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Soils Review Sheet
Courtesy of: Pamela J. Shlachtman, Miami Palmetto High School
Soil Formation
Parent materials - ancient layers of rock that have been broken down by
weathering
Chemical weathering - CO2 and H2O form Carbonic Acid which works on the rock
to break it down, especially limestone. Oxidation reactions work on ores and metallic
rocks. Acids and acid rain wear away rock
Physical ( Mechanical ) weathering - frost action occurs when water gets into the
cracks of rocks and freezes. The expansion of the ice breaks the rock apart. Heat
and cold on a rock cracks it as well. Large sheets can "peel" off called exfoliation.
Decayed Matter - Humus - is mixed into the rock layers and into the sediments
formed by weathering. It keeps the pH low as most plants favor 5.5 to 7.0 which is
acidic, not basic.
Burrowers - help cut the soil particles into smaller pieces. Worms literally eat their
way through the rocks and particles making the rocks smaller and more organic.
Roots of Plants - separate the soils, creates aeration in the soil as do burrowers.
Soil Properties
Texture - size of the rock. Gravels are the largest, Sand, Silt and Clay are the
remainder. Of course, gravels have the most interrock space ( porosity ) and clays
have the least.. The ideal texture is called LOAM which has a good aeration and
many types of soil within it.
Structure - How particles cling together - Sand has no structure. The particles do
not stick together. Clays and silt do. Good soils clump together but only in small
clumps and easily crumble. The ability to crumble is the friability. Sandy soils are
very friable, clay soils are not.
Soil Profile
Each layer of soil is called a horizon. The upper horizon or horizon A is the top soil. It has a lot of
organic materials in it ( humus ). Most organisms that live in the soil live here. The lower part of
horizon A has few nutrients because they have been carried to horizon B by leaching.
Subsoils or horizon B soils have much less organic matter very few organisms but a lot of nutrients.
The rainfall determines how much nutrient horizon B has. Too much or too little hurt horizon B's
nutrients.
Horizon C has no organic material. It has only nutrients. C helps determine the pH of B which
determines the pH of A. It also determines the rate of absorption and retention of water.
There are thousands of types of soils, all carefully classified by geologists but most of the soils can
be narrowed down to a few. Most are grassland or forest soils.
Grassland soils - deep Horizon A, low rainfall keeps leaching at a minimum . Thin
horizon B so root structure can not be large, thus trees are not common
Forest soils - higher rainfall causes leaching which increases horizon B, giving
structure for large roots.
Desert soils - poorly developed horizons. Little rain so leaching is non existent. Low
plant growth so the humus level is low. They are very poor unless fertilizer and/or
humus is added.
Cold Wet soils - accumulation of organic materials with very little actually going
into Horizon A. Not much decomposition goes on and the acidic nature of Horizon A
is high. A large horizon B creates tree producing soils.
Hot humid soils - tropical rainforests have so much rainfall that the soluble organic
materials are carried off quickly or absorbed into the plants that are there. Hot
humid soils are very poor.
Slope soils - accumulation of humus is rare as it is carried away as fast as it is
made. Slope soils are not very good.
Erosion
Rock wearing away and being transported to a new place by water, wind or gravity is called erosion.
The Mississippi River carries 325 million metric tons of topsoil into the Gulf of Mexico each year.
Worldwide, 25.4 billion tons of topsoil is transported. Unfortunately, most of it is not transported
to where we want it or can use it. Most of it goes to the sea. Wind erosion is extensive in dry areas.
Asia and Africa have huge amounts of wind erosion. Wind erosion from Northern Africa sometimes
reaches South Florida. Exposed soils from fires, farming or plowing can be eroded by the wind with
ease. Deserts, farms, overgrazed lands, mined lands and areas cleared for buildings and roads are
especially susceptible. Ethiopia and Nepal have lost over 1/3 of their topsoil to wind erosion. The
Great Plains of USA have had 4 huge droughts causing mass wind erosion since 1800. The last one
was in the 1930s and the area was referred to as the Great Dust Bowl, blowing dust all the way to
the Appalachian Mountains. Over 30 million hectares were damaged.
Sector
Cropland %
Pasture %
World
11
24
North America
13
16
USA
21
26
Africa
6
26
South America
8
26
Asia
17
24
Japan
13
1.6
Europe
30
18
We work as hard as we can to make the grasslands ecosystem croplands. There just isn't enough.
Most of our grasslands have been destroyed and made into farms. Marginal lands are being
converting into farms too. Chaparrals and deserts are being fertilized and irrigated to make farms.
Forests are being cut to make farms and pastures. If population does not slow down, all the world
will be farms , pastures, cities, roads and mountains. There are ways to use land better. Some of
the methods are designed to stop erosion and some are to increase the humus content of the soil.
Contour Plowing - plowing so that the tilling is at a right angle of the slope causes
the water to run along the tilling instead of straight down the slope. This slows down
the water and allows it to sink in rather than run off. Contour plowing is done on
gentle slopes and it can reduce erosion by at least 50%. On marginal lands, it
drastically increases the yield because water is at a premium.
Strip Cropping - When a slope is too steep for contour plowing, strip cropping is
done. Crops that are very porous to water are intermeshed with strips of crops that
absorb water. Hay, grass, alfalfa are alternated with corn or soybeans. The water
running through the corn hits the grass and slows down enough to soak in.
Terracing - Used on very steep slopes. Terraces are made so that they can be
planted . They are very expensive to make, need constant repair and maintenance.
Too much rain is very damaging. They are primarily used by places where hand labor
is in large amounts and time is not a problem. Some Chinese terraces took hundreds
of years to make and they still use them.
Waterways - protected channels of water collects. Must be closely watched and
maintained as they can become gulleys and wash water away too fast.
Windbreaks - Windbreaks reduce wind erosion a great deal. 35 mph winds can be
reduced to 8 mph or less. We used the Australian pine tree too much and it became
a mess. WE now use avocado and citrus trees.
Conservation Practices
Rotation of Crops - using a nitrogen producing crop such as alfalfa and clover,
after a high removing crop, such as cotton and tobacco. The land does not get totally
killed. Mass crops are taking a toll on rotation plans but small farmers rotate very
successfully.
Plowing weeds or crop stubble into the soil adds nutrients to the soil and reduces
the weeds in the next crops.
Strip tillage - till only the strip that seeds will be. Usually involves an herbicide.
No-till farming - Special planters or seeds in slits in crop residue or on plastic
sheets. Common where funguses are a problem. This leaves the rubble for wildlife,
creates less runoff, cleaner water, reduced costs and low value pasture can be used
to a point. It does ,however, slow down warming of the soil and reduces the
evaporation level. Pests in the rubble are a problem too.