Soil Notes

General Soil
Information
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%)

Importance
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.

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

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.

Renewable or Not?
Decomposition produces new soil
But, in the tropical rainforests, all of
the nutrients are caught in the trees and
when cut down & burned the soil
cannot get the nutrients back.

Texture
The percentages (by weight) of different
sized particles of sand, silt and clay that it
contains.
Soil Properties:

Texture (Cont.)
>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
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.

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.

Some Soil Properties
Soils vary in the
size of the particles
they contain, the
amount of space
between these
particles, and how
rapidly water flows
through them.
Figure 3-25Figure 3-25

Fig. 3-25, p. 70
0.05–2 mm
diameter
High permeability Low permeability
WaterWater
Clay
less than 0.002 mm
Diameter
Silt
0.002–0.05 mm
diameter
Sand

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.

pH
The pH of most soils ranges from 4.0 to
8.0.
But, the soil of the Pygmy Forest in
California is extremely acidic (2.8-3.9)
and in Death Valley, California, it is
very basic (10.5).
Plants are affected by pH because of the
solubility of nutrient minerals.

Slope
Steep slopes often have little or no soil
on them because of gravity.
Runoff from precipitation tends to
erode the slope also. Moderate slopes
and valleys may encourage the
formation of deep soils.

Depth
Some soils are very shallow (like in
some places in San Antonio). It can be
only two inches of soil and then you hit
rock. Other areas can have soil 36
inches deep or more.

Color
Dark soil is rich with lots of
organic matter.
Light soil (like sand) is not so rich
with very little organic matter.

Organic Layer (O-horizon)
The uppermost layer; it is rich in
organic material.
Plant litter accumulates in the O-
horizon and gradually decays.
In desert soils the O-horizon is
completely absent, but in certain
organically rich soils it may be the
dominant layer.

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.

Subsoil (B-horizon)
The light-colored
subsoil beneath the A-
horizon; it is often a
zone of illuviation
where nutrient
minerals have leached
out of the topsoil and
litter accumulate.
It is typically rich in
iron and aluminum
compounds and clay.

Parent Material (C-horizon)
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.

Fig. 3-23, p. 68
Fern
Mature soil
Honey
fungus
Root system
Oak tree
Bacteria
Lords and
ladies
Fungus
Actinomycetes
Nematode
Pseudoscorpion
Mite
Regolith
Young soil
Immature soil
Bedrock
Rock
fragments
Moss and
lichen
Organic debris
builds upGrasses and
small shrubs
Mole
Dog violet
Wood
sorrel
Earthworm
Millipede
O horizon
Leaf litter
A horizon
Topsoil
B horizon
Subsoil
C horizon
Parent
material
Springtail
Red Earth
Mite

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.

Fig. 3-24a, p. 69
Mosaic of
closely
packed
pebbles,
boulders
Weak humus-
mineral mixture
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
Desert Soil
(hot, dry climate)
Grassland Soil
semiarid climate)

Fig. 3-24b, p. 69
Tropical Rain Forest Soil
(humid, tropical climate)
Acidic
light-colored
humus
Iron and
aluminum
compounds
mixed with
clay

Fig. 3-24b, p. 69
Deciduous Forest Soil
(humid, mild climate)
Forest litter leaf
mold
Humus-mineral
mixture
Light, grayish-
brown, silt loam
Dark brown
firm clay

Fig. 3-24b, p. 69
Coniferous Forest Soil
(humid, cold climate)
Light-colored
and acidic
Acid litter
and humus
Humus and
iron and
aluminum
compounds

Case Study: Industrialized Food
Production in the United States
Industrialized agriculture uses about 17% of
all commercial energy in the U.S. and food
travels an average 2,400 kilometers from
farm to plate. Figure 13-7Figure 13-7

Fig. 13-7, p. 277
4%
Food production
Food distribution
and preparation
Food
processing
LivestockCrops
5%6%2% 17%
of total U.S.
commercial
energy use

Traditional Agriculture: Low Input
Polyculture
Many farmers in developing countries use low-
input agriculture to grow a variety of crops on each
plot of land (interplanting) through:
Polyvarietal cultivation: planting several genetic
varieties.
Intercropping: two or more different crops grown at the
same time in a plot.
Agroforestry: crops and trees are grown together.
Polyculture: different plants are planted together.

6.4 billion tons of soils are eroded from
the U.S. each year; this would fill 320
million average-sized dump trucks that,
if parked end-to-end, would extend to
the moon and ¾ of the way back!
Erosion

Definition
Erosion is the movement of soil
components, especially surface
litter and topsoil, from one place to
another.

Importance
In undisturbed ecosystems, the roots of plants
help anchor the soil, and usually soil is not lost
faster then it forms.
But, farming, logging, construction, overgrazing
by livestock, off-road vehicles, deliberate
burning of vegetation etc. destroy plant cover
and leave soil vulnerable to erosion. This
destroys in a few decades what nature took
hundreds to thousands of years to produce.

SOIL EROSION AND
DEGRADATION
Soil erosion lowers soil fertility and can
overload nearby bodies of water with
eroded sediment.
Sheet erosion: surface water or wind peel off
thin layers of soil.
Rill erosion: fast-flowing little rivulets of
surface water make small channels.
Gully erosion: fast-flowing water join together
to cut wider and deeper ditches or gullies.

SOIL EROSION AND
DEGRADATION
Soil erosion is the
movement of soil
components,
especially surface
litter and topsoil,
by wind or water.
Soil erosion increases through activities such as
farming, logging, construction, overgrazing, and
off-road vehicles.

Global Outlook: Soil Erosion
Soil is eroding faster than it is forming on more
than one-third of the world’s cropland.

Case Study: Soil Erosion in the U.S. –
Some Hopeful Signs
Soil erodes faster than it forms on most U.S.
cropland, but since 1985, has been cut by
about 40%.
1985 Food Security Act (Farm Act): farmers
receive a subsidy for taking highly erodible
land out of production and replanting it with
soil saving plants for 10-15 years.

Water Erosion
Splash – water hits the soil at a severe angle
(based on slope)
 This can erode soil.
Sheet – when surface water moves down a
slope or across a field in a wide flow and peels
off fairly uniform sheets of soil.
Because the topsoil disappears evenly, sheet
erosion may not be noticeable until too much
damage has been done.

Water Erosion (Cont.)
Mass Slippage – (like in California) where
it is very wet and large amounts of soil slip
away in large chunks (mud slides).
Rill – concentrated flow across the surface
of soil. Leaves rivets (micro channels).

Gully – rivulets of fast-flowing water join
together and, with each succeeding rain, cut
the channels wider and deeper until they
become ditches or gullies. Gully erosion
usually happens on steep slopes where all
or most vegetation has been removed.

Wind Erosion
Saltation – one particle hitting
another and being blown across the
surface of the soil.

Suspension – airborne soil. Ex. soil
from Lubbock is found in Temple,
Texas.

Surface Creep – mountains/sand
dunes; surface creeping slowly
across. Landslides are an example
of a very fast surface creep.

Desertification: Degrading
Drylands
About one-third of the world’s land has lost some
of its productivity because of drought and human
activities that reduce or degrade topsoil.

Salinization
and
Waterlogging
Repeated
irrigation can
reduce crop
yields by
causing salt
buildup in the
soil and
waterlogging
of crop plants.

Fig. 13-13, p. 281
Evaporation
Transpiration
Evaporation
Evaporation
Waterlogging
Salinization Waterlogging
1. Irrigation water contains small
amounts of dissolved salts
2. Evaporation and transpiration
leave salts behind.
3. Salt builds up in soil.
1. Precipitation and irrigation
water percolate downward.
2. Water table rises.
Less permeable clay layer

Fig. 13-15, p. 281
CleanupPrevention
Soil Salinization
Solutions
Reduce irrigation
Switch to salt-
tolerant crops
(such as barley,
cotton,
sugarbeet)
Flush soil
(expensive and
wastes water)
Stop growing crops
for 2–5 years
Install underground
drainage systems
(expensive)

Salinization and Waterlogging of
Soils: A Downside of Irrigation
Example of
high
evaporation,
poor drainage,
and severe
salinization.
White alkaline
salts have
displaced cops.

Erosion Control (see Miller pg. 282)
Shelterbelts – can reduce wind erosion.
Long rows of trees are planted to partially
block the wind. They can also help retain
soil moisture, supply some wood for fuel,
and provide habitats for birds.

Minimum Tillage – (conservation
tillage) to disturb the soil as little as
possible while planting crops.
Special tillers break up and loosen the
subsurface soil without turning over
the topsoil, previous crop residues,
and any cover vegetation.

SUSTAINABLE AGRICULTURE
THROUGH SOIL
CONSERVATION
Modern farm machinery can plant crops
without disturbing soil (no-till and
minimum tillage.
Conservation-tillage farming:
• Increases crop yield.
• Raises soil carbon content.
• Lowers water use.
• Lowers pesticides.
• Uses less tractor fuel.

Contour Farming –sloping your growing
crops, etc.
You run terraces parallel to the ground to
stop soil from running down a steep slope.
Plowing and planting crops in rows across,
rather than up and down, the sloped
contour of the land.

Terracing – (what you
use for contour farming.)
Dirt goes up to hold the
dirt in place. Broad,
nearly level terraces that
run across the land
contour. Helps to retain
water for crops at each
level and reduce soil
erosion by controlling
runoff.

THROUGH SOIL
CONSERVATION
Terracing, contour
planting, strip
cropping, alley
cropping, and
windbreaks can
reduce soil erosion.

Strip Cropping – a row crop such as
corn alternates in strips with another
crop that completely covers the soil,
reducing erosion. It catches and
reduces water runoff and helps prevent
the spread of pests and plant diseases.

Cover Cropping (alley cropping) – several crops
are planted together in strips or alleys between trees
and shrubs that can provide shade (which reduces
water loss by evaporation) and helps to retain and
slowly release soil moisture.

Irrigation Techniques
Conventional center-pivot irrigation- allows
80% of the water input to reach crops
Gravity-flow irrigation- Valves that send
water down irrigation ditches.
Drip irrigation- Can raise water efficiency
to 90-95% and reduce water use by 37-
70%.
Floodplain irrigation- allowing the natural
floods to irrigate the crops. Soils in flood

Macronutrients
Macronutrients are larger in atomic
structure. Ex. Nitrogen, Phosphorus
& Potassium.
Micronutrients
These are smaller in atomic structure.
Plants need them in small amounts.
Ex. Selenium, Zinc & Iron.
Soil Nutrients

Fertilizers and Labels
Organic Fertilizers – animal manure, crop
residues, bone meal, and compost
Inorganic Fertilizers – man-made 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.

Definition
Hydroponics are growing plants in
fertilized water.
Hydroponics:
Method of suspending plants in
water and the solutions involved.
Ex. cranberries are grown this way.

Costs of Hydroponics:
It is labor-intensive and expensive.
You can control the
environment & grow plants
where there is no soil; NASA is
looking into this.
Benefits:

THROUGH SOIL
CONSERVATION
Fertilizers can help restore soil nutrients,
but runoff of inorganic fertilizers can cause
water pollution.
Organic fertilizers: from plant and animal
(fresh, manure, or compost) materials.
Commercial inorganic fertilizers: Active
ingredients contain nitrogen, phosphorous, and
potassium and other trace nutrients.

THE GREEN REVOLUTION AND
ITS ENVIRONMENTAL IMPACT
Since 1950, high-input agriculture has produced
more crops per unit of land.
In 1967, fast growing dwarf varieties of rice and
wheat were developed for tropics and subtropics.

Lack of water, high costs for small farmers, and
physical limits to increasing crop yields hinder
expansion of the green revolution.
Since 1978 the amount of irrigated land per
person has declined due to:
Depletion of underground water supplies.
Inefficient irrigation methods.
Salt build-up.
Cost of irrigating crops.

Modern agriculture has a greater harmful
environmental impact than any human activity.
Loss of a variety of genetically different crop and
livestock strains might limit raw material needed
for future green and gene revolutions.
In the U.S., 97% of the food plant varieties available
in the 1940 no longer exist in large quantities.

Fig. 13-18, p. 285
Biodiversity Loss Soil Water Air Pollution Human Health
Loss and
degradation of
grasslands,
forests, and
wetlands
Erosion Water waste Greenhouse gas
emissions from
fossil fuel use
Nitrates in
drinking water
Loss of fertility Aquifer depletion
Pesticide
residues in
drinking water,
food, and air
Salinization Increased runoff and
flooding from cleared
land
Other air pollutants
from fossil fuel use
Fish kills from
pesticide runoff
Waterlogging
Sediment pollution from
erosion Greenhouse gas
emissions of
nitrous oxide from
use of inorganic
fertilizers
Contamination of
drinking and
swimming water
with disease
organisms from
livestock wastes
Desertification
Killing wild predators to
protect livestock
Fish kills from pesticide
runoff
Surface and groundwater
pollution from pesticides
and fertilizers Belching of the
greenhouse gas
methane by cattle
Loss of genetic diversity
of wild crop strains
replaced by monoculture
strains
Bacterial
contamination of
meat
Overfertilization of
lakes and rivers from
runoff of fertilizers,
livestock wastes, and
food processing wastes
Pollution from
pesticide sprays

THE GENE REVOLUTION
To increase crop yields, we can mix the
genes of similar types of organisms and mix
the genes of different organisms.
Artificial selection has been used for centuries
to develop genetically improved varieties of
crops.
Genetic engineering develops improved strains
at an exponential pace compared to artificial
selection.
Controversy has arisen over the use of
genetically modified food (GMF).

Mixing Genes
Genetic
engineering
involves splicing a
gene from one
species and
transplanting the
DNA into another
species.

Fig. 13-19, p. 287
Projected
Disadvantages
Irreversible and
unpredictable genetic
and ecological effects
Need less fertilizer
Need less water
More resistant to
insects, disease,
frost, and drought
Harmful toxins in food
from possible plant cell
mutations
Grow faster New allergens in food
Can grow in
slightly salty soils
Lower nutrition
Less spoilage
Increased development
of pesticide-resistant
insects and plant
diseases
Need less pesticides
Can create herbicide-
resistant weeds
Better flavor
Tolerate higher
levels of herbicides
Can harm beneficial
insects
Lower genetic diversityHigher yields
Trade-Offs
Genetically Modified Crops and Foods
Projected
Advantages

PRODUCING MORE MEAT
About half of the world’s meat is produced
by livestock grazing on grass.
The other half is produced under factory-
like conditions (feedlots).
Densely packed livestock are fed grain or fish
meal.
Eating more chicken and farm-raised fish
and less beef and pork reduces harmful
environmental impacts of meat production.

Fig. 13-21, p. 289
Trade-Offs
Animal Feedlots
Advantages Disadvantages
Increased meat
production
Need large inputs
of grain, fish
meal, water, and
fossil fuelsHigher profits
Concentrate
animal wastes
that can pollute
water
Less land use
Reduced overgrazing
Reduced soil
erosion
Antibiotics can
increase genetic
resistance to
microbes in
humans
Help protect
biodiversity

How Many People can the World
Support? Food Production and
Population
The number of people the world can
support depends mostly on their per capita
consumption of grain and meat and how
many children couples have.
Research has shown that those living very low
on the food chain or very high on the food
chain do not live as long as those that live
somewhere in between.

PRODUCING MORE MEAT
Efficiency of converting grain into animal
protein.

CATCHING AND RAISING MORE
FISH AND SHELLFISH
After spectacular increases, the world’s
total and per capita marine and freshwater
fish and shellfish catches have leveled off.

CATCHING AND RAISING MORE
FISH AND SHELLFISH
Government subsidies given to the fishing
industry are a major cause of overfishing.
Global fishing industry spends about $25
billion per year more than its catch is worth.
Without subsidies many fishing fleets would
have to go out of business.
Subsidies allow excess fishing with some
keeping their jobs longer with making less
money.

Aquaculture: Aquatic Feedlots
Raising large numbers of fish and shellfish
in ponds and cages is world’s fastest
growing type of food production.
Fish farming involves cultivating fish in a
controlled environment and harvesting them
in captivity.
Fish ranching involves holding anadromous
species that live part of their lives in
freshwater and part in saltwater.
Fish are held for the first few years, released,
and then harvested when they return to spawn.

Fig. 13-24, p. 292
Trade-Offs
Aquaculture
Advantages Disadvantages
High efficiency Needs large inputs
of land, feed, and
water
High yield in
small volume
of water
Large waste
output
Destroys
mangrove forests
and estuaries
Can reduce
overharvesting
of conventional
fisheries Uses grain to feed
some species
Low fuel use Dense populations
vulnerable to
disease
Tanks too
contaminated to
use after about 5
years
High profits
Profits not tied
to price of oil

Fig. 13-25, p. 293
Solutions
More Sustainable Aquaculture
• Use less fishmeal feed to reduce depletion of other fish
• Improve management of aquaculture wastes
• Reduce escape of aquaculture species into the wild
• Restrict location of fish farms to reduce loss of
mangrove forests and estuaries
• Farm some aquaculture species in deeply submerged
cages to protect them from wave action and predators
and allow dilution of wastes into the ocean
• Certify sustainable forms of aquaculture

SOLUTIONS: MOVING TOWARD
GLOBAL FOOD SECURITY
People in urban areas
could save money by
growing more of their
food.
Urban gardens provide
about 15% of the
world’s food supply.
Up to 90% of the
world’s food is wasted.

Solutions: Steps Toward More
Sustainable Food Production
We can increase food security by slowing
populations growth, sharply reducing
poverty, and slowing environmental
degradation of the world’s soils and
croplands.

Soil Notes

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