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1. Loam—When rubbed between the
thumb and fingers, approximately equal influence of sand, silt,
and clay is felt. Makes a weak ribbon (less than 2.5 cm long).
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Textural Group |
Textural Classes |
| Sandy |
Coarse |
Sand
Loamy Sand |
| Loamy |
Moderately Course |
Sandy Loam |
| Medium |
Loam
Silt Loam
Silt |
| Moderately Fine |
Clay Loam
Sandy Clay Loam
Silty Clay Loam |
| Clayey |
Fine |
Sandy Clay
Silty Clay
Clay |
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2. Sandy loam—Varies from very
fine loam to very coarse. Feels quite sandy or gritty, but
contains some silt and a small amount of clay. The amount of silt
and clay is sufficient to hold the soil together when moist.Makes
a weak ribbon (less than 2.5 cm long). |
3. Silt loam—Silt is the dominant
particle in silt loam, which feels quite smooth or floury when
rubbed between the thumb and fingers. Makes a weak ribbon (less
than 2.5 cm long). |
4. Silty clay loam—Noticeable
amounts of both silt and clay are present. Makes a medium ribbon
(2.5 to 5 cm long). |
5. Clay loam—Clay dominates a clay
loam, which is smooth when dry and slick/sticky when wet. Silt and
sand are usually present in noticeable amounts in this texture of
soil, but are overshadowed by clay. Makes a medium ribbon (2.5 to
5 cm long). |
In general, the finer the texture: the more difficult a
soil is to work or till, the greater the water holding capacity,
the slower water will enter and move through the soil profile, the
more difficult plant root penetration, the more readily surface
soil will crust, and the more nutrient rich the soil. |
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Regardless of textural class, all soils in Arizona
contain sand, silt, and clay, although the amount of a particular
particle class may be small. |
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Other textural designations of surface soils are sands,
loamy sands, sandy clay loams,silty clay, siltss, and clays. In
each textural class there is a range in the amount of sand, silt,
or clay that class may contain. These ranges can be expressed as a
percentage for each soil texture. The percentages converge on the
soil triangle to determine soil textural classes. The composition
of each textural class does not allow for overlap from one class
to another. |
Single Grain
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Usually individual sand grains not held together. Found
in sandy or loamy textures. |
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Texture of soil influences many different
characteristics. A brief comparison between sandy and clay soils
will highlight these points. In general coarse-textured or sandy
soils allow water to enter at a faster rate and to move more
freely in the soil. In addition, the relatively low water-holding
capacity and the large amount of air present in sandy soils allows
them to warm up faster than fine-textured soils. However, sandy
soils must also be irrigated more often than high clay soils.
Sandy soils are also more easily tilled. They are well-suited for
the production of vegetables, landscape plants and many types of
fruit.
Cation Exchange
Clay psrticles are composed of minerals that, in general, possess a
negative electrical charge. In some mineral particles this charge is
built in to the clay crystals, and is unaffected by changing soil
conditions. This is called fixed charge. In other clay
minerals, and in organic particles, the charge is altered by changes
in soil acidity. This charge is called variable charge.
Variable charge is greatest under alkaling conditions, and least
under acidic conditions.
The negative charges of clay particles give them the ability to attract and
hold positively-charged molecules ( cation). Nutrients such as
calcium, magnesium, potassium, and many other soil elements are
cations. The ability of the soil to hold on to cations is cation
exchange capacity (CEC). Nutrients retained by CEC are prevented
from leaching out of the rooting zone, yet are held lossely enough
to be available to growing plants.
The soil properties most closely related to CEC are soil texture and
organic matter content. Fine textured soils have more CEC than
coarse-textured soils, and soils high in organic matter have more
CEC than low organic matter soils. Also, non-acidic soils have more
CEC than acidic soils.
Some soil particles have a small positive electrical charge, and can
attract anions (negatively-charged molecules.) Anion
exchange capacity is usually insignificant in Arizona soils. |
Granular
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Porous granuals held together by organic matter and
some clay. Found in A horizons with some organic matter. |
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Structure
Most soil particles are grouped together to form structural pieces called
peds or aggregates, although coarse soil particles may be
unaggregazed and exist as single grains. In surface soil, the
structure usually will be granular unless it is very sandy. The
soil aggregates will be rounded and vary in size from that of a
very small shot to that of a large pea. If organic matter content
is low and the soil has been under continuous cultivation, the
soil structure may be quite indistinct. If the soil is
fine-textured with high organic content, it may have a blocky
surface structure. Substance aggregate types are platy, blocky,
prismatic, and massive. |
Platy
|
Aggregates that have a thin vertical dimension with
respect to lateral dimensions. Found in compacted layers and
sometimes E horizons. |
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Air and water movement within the soil is closely
related to its structure. Good structure allows rapid movement of
air and water, while poor structure slows down this movement.
Other things being equal, water can enter a surface soil that has
granular structure more rapidly than one that has little
structure. Soil compaction due to excessive vehicle or foot
traffic or excessive tillage can greatly reduce soil aggregation
and water infiltration. Since plant roots move through the same
channels in the soil as air and water, good structure allows
extensive root development whereas poor structure discourages it.
Water, air, and plant roots move more freely through subsoils that
have blocky structure than those with a flaky horizontal
structure. Good structure of the surface soil is promoted by an
adequate supply of organic matter. Soil structure can be
protected, by working the soil only when moisture conditions are
correct. |
Blocky
|
Roughly equidimensional pads usually higher in clay
than other structural aggregates. Found in Bhorizons
with clay. |
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Growing plants also change the soil structure as they
send their roots into the soil for mechanical support and to
gather water and nutrients. The roots of plants, as they grow,
tend to enlarge the openings in the soil. When they die and decay,
they leave channels for movement of air and water. In addition to
the plants that we see, there are bacteria, fungi, nematodes, and
other very small organisms growing in the soil which can be seen
only with the aid of a microscope. Even these organisms enrich the
soil as they die. |
Prismatic
|
Structural aggregates that have a much grater vertical
than lateral dimension. Found in some B horizons. |
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Drainage
Soil drainage is defined as the rate and extent of water movement in the
soil, including movement across the surface as well as downward
through the soil. Slope is a very important factor in soil
drainage. Other factors include texture, structure, and physical
condition of surface and subsoil layers. Soil drainage is
indicated by soil color. Clear, bright colors indicate
well-drained soils. Mixed, drab, and dominantly gray colors
indicate poor drainage. Low-lying areas within the landscape
receive run-off water. Frequently, the water from these areas must
escape by lateral movement through the soil or by evaporation from
the surface, as poor structure and other physical influences do
not allow drainage through the soil. Continuous, cemented hardpan
layers such as caliche also greatly reduce the internal drainage
through a soil profile. |
Massive
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No definite structure or shape; usually hard. Found in
C horizons or compact transported material. |
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Too much or too little water in the soil is equally
undesirable. With too much water, most plant roots will suffocate
due to a lack of oxygen. Where there is too little water, plants
will wilt and eventually die. The most desirable soil moisture
situation is one in which approximately one-half of the pore space
of the soil is occupied by water. |
To test soil drainage, dig a hole the size of a
five-gallon bucket. Fill the hole with water. Let the soil absorb
the water for an hour or two. Then fill it again with water. The
hole should drain within 24 hours. If not, then plants should not
be planted in that location. Perhaps caliche is a problem or some
other physical barrier is present. |
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Compare How Much Water Different Soils Hold |
Objectives:
Students will be able to:
- determine that soils higher in organic matter have
higher water holding capacities,
- examine texture and structure influence water holding
capacity.
Activity:
You will need 2 cans of equal size (coffee cans will do); two 18-inch
squares of cloth; some heavy string; a package or similar
scale that weighs up to 64 ounces or 2,000 grams; and a
container of water, such as a 2 or 3 gallon bucket or a
5-quart oilcan with the top cut out; old fashioned lamp
chimneys or cylinders.
Put equal volumes of soil in the two cans. Take the soil for one from a
field or garden that has been cultivated for several years
and that shows lack of organic matter. This sample should be
hard and cloddy. Get the other from a well-managed field
where grasses and legumes have been grown, or from a good
pasture or similar location. This sample should be crumbly
and free from clods.
First allow the soils to dry. |
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Empty the two soil samples on the cloth squares, pull
the corners together, and tie with a heavy string. Weigh
each sample and record the weight.
Saturate each bag of soil by holding it in the water long enough to soak
thoroughly. Remove the soil samples from the water and allow
them to drain off the free water for a few minutes. Then
weigh again and record the weights. Calculate the difference
in weight.
Another way to measure the water-holding capacity of soils is to use two
old-fashioned lamp chimneys or cylinders. Tie a cloth over
the top, turn them upside down, and fill them about
two-thirds full with the same two soils. Be sure the soils
are both dry. |
Place the chimneys in small-mouth fruit jars, as shown
in the drawing. Pour a pint of water into each chimney. Then
note how long it takes the water to begin to drip into the
jars, how much water comes from each soil, and how long the
water continues to drip.
Extension: add sawdust or other organic matter to soil and run the test
again. Compare the results.
Questions:
- Which soil held more water?
- Which soil took longer for the water to infiltrate?
- Which soil would have more erosion?
- Which soil would plants grow in best?
- What did adding organic matter to the soil do?
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