Soil
Texture and Soil Structure
Figure 15. Textural Triangle. The textural triangle describes the relative proportions of sand, silt and clay in various types of soils.
The major textural classes for the soils of Maui are provided in Table 3. Each of the textural classes listed in Table 3 make up finely textured soils. As you can see, soil surveys show that more than 90% of Maui’s soils are finely textured. This is largely due to the type of parent material of most Hawaii soils, which is basalt. Since basalt is a finely textured rock, it weathers into finely textured soils. The relative amount of clay has great importance in the soil.
Soil texture and soil structure are both unique properties of the soil that will have a profound
effect on the behavior of soils, such as water holding capacity, nutrient
retention and supply, drainage, and nutrient leaching.
In
soil fertility, coarser soils generally have a lesser ability to hold and
retain nutrients than finer soils. However, this ability is reduced as
finely-textured soils undergo intense leaching in moist environments.
Soil Texture
Soil
texture has an important role in nutrient management because it influences
nutrient retention. For instance, finer textured soils tend to have greater
ability to store soil nutrients.
In
our discussion on soil mineral composition, we mentioned that the mineral
particles of a soil are present in a wide range of size. Recall that the fine
earth fraction includes all soil particles that are less than 2 mm. Soil
particles within this fraction are further divided into the 3 separate size
classes, which includes sand, silt, and clay. The size of sand particles range
between 2.0 and 0.05 mm; silt, 0.05 mm and 0.002 mm; and clay, less than 0.002
mm. Notice that clay particles may be over one thousand times smaller than sand
particles. This difference in size is largely due to the type of parent
material and the degree of weathering. Sand particles are generally primary
minerals that have not undergone much weathering. On the other hand, clay
particles are secondary minerals that are the products of the weathering of
primary minerals. As weathering continues, the soil particles break down and
become smaller and smaller.
TEXTURAL
TRIANGLE
Soil texture is the relative proportions of sand, silt, or clay in a soil. The soil textural
class is a grouping of soils
based upon these relative proportions. Soils with the finest texture are called
clay soils, while soils with the coarsest texture are called sands. However, a
soil that has a relatively even mixture of sand, silt, and clay and exhibits
the properties from each separate is called a loam. There are different types
of loams, based upon which soil separate is most abundantly present. If the
percentages of clay, silt, and sand in a soil are known (primarily through
laboratory analysis), you may use the textural triangle to determine the texture
class of your soil.
Figure 15. Textural Triangle. The textural triangle describes the relative proportions of sand, silt and clay in various types of soils.
The major textural classes for the soils of Maui are provided in Table 3. Each of the textural classes listed in Table 3 make up finely textured soils. As you can see, soil surveys show that more than 90% of Maui’s soils are finely textured. This is largely due to the type of parent material of most Hawaii soils, which is basalt. Since basalt is a finely textured rock, it weathers into finely textured soils. The relative amount of clay has great importance in the soil.
Table 3. Major textural classes of Maui soils
Textural Class
|
Percentage of Maui soils that fall within the major textural
classes
|
Silty clay
|
44%
|
Silty clay loam
|
23%
|
Silty loam
|
11%
|
Loam
|
10%
|
Clay
|
5%
|
To
learn more about the textural triangle and textural classifications of soil,
click on the North Carolina State University animation below:
Importance of Clay and
Other Particles of Similar Size
Clay
particles, as well as other particles of similar size, are important components
of a soil. There is a fundamental difference between soils that contain large
amounts of sand particles and soils that contain large amounts of very small
particles, such as clay. That difference is surface area. The total surface
area of a given mass of clay is more than a thousand times the total surface
area of sand particles with the same mass. To put this idea into perspective,
imagine a single cube with 6 sides. This cube represents a sand particle. Now,
imagine that you break this single cube up into 100 smaller cubes, which
represent 100 clay particles. These 100 cubes each have 6 sides. Essentially,
by breaking up the larger cube, you have exposed many more surfaces. Thus, the
total surface area of the smaller cubes will be much greater than the surface
area of the single cube.
To
explore this concept further, view a brief animation by clicking the following
link to North Carolina State University:
This
increase in surface area has an important implication in nutrient management
because it provides many places for soil particles to retain and supply
nutrients (such as calcium, potassium, magnesium, phosphate) and water for
plant uptake
Types of Very Small Particles within the Soil
- The most common clay minerals
in Maui’s soil are called layered
silicate clays, or phyllosilicates. There are different types of layered silicates, such
as kaolinite, halloysite, montmorillonite, and vermiculite. The various
types of layered silicates differ greatly, as we will discuss later.
For
more details about the various layered silicate clay minerals, click on the
link below and scroll down to the “Phyllosilicate Room:”
Amorphous minerals, such as allophane, imogolite, and ferrihydride, may be found in the volcanic soils of Hawaii that developed from volcanic ash. Like silicate clays, these minerals have a very high surface area. As a result, soils with amorphous minerals hold large amounts of water and stored nutrients, depending on the degree of weathering.
Amorphous minerals, such as allophane, imogolite, and ferrihydride, may be found in the volcanic soils of Hawaii that developed from volcanic ash. Like silicate clays, these minerals have a very high surface area. As a result, soils with amorphous minerals hold large amounts of water and stored nutrients, depending on the degree of weathering.
- Aluminum
and iron oxides are typically found in the highly-weathered soils of
the tropics. As clay minerals are intensely weathered, the structure of
silicates clays change. Particularly, the silicate clays lose silica. What
remains in the soil are aluminum and iron oxides. Gibbsite is an example
of an aluminum oxide, which has a grayish, whitish hue. Goethite is an
example of an iron oxide, which imparts a reddish color to the soil.
Properties of oxides
- Oxides are fairly stable and
resistant to further weathering.
- Oxides can act like a glue and
hold other soil particles together.
- Oxides can tie up nutrients,
such as phosphorus.
- Oxides have a high anion
exchange capacity (AEC).
- Humus is the portion of organic matter that is mostly
resistant to decomposition and remains in the soil. Humus is composed of
small particles, with tremendous surface area. These particles have a very
great capacity to retain and supply nutrients, as well as hold water.
Soil Structure
Soil
structure is the arrangement of soil particles into groupings. These groupings
are called peds or aggregates, which often form distinctive shapes typically
found within certain soil horizons. For example, granular soil particles are
characteristic of the surface horizon.
Soil
aggregation is an important indicator of the workability of the soil. Soils
that are well aggregated are said to have “good soil tilth.” The various types
of soil structures are provided in Table 4.
Table 4. Types of Soil Structures in Soils
SOIL
AGGREGATES
Generally,
only the very small particles form aggregates, which includes silicate clays,
volcanic ash minerals, organic matter, and oxides. There are various mechanisms
of soil aggregation.
Mechanisms of soil
aggregation
- Soil microorganisms excrete
substances that act as cementing agents and bind soil particles together.
- Fungi have filaments, called
hyphae, which extend into the soil and tie soil particles together.
- Roots also excrete sugars into
the soil that help bind minerals.
- Oxides also act as glue and
join particles together. This aggregation process is very common to many
highly weathered tropical soils and is especially prevalent in Hawaii.
- Finally, soil particles may
naturally be attracted one another through electrostatic forces, much like
the attraction between hair and a balloon.
Aggregate Stability
Stable
soil aggregation is a very valuable property of productive soils. Yet, the
stability of soil aggregation is very reliant on the type of minerals present
in the soil. Certain clay minerals form very stable aggregates, while other
clay minerals form weak aggregates that fall apart very easily.
- Highly weathered silicate
clays, oxides, and amorphous volcanic materials tend to form the most
stable aggregates. The presence of organic matter with these materials
improves stable aggregate formation. In nutrient management, the aggregate
stability is important because well-aggregated minerals are well drained
and quite workable.
- In contrast, less weathered
silicate clays, such as montmorillonite, form weak aggregates. Some
silicate clays are said to have a shrink-swell potential. This means that
the soil minerals expand, or swell, when wet, causing the soil to become
sticky and drain poorly. When dry, these soils shrink and form cracks. The
make-up of the lattice structure of silicate clays determines the
shrink-swell potential. Although there are no soils with a shrink-swell
potential in Maui, these soils may be found on Molokai.
For
a simple discussion of the chemistry of soil clays, click on the following
link:
To
learn more detail about the structure of silicates clays, click on the next
link from the University of Florida:
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