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

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

4%Food distribution and preparation

Food processing

LivestockCrops5%6%2%

17% of total U.S. commercial energy use

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.

Micronutrientso 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.