General Soil Information
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.
o 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.
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.
 >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.
 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.
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.
Soil Horizons
 Organic Layer (O-horizon)
o The uppermost layer; it is rich in organic material.
o Plant litter accumulates in the O-horizon and gradually decays.
o In desert soils the O-horizon is completely absent, but in certain
organically rich soils it may be the dominant layer.
 Topsoil (A-horizon)
o It is dark and rich in accumulated organic matter and humus.
o 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)
o 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.
o It is typically rich in iron and aluminum compounds and clay.
 Parent Material (C-horizon)
o 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.
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.
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.
Food production
4%
Crops
2%
Livestock
6%
Food
processing
17%
of total U.S.
commercial
energy use
5%
Food distribution
and preparation
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:
o Polyvarietal cultivation: planting several genetic varieties.
o Intercropping: two or more different crops grown at the same time in
a plot.
o Agroforestry: crops and trees are grown together.
o Polyculture: different plants are planted together.
Erosion
 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!
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.
o Sheet erosion: surface water or wind peel off thin layers of soil.
o Rill erosion: fast-flowing little rivulets of surface water make small
channels.
o 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.
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%.
o 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.
 Mass Slippage – (like in California) where it is very wet and large amounts
of soil slip away in large chunks (mud slides).
Wind Erosion
 Saltation – one particle hitting another and being blown across the surface of
the soil.
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.
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
 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.
SUSTAINABLE AGRICULTURE THROUGH SOIL CONSERVATION
 Modern farm machinery can plant crops without disturbing soil (no-till and
minimum tillage.
o 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.
SUSTAINABLE AGRICULTURE 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.
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 zones tend to be nutrient rich and fertile.
Soil Nutrients
 Macronutrients
o Macronutrients are larger in atomic structure. Ex. Nitrogen,
Phosphorus & Potassium.
 Micronutrients
o These are smaller in atomic structure. Plants need them in small
amounts.
o Ex. Selenium, Zinc & Iron.
Fertilizers and Labels
 Organic Fertilizers – animal manure, crop residues, bone meal, and compost
 Inorganic Fertilizers – man-made from chemical
compounds
o Benefits – exact compositions are known; they are soluble & thus
immediately available to the plant
o 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.
 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.
Benfits
 You can control the environment & grow plants where there is no soil;
NASA is looking into this.
SUSTAINABLE AGRICULTURE THROUGH SOIL CONSERVATION
 Fertilizers can help restore soil nutrients, but runoff of inorganic fertilizers
can cause water pollution.
o Organic fertilizers: from plant and animal (fresh, manure, or
compost) materials.
o 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:
o Depletion of underground water supplies.
o Inefficient irrigation methods.
o Salt build-up.
o 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.
o In the U.S., 97% of the food plant varieties available in the 1940 no
longer exist in large quantities.
THE GENE REVOLUTION
 To increase crop yields, we can mix the genes of similar types of organisms
and mix the genes of different organisms.
o Artificial selection has been used for centuries to develop genetically
improved varieties of crops.
o 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.
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).
o 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.
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.
o 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.
 Government subsidies given to the fishing industry are a major cause of
overfishing.
o Global fishing industry spends about $25 billion per year more than
its catch is worth.
o Without subsidies many fishing fleets would have to go out of
business.
o 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.
o Fish are held for the first few years, released, and then harvested when
they return to spawn.
SOLUTIONS: MOVING TOWARD GLOBAL FOOD SECURITY
 People in urban areas could save money by growing more of their food.
o 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.