Soils 2
Chapter 10 Soil Mineralogy
98% of the earth’s crust is made up of 8 chemical elements , see Figure 10.1 in book (pg 149), these
elements combine with one another and others to form the minerals that exist in rocks. The dominant
minerals are feldspars, amphiboles, pyroxenes, quartz, mica, apatite, clay, iron oxides (goethite) and
carbonate minerals.
As these minerals are weathered I rocks new minerals are synthesized in the weathering processes. Those
minerals least resistant to weathering disappear first. Quartz is the most resistant so that there is more
quartz remaining in soils than other primary minerals. Weathering processes are constant and observable
all the time…weathering is stimulated by the naturally occurring acids in nature. As primary minerals are
weathered nutrients are released in the form of ions available for plant life. The primary minerals disappear
and secondary minerals are formed. Clay minerals begin to accumulate.
Chapter 3 Soil Physical Properties
SOIL TEXTURE
Soil texture is the relative proportions of sand, silt and clay in a soil.
Soil separates are the size groups of mineral particles less than 2 millimeters (mm). See the chart on page
23, Table 3.1). See Textural Triangle on pg 25.
Sand is the 2.0 to .05 millimeter fraction. Under the USDA system it is divided into very fine,
fine, medium, coarse and very coarse sand separates.
Silt is the .05 to .002 millimeter (2 microns) particle size separation. Note that it is difficult to feel
the difference between fine or very fine sand and silt. Generally, silt feels smooth like powder while sand
is gritty.
Clay particles are separates with a diameter of less than .002 millimeters or 2 microns. Clays tend
to be plate shaped not spherical and are very small but have a very large surface area per gram. Note the
differences in surface area in Table 3.1, pg 23. It is the shape and surface area of clays which give them
their unique properties. Water between clay particles tend to act as a lubricant. Surface area is also an
important aspect of water absorption; clays absorb much more water than sands or silts. When clays are
dried there is a greater surface area of contact between particles and hard clods con be formed.
Particle Size Analysis pg 24./ Field Method pg 25
Soil Textural Classes (pg 25 Textural Triangle)
Percentages of sand, silt and clay detyermine which of 12 Textural Classes the soil is in. look on Soil
Triangle. A soil that is 15% clay, 65% sand and 20% silt is in which class? (Sandy loam)
A soil containing equal amount of sand silt and clay is a clay loam. Use the chart.
Use of Soils
Plasticity, permeabiltiy, ease of tillage, droughtiness, fertility and productivity are all related to
soil texture.
Clay soils may have a tendency to shrink and swell with wetting and drying.
SOIL STRUCTURE
Soil structure and texture are not the same thing. Where soil texture refers to the size of soil
particles, soil structure refers to how they are arranged. Soil particles arrange themselves into secondary
structures called peds or aggregates. Soil structure is described by describing the shape and size of the
peds.
Structure is important because it modifies the effect of soil texture with regard to water and air and
root penetration. Peds are identified by the fact that the spaces between the aggrgate are larger than the
spaces between the individual soil particles.
Structures are described by their shape. There are four basic structural types:
1.
spheroid
2.
platelike
3.
blocklike
4.
prismlike
These shapes in turn give rise to granular, platy, blocky and prismatic types of structure.
Columnar structure is prismatic shaped peds with rounded tops.
Generally speaking soils have no structure inherent in their parent materials. Structureless conditions are
referred to as single grained (sand) or massive (clayey with no discernible structure).
Because of the level of activity at the A horizon (weather, organisms, etc.) the structure tends to be
granular. As you go deeper there is less influence of wetting and drying cycles, fewer organisms and the
additional effect of weight from the upper layers, this layer tends to be more blocky or prismatic.
GRADE and CLASS (pg 29)
When a soil structure is described the description includes:
1.
the soil structure type, that is the shape and arrangements of the peds,
2.
the class, which refers to the ped size,
3.
the grade which indicates the distinctiveness of the peds
A Table of peds types and class is shown in Appendix 2.
Grades are described as:
0.
Structureless – no discernable aggregation. Massive of single grained
1.
Weak.- poorly formed, indistinct, barely observable
2.
Moderate- well formed distinct, moderately durable
3.
Strong- durable peds
SOIL CONSISTENCE
Soil Consistence is the resistance of a soil to deformation or rupture. It is a function of the cohesive and
adhesive properties of the soil. Where soil =structure is concerned with the shape, size and distinctiveness
of soil aggregates, Consistence is concerned with the strength of the forces between the sand, silt and clay
constituents.
Consistence is important for tillage and traffic considerations. Sand dunes show minimal consistence, little
cohesive or adhesive properties. Sand is easily deformed (footprints, cars get stuck in it)
Clay soils show more resistance and are not as easily deformed however, they get sticky when wet and are
difficult to till.
Soil Consistence is described in terms of moisture – wet, moist and dry, so that a oil might be described as
sticky when wet, firm when moist and hard when dry. Some terms used to describe soil consistence are:
1.
Wet soil may be sticky, nonsticky, plastic or nonplastic
2.
Moist soil may be loose, friable, firm
3.
Dry soil may loose, soft or hard
Cemented soils are held together by mineral cementing agents such as calcium carbonate, silica, iron oxide
or aluminum oxide. When a cementatious soil becomes so hard a hammer is required to break it up it is
called an indurated soil. These tend to be very old soils…
DENSITY AND WEIGHT RELATIONSHIPS
Soil density is described in 2 ways: particle density and bulk density
Particle density is the average density of the particles, and bulk density is the density of the soil in its
natural state which includes pore space as well as particles.
Density is a comparsion to the weight of water. A cubic foot of water weighs 62.4 pounds.
Average particle density for mineral soils is (2.65 g/cm3) 2.65 grams per cubic centimeter (or 2.65 x 62.4
= 165.36 ). When samples are collected for bulk density analysis they are taken with core samplers to leave
the sample as close to possible in its natural condition. Cores are taken of various depths but typically are
about 2 to 2.5 inches in diameter.
Bulk density = mass oven dry/volume
The bulk density of a soil is inversely related to porosity. The more pore space the less mineral soil…
Soils that have no structure may have a bulk density of about 1.6 or 1.7g/cubic centimeter. As structure is
formed pore spaces are formed leading to an increase in porosity and a decrease in bulk density so that a
loam surface soil may have a density of 1.3 g/cubic centimeter. As clay content of surface soils increases
structural development increases and bulk density decreases…but as illuviation occurs and the pore spaces
of B horizon soils is filled with clay, forming the Bt Horizon, the bulk density increases…
NOTE THE RELATIONSHIP BETWEEN BULK DENSITY AND PORE PSACE IN THE FIGURE 3.10,
pg 33.
Furrow-Slice
Erosion is usually expressed in terms of tons of soil per acre but without knowing the weight of a soil the
figure is meaningless. An acre-furrow-slice is consider to be a layer of soil about 7 inches thick over an
acre or about 25,410 cubic feet (43560 sq ft x 7 inches)
Exercise on pg 34
Porosity
The fact that particle densities may be as high as 2.65 and bulk densities as low as 1.3 is an indiction that
the soil is approximately 50% pore space or 50% porosity. Pore spaces vary in size however and the size of
the pore space may be as important as the 5 porosity.
See method of determining porosity on page 34.
Although clay soils have more porosity, more pore space than surface soils watr moves more slowly
through them because the pore spaces of clay are so small. Macropores, such as tha pore spaces in sand are
large enough that they cannot contain water against gravity. Pores that can hold water fter wetting are
caled micropores or capillary pores.
Soils with macropores are well aerated but droughty, where as soils with micropores may be wet and
airless.
SOIL COLOR
Soil color isimportant for several reasons: first it is an observable measure of the orgainic content, but it
may also be an indicator of drainage and aeration. To the trained eye soil color may also indicte the history
of the soil.
Soil color are important features and are used as part of a soil description. Soil colors are compared to
color chips in a soil color book, usually the Munsell soil-color book. Colors are arranged according to hue,
value and chroma.
Hue refers to the dominant wavelength or color, Value refers to the brilliance of the color from
dark to light. Chroma refers to the purity of the light. The three properties are always reported in the order
of hue, value and chroma. In the notation 10YR 6/4 10YR is the hue, 6 is the value and 4 is the chroma.
The value of this system is that you can communicate the information to anyone in the world across time
and space.
Factors Affecting Color
Factor
Affect
Organic Matter
Raw Peat
Humus
Manganese oxides
Iron oxide (hematite)
Hydrated iron oxide (goethite)
Reduced Iron oxide
Cycles of water saturation
brown
very dark, black
black
brown, reddish, rust colored
yellow, yellow-brown
gray (indicative of water-saturation)
mottled soils
SOIL TEMPERATURE
Temperature considerations are important for several reasons: as temperatures drop organic activity slows
and chemical processes slow. Freeze- thaw cycles are important in soil development
Heat Balance – refers to the gains and losses of heat. Soil is warmed by the sun; some energy is reflected
but some is absorbed. A dark colored soil may absorb as much as 80% of incoming insolation while a light
colored soil my only absorb 30% Book says that 34% of the available solar radiation is reflected of back
into space, 19% is absorbed by the atmosphere and 47% is absorbed by the land.
Soils lose heat through:
1.
evaporation of water (the amount of heat needed is related to water content, while it takes
only .2 calories /gram of energy top raise the
2.
temp of one gram of dry soil 1 degree Celsius, it takes 1 calorie per gram of water…) in
general than sandy soils retain less water and heat quicker in spring to allow earlier
planting…
3.
radiation back to the atmosphere
4.
heating the air in contact with the soil
5.
heating other soil
Overtime the gains and losses balance but there are seasonal gains and losses. Other factors include site
aspect (north facing slopes get less sun…), location relative to large bodies of water…
Modifications that affect soil temperature- change moisture content (drain soils) and change surface color
(light colored mulch will reflect light but will reduce water loss through evaporation and reduce heat loss).