Soils
I. The formation of soil, the thin plant-growth-supporting surface layer of the Earth's crust, results from
interactions among five soil-forming factors: parent material, climate, organisms, time, and
topography (a region's surface features: slopes, low-lying spots, etc.).
A. Biological, chemical, and physical weathering processes slowly break parent material into
smaller and smaller particles. Parent material was the mixture of starting materials in
which a soil formed. Sometimes, soils form by weathering into bedrock. In New
England, most soils formed in parent materials consisting of ground rock bull-dozed into
place (or spewed as glacial outwash onto the landscape) by glaciers. Some of the
world’s best soils formed in aeolian (wind-blown) silt deposits called loess.
B. Soil is composed of inorganic minerals and organic materials as well as air and water
contained in the pores or void spaces between solid particles.
1. Humus is the dark brown or black organic material that remains after much
decomposition of plant debris.
2. Soil water that is not absorbed by roots moves downward, leaching dissolved
nutrients from the soil.
C. Soil is organized into horizontal layers called soil horizons. The horizons commonly, but not
always, present in most soils include (from top to bottom):
1. O horizon - a litter or thatch layer at the soil surface.
2. A horizon - the mostly mineral, but organic-matter-rich, dark brown topsoil.
Materials other than organic matter (incl. clay, iron, and aluminum) have often
been removed by leaching or eluviation from this layer.
3. E horizon - a strongly leached, often white or light yellow, zone of eluviation too
deep to have accumulated organic matter.
4. B horizon - A depth zone where materials eluviated from shallower depths in the
soil profile have been deposited by illuviation. (Incidentally, “illuviation” and
“eluviation” are words like “immigration” and “emigration”. The “i” + double
consonant means something or someone moves in; the “e” + single consonant
means someone or -thing moves out.) B horizons are often orange or red in
color. A black horizontal stripe of a special kind of leachable organic matter
often occurs in the B horizons of conifer-dominated forest soils (Spodosols).
Together with the E horizon, if present, this is the subsoil.
5. C horizon - parent material that was too deep to be much affected by weathering
and has changed little except perhaps to be broken into smaller chunks. The C
horizon is sometimes called the substratum.
6. R horizon - bedrock.
D. Soil organisms such as plants, algae, fungi, worms, insects, spiders, and bacteria are
important not only in forming soil, but also in cycling nutrient minerals.
1. Earthworms bring nutrient minerals (esp. calcium) from deeper soil layers to upper
layers when they deposit castings on the soil surface.
2. Mycorrhizae are symbiotic relationships between soil-dwelling fungi and the roots
of vascular plants that greatly enhance the ability of plants to extract phosphate
from soil.
3. In a balanced ecosystem, nutrient cycling occurs in which the nutrient elements
(e.g., N, P, K, Ca, Mg, S) removed from the soil by plants are returned when
plants or the animals that eat plants die and are decomposed by soil
microorganisms.
E. The texture of a soil depends on the relative amounts of sand, silt, and clay in it. Along
with soil pH, soil texture strongly affects many soil properties that control plant growth,
such as water-holding capacity, aeration, easy of root penetration, tendency to
aggregate, and nutrient availability.
1. Sand, silt, and clay are size categories for the mineral particles in soil. The upper
and lower limits for the diameters of these particles are:
clay < 2 :m (0.002 mm) < silt < 50 :m (0.05 mm) < sand < 2 mm.
Any particle > 2 mm is a “coarse particle”. The relative sizes of sand, silt, and
clay are shown in Fig. 14-6. Soils in which clays predominate are called “finetextured” and sand-rich soils are called “coarse-textured”, in part because they
feel scratchy.
2. Soil textural classes are categories for soils of similar textures which are defined
based on the relative abundances of sand, silt, and clay. For example, a soil
with a nearly ideal texture contains a sand:silt:clay ratio of about 2:2:1 and is
called a loam (a textural class). Nearly, but not quite ideal textural classes have
names like sandy loam, silt loam, or clay loam to indicate that they were almost
loams but have a little too much sand, silt, or clay, respectively. The soil
textural triangle shows all the possible classes.
3. Loam does not mean topsoil! Topsoil refers to a soil A horizon. Loam is a
textural class. You can have C horizon soil that is a loam. Only in New
England do you see advertisements for “screened loam” when they are really
selling topsoil.
4. The clay fraction of a soil has the greatest influence on the overall physical and
chemical properties of soils. Unlike silts or sands, clay-sized particles in soil
were mostly formed by precipitation from soil solutions rather than by being
broken off from rocks. Other differences include:
a. Clays have many negative charges locked tightly in their crystalline interiors.
This means that everywhere that a negative charge is locked inside a
clay, right next to it sitting on the clay surface is a positively charge ion
(a cation) to balance the clay’s charge. The cation cannot
spontaneously leave, but it can be replaced by another cation from the
soil solution. Each kind of clay has its own cation exchange capacity
which is the number of cmol (centimoles / hundredths of a mole) of
positive charge that a kg of that clay will retain. The most ancient clays
(e.g., those from the Amazon basin) have very little cation exchange
capacity compared to clays from the American Midwest.
b. Clay-rich soils with many cation exchange sites buffer (insulate against large
changes in) cation concentrations. The clays keep most of the nutrient
ions (like potassium and ammonium) out of the soil solution so they
leach less slowly than in a sandy soil.
c. Hydrogen ions (H+), which are acidic cations, are also buffered by cation
exchange sites. Thus clay-rich soils are more slowly acidified by acid
rain than sandy soils. Conversely, an acidic clay soil needs a lot more
lime to bring its pH back to neutrality than a sandy soil with the same
starting pH.
d. Clay particles stick to each other (cohere ) far more tightly than do sand or
silt particles. Unlike very sandy soils, soils with a decent amount of clay
can contain soil structures.
5. The relationship between soil textural class and other important soil properties is
shown in Table 14-2.
F. Soil structures are aggregations primarily of clay together with included silts and sand
particles.
1. Some structures in B horizons are fairly large columns or blocks several inches tall
or wide.
2. In topsoil, the most important soil structures are little spheroidal aggregates called
crumbs or granules, whose presence opens up otherwise very tight clay-rich
soils.
a. The macropores between the granules of a well-aggregated topsoil allow
easy entry for air and water into soil.
b. These aggregates are stabilized by soil organic matter that helps keep them
glued together.
c. Well-aggregated silt-rich soils resist erosion far better than similar soils with
few crumbs or granules.
d. Compaction of soil by traffic or tillage destroys these structures to the
detriment of soil productivity.
(Simkins shows mercy! Section I.G. was omitted from the lecture and cannot be included on the
exam. It appears below only for your amusement.)
G. A taxonomy exists for classifying soils into a nested series of categories analogous to the
kingdom, phylum, class, order, family, genus, species system familiar from biology. The
first and broadest division in this scheme is to classify all soils into one of 12 soil
orders, all of which have names ending in the suffix “...sols”. Some of these orders are
reserved for soils whose nature has been largely dictated by the overwhelming influence
of parent material (e.g., recent volcanic), time (very young soils), or topography (lowlying wet spots give swampy soils). However, five soil orders (Fig. 14-8) are
characteristically formed by the interacting effects of climate and vegetation associated
with five of the great terrestrial biomes.
1. Spodosols form under taiga. Unless a nearby river or volcano has messed things up
locally, the acidifying effects of coniferous vegetation will form soils in which the
acidity has leached the iron, aluminum, and a lot of the organic matter out of the
A and E horizons and deposited all three in the B horizon. Some of these soils
2.
3.
4.
5.
6.
have profiles with a 3-inch strip of brown A horizon, followed by a white E
horizon underlain by a wide orange (rusty) B horizon with a thin stripe of black
organic matter running through it.
Northern, temperate deciduous forests characteristically create alfisols. These soils
are less acidic than spodosols and have higher levels of nutrients. They also
have clay-rich B horizons. Forest soils, whether alfisols or spodosols, usually
have fairly thin A horizons.
Mollisols form under temperate grasslands and include many of the richest soils in
the world. Unlike forest soils, mollisols have thick, black, well aggregated, soft,
A horizons, which under tall-grass prairies can be 2 feet thick or deeper.
Most desert soils are classified as aridisols. Formed in areas of little rain or natural
vegetation, aridisols naturally contain little organic matter. However, these
unleached soils tend to contain an abundance of salts that can raise their pH to
quite alkaline levels. Instead of a litter layer, desert soils tend to have a “desert
pavement”, a coating of pebbles that are left behind when unaggregated sand
particles are blown away.
Oxisols form below tropical rainforests. These most intensely weathered of all soils
have been almost entirely leached of all nutrient ions, and only the most
weathering resistant minerals (iron and aluminum oxides and the most stable
kinds of clays) survive. These soils have been weathered to great depths (3
m+) and under their thin A horizons are almost always bright orange or red.
The text does not mention it, but the tundra biome now has its own characteristic
soil order, the gelisols, soils formed over permafrost.
II. Sustainable soil use is the wise use by humans of soil resources without a reduction in the amount
or fertility of soil, so that it is productive for future generations.
A. Soil erosion is the removal of soil from the land by the actions of water, wind, ice, or other
agents.
1. Soil erosion is a natural process that is often accelerated by human activities such as
farming on arid land and deforestation.
a. Because tillage (e.g., plowing) accelerates bacterial and fungal decomposition
of soil organic matter, conventional farming invariably leads to a
decrease in soil organic matter levels compared to those in comparable
virgin (i.e., never plowed) soils.
b. With less organic matter to stabilize soil aggregates, the unglued, isolated soil
particles in farmed soils are more easily swept away by wind and
water.
c. Plowed soils also lack litter layers that seal natural soils away from wind and
runoff.
d. Silt particles are blown and washed away more easily than sand or clay.
Sand grains are bigger and heavier, and clay particles stick together.
2. The Dust Bowl that occurred in the western United States during the 1930s is an
example of accelerated wind erosion caused by human exploitation of marginal
land for agriculture. (See Fig. 14-10)
a. Some of the soils that were the most intensely eroded during the Dust Bowl
were silt loams that formed from wind-deposited (aeolian) parent
materials called loess consisting almost entirely of silt.
b. Mini-Dust Bowls still occur. The text cites a 1989 dust storm that forced
the closure of 150 mi of Interstate 70 in Kansas.
c. Major dust bowls in the former short-grass prairie regions have not
occurred recently for two primary reasons:
i. After World War II, increasing use of the Ogallala aquifer for
irrigation kept these soils moist and productive even during the
inevitable drought years.
ii. After WW II, increased inorganic fertilizer use caused increased
crop growth and, consequently, increased amounts of plant
residues plowed back into the soil every year. This led to
approximately a doubling in the organic matter contents of these
soils since that time.
B. Mineral depletion occurs in all soils that are farmed. It is a particularly serious problem
when tropical rain forests are removed, because the nutrient minerals in the soil are
quickly leached out.
1. Mineral depletion occurs in farmed soils much more rapidly than in their virgin
counterparts because the network of roots between newly planted row crops is
incomplete, and less “tight” than in perennial grasslands or forests. The rootfree areas of soil early in the growing season leave superhighways for
unimpeded leaching of plant nutrients out of the rooted depth.
2. The rules for cation exchange say nutrient cations cannot just leave the clays behind
naked; they must be replaced by other cations. The only newly arriving cations
available to replace the leaching nutrients are H+ ions from carbonic acid in
rainwater. So a consequence of leaching loss of nutrient is acidification of soil.
This is why nutrient-tight natural ecosystems get along nicely without needing to
be limed, but farmed soils must be limed every few years.
C. Desertification is the degradation of once-fertile rangeland or forest into nonproductive
desert; it is caused partly by soil erosion, overgrazing, overcultivation, and forest
removal. A positive-feedback loop appears to be partly responsible for desertification.
Grazing º less plant biomass º less soil organic matter º poorer soil support of
plant growth º less plant biomass º ...
D. In 1992, the first global assessment of soil conditions reported that 17% of the Earth's total
vegetated surface area had been degraded since World War II, and we may have
permanently lost 11% of our once vegetated soil.
III. Soil conservation practices can minimize soil erosion and mineral depletion in healthy soils and help
restore damaged soils.
A. In conservation tillage residues from previous crops are left in the soil as a mulch, partially
covering it and helping to hold it in place. Instead of plowing and discing soil to prepare
it as a seed bed, special equipment drills seeds for the next crop through the
undisturbed residues of the previous crop. This practice increases the organic matter
(no plowing!) and, therefore, improves the soil's water-holding capacity but requires
greater use of herbicides to control weeds. Originally, this practice was called
“chemical tillage” back in the 1960s but that term was found to have lousy PR value, so
they changed it.
B. Crop rotation is the planting of a series of different crops in the same field over a period of
years. Crop rotation prevents depletion of certain nutrient minerals that occurs when the
same crop is grown year after year. Legumes in a crop rotation contribute ammonium
to the soil when they die. Some crops have mycorrhizal fungal symbionts that help
improve phosphate availability. Crop rotation also reduces the need for fungicides and
insecticides by avoiding the intense buildup of disease spores and pests in the soil that
results from growing the same monoculture in the same soil year after year.
C. In contour plowing fields are plowed and planted in curves that conform to the natural
contours of the land. Rows formed by plow straight up and down slopes allows water
to rapidly run downhill. Plowing across a slope puts barriers to water flow, slowing it
down and giving it time to filter into the soil. Strip cropping is a special type of contour
plowing with alternate strips of different crops planted along natural contours. Soil
eroded from a strip of soybeans may be trapped down slope in a wheat strip.
D. Terracing can be used to help control erosion in mountainous terrain that is being
cultivated.
E. The use of organic fertilizers (animal manure, crop residues, bone meal, and compost) is
preferred over commercial inorganic fertilizers because organic fertilizers are slow
acting, long lasting, and may suppress microorganisms that cause certain plant diseases.
Researchers at UMass found that strawberries grown with compost are much more
disease resistant than their conventionally grown counterparts. Other researchers at
UMass have found that a green manure mixture of oats and hairy vetch planted in the
fall will hold the soil in place, enrich it with organic matter, and fix enough nitrogen to
support a corn crop, before it is plowed under in the spring in time to plant the corn.
F. Soil that has been badly damaged by erosion can be reclaimed.
1. Providing plant cover and shelterbelts, rows of trees that lessen the impact of wind,
helps to stabilize the land.
2. Restricting land use until the soil has time to recover helps restore soil fertility.
IV. The Natural Resources Conservation Service (formerly the Soil Conservation Service) works with
U.S. citizens to conserve natural resources on private lands.
A. The conservation compliance program requires farmers with highly erodible land to adopt
erosion-control measures or lose certain federal benefits.
B. The Conservation Reserve Program, which is voluntary, pays farmers to stop producing
crops on highly erodible (e.g., steeply sloping or sloping and silt rich) farmland. The
1996 Farm Bill provided funding for the Conservation Reserve Program until 2002.
C. Historically, farmers have shown greater interest in voluntary, incentive-based soilconservation programs when agricultural commodity prices are low.