposed
organic matter, inorganic
A
minerals, and living organisms.
• It is dark and rich in accumulated
organic matter and humus.
• It holds water & nutrients for
plants
• Color indicates richness
– dark brown or black usually =
nutrient rich = good for farming
– gray, yellow or red = low organic
matter = poor for farming
• Zone of biological activity Bacteria, insects, earthworms,
fungi
E horizon – not always present or noted
• Zone of leaching
• Zone present in acidic soils,
– either between the O and A
– Or A and B
– always above the B horizon
• Nutrients and minerals move
quickly through this layer are
deposited in the B horizon
B-Horizon
Yellow
alum
oxides
=
• Sub Soil
• Light colored
• Minerals leached down
from A horizon
Red
=
iron oxides
• Mostly inorganic –Fe, Al,
and humic compounds
• Almost no organic matter
White
=
calcium
carbonate
• Roots may extend into this
layer
C-Horizon
• Parent material
• Broken rock fragments
• This contains
weathered pieces of
rock and borders the
unweathered solid
parent material. Most
roots do not go down
this deep and it is often
saturated with
groundwater.
R-Horizon
•
•
•
•
•
Bedrock
Unbroken rock
Sandstone
Granite
Limestone
Oak tree
Earthworm
Grasses and
small shrubs
O horizon
Leaf litter
Organic debris
builds up
Rock
fragments
Moss & lichen
Honey
fungus
A horizon
Topsoil
B horizon
Subsoil
Mole
Immature
soil
Young
soil
C horizon
Weathered
Parent
material
R - Bedrock
Mature soil
Fungus
Bacteria
Fig. 3-23, p. 68
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.
Soil Texture
• Textures
– Sand - biggest particles (0.05 – 2.00mm)
– Silt – (.002-.05mm)
– Clay – smallest components (less than .002mm)
• Determined by the % of each type of particle
– Gritty = sand
– Smooth = silt
– Sticky = clay
Texture: Particle Size
• The combination of sizes
gives the soil its texture.
Sand: particle diameter 0.05mm > 2mm
• Sand (largest) feels gritty,
Silt: particle diameter 0.05mm > 0.002mm
• Silt (medium) feels smooth,
• Clay (small) feels very smooth
& sticky.
Clay: particle diameter < 0.002mm
Photo: Infrogmation, Creative Commons share alike 2.5
Photo: Bobannye
• To tell the difference in soil,
take the soil, moisten it, and
rub it between your fingers
and thumb.
Photo:Siim Sepp, Creative Commons share alike 3.0
• Soils are made of many
different sized mineral
particles and other material
Soil Texture
• Impacts
– Too sandy = rapid infiltration and nutrient loss
– Too much clay = no infiltration, too hard,
roots suffocate, drown, or can’t penetrate
– Too silty = compact easily, crusty surface
• Loam
– a mixture of sand - silt - clay
• 40-40-20
– BEST for farming!
– Promotes drainage while also
retaining nutrients
• Porosity
– The percentage of open
pore space in soil
– Silty soil holds water well
– Coarse soil holds air
• Permeability
– The rate at which water
flows through soil
– Determined by porosity
and structure
Sand
0.05–2 mm
diameter
Silt
0.002–0.05 mm
diameter
Water
High permeability
Clay
less than 0.002 mm
Diameter
Water
Low permeability
Soil pH
• The soil pH is a measurement of the
acidity or alkalinity within the soil.
• Soil pH is the negative logarithm of
the hydrogen ion concentration.
pH correction
Too acidic
• Add lime to neutralize
• Must be used with organic fertilizer
Too Basic
• Add sulfur which is converted to
sulfuric acid
• Very SLOW
• The pH scale goes from zero to
fourteen.
< 7 = acidic
7 = neutral
> 7 = basic
• A pH of 6-7.5 is best for most
crops
Acidic soils
•
•
•
Common Name Optimum
pH
Acidic soils
Range
– Hinder nutrient availability
Asparagus
6.0-8.0
– Increase availability of toxic heavy metals
Bean, pole
6.0-7.5
• Major problem for urban gardens in the
Beet
6.0-7.5
northeast US
Broccoli
6.0-7.0
– Increase pesticide runoff
6.0-7.5
– Decrease bacteria populations less nitrogen Brussels sprout
Carrot
5.5-7.0
fixation
Cauliflower
5.5-7.5
Soil in California tends to be basic
Celery
5.8-7.0
Soil in the northeastern US is more acidic Chive
6.0-7.0
Cucumber
5.5-7.0
Garlic
5.5-8.0
Kale
6.0-7.5
Lettuce
6.0-7.0
Pea, sweet
6.0-7.5
Pepper, sweet
5.5-7.0
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.
Renewable or Not?
• Decomposition produces new soil
• Depends on climate & local conditions
– May take decades or thousands of years
• Tropical rainforests: all of the nutrients
are caught in the trees and when cut
down & burned the soil cannot get the
nutrients back.
What determines the type of soil?
•
Parent Material – the type of rocks naturally found in an area
– Quartz sand based rocks create nutrient depleted soil that is not good for farming
– Soil with calcium carbonate parent material will have plenty of calcium, a high pH and be
good for farming
•
Climate
– need non-freezing temps to encourage decomposition plus climate determines
vegetation which provides the organic matter for soil
•
Topography – geographical features of the area
– Steep slopes will constantly erode leading to poor soil
– River deltas have seasonal flooding that deposit nutrients and silt which lead to good
soil
•
Organisms
– Organisms help churn soil mixing nutrients evenly plus they aid in decomposition and
nutrient cycling
•
Time
– It takes a long time for soil to form, so in general older soils are better and more
established, but it depends on the vegetation.
– Desert soil might be old, the lack of vegetation means it does not improve much with
age
Soil Development
• The character and composition of the parent
material is important in determining the
properties of a soil.
• Parent materials include
– volcanic deposits,
– sediments deposited by wind, water, or
glaciers.
• Granite will take longer to break down
• Limestone will have more nutrients
Soil Development
• Climate affects vegetation, influencing soil
development.
– Moist soils with a high organic content tend to be
higher in biological activity because of the
opportunity for shelter and food.
– length of Growing season
– The occurrence of freeze-thaw and wet-dry
cycles, are important in the development of soils.
Slope
• The topography of the land
influences soil development by
affecting soil moisture and
tendency towards erosion.
• Steep slopes often have little or no
soil
• Runoff from precipitation tends to
erode the slope also.
• Moderate slopes and valleys may
encourage the formation of deep
soils.
Soil Development
• Organisms - Plants, animals, fungi, and bacteria help to
create a soil both through their activities and by
adding to the soil's organic matter when they die.
Color
• Dark soil is rich with
lots of organic
matter.
Photo: USDA
• Light soil (like sand)
is not so rich with
very little organic
matter.
Photo: USDA
Soil Testing
• to determine the nutritional value of soils.
• Chemical tests include:
pH (acidity or alkalinity)
Salinity (salt content)
Organic content (humus)
Major elements: nitrogen, phosphorous,
potassium, or sulfur
Trace elements: iron, cobalt, calcium,
magnesium, selenium, or aluminum
Chemical testing of soil
• Physical tests include:
Texture, composition, particle size (% sand,
silt, clay)
Water holding capacity, porosity
Percolation rate, infiltration, permeability
Physical testing of soil
Soil Profiles by Biome
Mosaic of
closely
packed
pebbles,
boulders
Weak humusmineral mixture
Desert Soil
(hot, dry climate)
Dry, brown to
reddish-brown
with variable
accumulations
of clay, calcium
and carbonate,
and soluble
salts
Alkaline,
dark,
and rich
in humus
Clay,
calcium
compounds
Grassland Soil
semiarid climate)
Acidic
light-colored
humus
Iron and
aluminum
compounds
mixed with
clay
Tropical Rain Forest Soil
(humid, tropical climate)
Fig. 3-24b, p. 69
Forest litter leaf
mold
Humus-mineral
mixture
Light, grayishbrown, silt loam
Dark brown
firm clay
Deciduous Forest Soil
(humid, mild climate)
Fig. 3-24b, p. 69
Acid litter
and humus
Light-colored
and acidic
Humus and
iron and
aluminum
compounds
Coniferous Forest Soil
(humid, cold climate)
Fig. 3-24b, p. 69
A toxic white crust runs through irrigated fields in Grand Valley,
Colorado: Moisture evaporating from the soil has drawn underground
salt to the surface. To keep the salt from damaging the roots of their
crops, farmers must add even more water.
Thick, six-foot-long roots of sunflowers, side-by-side with the roots of assorted prairie
grasses, delve deep into a plot of earth near Salina, Kansas. This soil has never been
broken by a plow. These perennials have root systems that expand and strengthen year
after year—unlike annual crops that demand much of the soil but provide little in return.
Such growth not only helps prevent erosion but also serves as a water-storage system
that enables the plants to survive during droughts.
Virgin Prairie—Kansas, United
States.
Rancher Jim Duggan holds a stalk of
big bluestem, one of the native
grasses growing on 40 acres of his
farmland that have never been
plowed. "This land is the best there
is," he says. "It's class-one riverbottom soil." Compared with tilled
fields, the parcel has deeper, richer
topsoil and soaks up more rain.
Reclaimed Fields—Keita
District, Niger.
Mariama Abdoulaye feeds
her family with millet she
grows on once barren land.
After severe droughts in the
1970s and ’80s, the UN Food
and Agricultural Organization
enlisted Abdoulaye and
10,000 other women to plant
millions of trees. Tree roots
block wind-driven erosion
and help rain penetrate the
earth.
Rice Terrace—Yunnan Province,
China.
Perched on an earthen retaining
wall, Zhu Minying holds cords used
to bundle harvested rice. Soil here
reflects human activities that began
with reshaping hillsides into grand
staircases of grain. Rice stubble left
to decay in the field, manure, and
fish raised in the paddy water, all
add nutrients to Zhu's soil.
Dry Land—Khanasser Valley, Syria.
Farmers like Ismail Hassoun Hariri struggle
to grow even hardy barley in this parched
land. Soil and rock eroded from
surrounding hills lie thick in the valley, but
annual rainfall averages only nine inches.
In some very dry years the barley crop
fails to mature and can only be used to
feed sheep and goats.
After losing a foot of soil from parts
of their Iowa corn farm, the Reed
family changed the way they prepare
fields for planting, to limit erosion.
Cletus Reed, 80, hopes his grandson,
Sam, will work these acres someday.
"The land takes care of us as we care
for it," he says.
Tiny earthworks stipple bare slopes in China's Zizhou County, each intended to cradle a
single sapling. Government mandated reforestation programs are intended to halt erosion,
but many earlier efforts here in the Loess Plateau failed when newly planted trees died.