Soil Texture and Soil Structure

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.
Source: http://soils.usda.gov/technical/manual/print_version/complete.html

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

To learn more about the textural triangle and textural classifications of soil, click on the North Carolina State University animation below:
http://courses.soil.ncsu.edu/resources/physics/texture/soiltexture.swf

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:
http://courses.soil.ncsu.edu/resources/physics/texture/soilgeo.swf

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:”
http://www.soils.wisc.edu/virtual_museum/silicates.html

• 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

Source: http://www.cst.cmich.edu/users/Franc1M/esc334/lectures/physical.htm

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:
http://www.aehsmag.com/issues/2002/june/soilclays.htm

To learn more detail about the structure of silicates clays, click on the next link from the University of Florida:
http://grunwald.ifas.ufl.edu/Nat_resources/silicates/silicates.htm