Soil Mineralogy

Soil mineralogy is closely related to soil fertility. Differences in soil mineralogy cause great differences in soil fertility. For instance, moderately weathered clays attract and retain greater amounts of nutrients than highly weathered clays and oxides. Though volcanic ash inherently has a low ability to hold nutrients, it interacts with organic matter to produce very fertile soils. Knowledge of mineralogy helps to determine the appropriate nutrient management strategy for your soil.

In our discussion on soil texture and structure, we mentioned that the very small particles form aggregates. On Maui, the major groups of small particles include silicate clays, organic matter, volcanic ash minerals, and oxides. In fact, this grouping of small particles is also used to describe the distinct categories of soil mineralogy (types of clay minerals).

Soil mineralogy:

• Layered silicate clays
• High activity
• Low activity
• Organic matter
• Volcanic ash material
• Oxides

Soil behavior is greatly influenced by these types of clay minerals and/or the amount of organic matter that a particular soil type contains.

Before discussing differences in soil mineralogy, it is very helpful to understand the concepts of cation and anion exchange capacities (CEC and AEC). CEC and AEC are properties that can help differentiate soil minerals.

Cation Exchange Capacity (CEC) and Anion Exchange Capacity (AEC)

CEC and Less Weathered Soils of Hawaii

Less weathered soils, that contain minerals such as montmorillonite, are said to have a ‘cation exchange capacity,’ or CEC, under acidic, neutral, and alkaline conditions. CEC is the soil’s ability to attract, retain, and supply nutrients, such as calcium, potassium, ammonium, and magnesium. These nutrients are positively charged atoms, known as cations. The surfaces of less-weathered clay minerals (such as montmorrillonite) generally have a negative charged. Much like a magnet, negatively charged soil surfaces attract positively charged cations. However, under acidic conditions, the soil will also have a tendency to attract aluminum and hydrogen cations. The presence of aluminum and hydrogen contributes to soil acidity. In contrast, under alkaline conditions, the soil attracts sodium which contributes to soil alkalinity.

Less-weathered soils that are founding in Maui County include Vertisols, Mollisols, Aridisols, and Inceptisols.

AEC and Highly Weathered Soils of Hawaii

Although the majority of the world’s soils have CEC, the highly weathered soils of the tropics are an exception. In addition to the having CEC, many tropical soils also have an ‘anion exchange capacity,’ or AEC, depending upon the pH of the soil. Under neutral and alkaline conditions, the soil has CEC, like the less weathered soils. However, under acidic conditions, these soils generate AEC. This means that the soil becomes positively charged and attracts, retains, and supplies negatively charged anions, such as sulfate, phosphate, nitrate, and chloride. For soils with AEC, proper management of pH is crucial in order to provide sufficient amounts of the nutrient cations (calcium, magnesium, ammonium, and potassium).

Minerals that exhibit AEC are highly weathered kaolinite, aluminum and iron oxides, organic matter, and the allophanes and imogolites of volcanic soils. Highly weathered Ultisols and Oxisols, volcanic Andisols, and organic Histosols all have AEC under acidic conditions. The pH at which these soils develop AEC differs depending upon the minerals within the soil. Since organic matter only generates AEC at a very low pH, it is still a good source of CEC.

Importance of CEC and AEC

CEC and AEC values are important measurements that provide us with important information regarding the soil’s ability to retain and supply certain nutrients to the plant. In addition to nutrient retention, CEC and AEC helps us predict the leaching potential of certain nutrients in areas with high rainfall. When the soil has a very high CEC, negatively charged nutrients such as nitrate are not be retained by the soil. Instead, nitrate leaches through the soil profile in areas with high amounts of precipitation. Likewise, soils with high AEC experience leaching of positively charged nutrients, such as calcium and potassium.

CEC is often expressed in centimoles of charge per kg of soil. By sending a sample of your soil to a soil testing laboratory, you can determine your soil’s CEC. The value of knowing a soil’s CEC cannot be underestimated in nutrient management. Without it, your soil would not be able to provide your plants with sufficient amounts of nutrients. However, one must be cautious when interpreting the laboratory results for CEC depending on the soil type being analyzed. You can obtain accurate results for most laboratory methods that measure CEC in less weathered soils with permanently negative charge. On the other hand, these laboratory methods can overestimate the CEC of highly weathered soils that have an AEC. This is because the pH of highly weathered soils affects the CEC of the soil. As the pH of highly weathered soils with AEC increases, the CEC of the soil also increases. Thus when the method used to determine the CEC uses solutions that raise the pH of the soil, the reported CEC is higher than its actual value in the field.

Layered Silicate clays

Layered silicate clays are secondary minerals that have formed as the result of weathering of parent material. There are two major categories of layered silicate clays within the soil: high activity clays and low activity clays.

High activity clays

Generally, soils with large amounts of high activity clays are not highly weathered. High activity clays have a high ‘cation exchange capacity’ (CEC), due to their large surface area. This means that these clays have a great capacity to retain and supply large quantities of nutrients, such as calcium, magnesium, potassium, and ammonium. Not only do these clays have a large CEC, but they will generate CEC under all soil conditions regardless of soil pH. As a result, these clays tend to produce highly fertile soils. Examples of these clays are montmorillonite (and other smectites), vermiculite, illite, and mica.

Cation Exchange Capacity or Anion Exchange Capacity?

• High activity clays have a cation exchange capacity (CEC). Although high activity clays will not have anion exchange capacity (AEC), the CEC increases as pH increases and decreases as pH decreases.

Montmorillonite

Some high activity clays, such as montmorrillonite, have a shrink and swell potential. This means that the clays will shrink and crack when dry, and expand and swell when wet. With little additions of nutrients, these soils may be very productive. However, the shrink and swell potential will result in poorer drainage. And so, proper management of irrigation is required.

Low activity clays

In contrast, low activity clays are more highly weathered. Thus, due their lesser surface area, low activity clays have a lower capacity to retain and supply nutrients. In addition to CEC, low activities clays can also have AEC, depending upon the pH of the soil. The AEC causes these clays to retain and supply nutrients, such as phosphate, sulfate, and nitrate, rather than the base cations, under acidic conditions. Yet, under neutral and alkaline conditions, these low activity clays generate a CEC.

Cation Exchange Capacity or Anion Exchange Capacity?

• Under acidic conditions, low activity clays have an AEC
• Under neutral and alkaline conditions, low activity clays have a CEC

Management of pH

In order to provide adequate amounts of the base cations, proper management of pH is crucial. If soil pH is low, additions of lime and/or organic matter may increase the CEC for soils high in low activity clays.

Low activity clays have a low shrink and swell potential. With additions of nutrients, these soils may be very productive soils.

Table 5. CEC and surface area of common soil minerals

* Positive sign indicates that the minerals no longer exhibit a cation exchange capacity, but rather an anion exchange capacity.
Adapted from Table 8.1, Brady and Weil (2002).

Organic matter

Most soil organic matter accumulates within the surface layer of the soil. This organic matter may be divided into two groups: non-humic matter and humic matter.

Non-humic matter includes all undecomposed organic material within the soil. Examples of non-humic matter are twigs, roots, and living organisms.

Humic matter includes humic acids, fulvic acids, and humin. (Humin is the dark material in soil that is highly resistant to decomposition.)

Importance of soil organic matter

• Due to its tremendous surface area, soil organic matter:

• Acts like a sponge to store water
• Retains and provides nutrients (CEC)
• Glues and binds soil particles into stable aggregates

• Reduces the occurrence of aluminum toxicities.

Like low activity clays, organic matter may have either CEC or AEC, depending upon soil pH. However, it will rarely have AEC. In fact, the pH must fall to approximately 2.0 before it will have AEC.

Cation Exchange Capacity or Anion Exchange Capacity?

• Soil organic matter may have both AEC and CEC. However, the charges on organic matter are dependent upon soil pH. For soil organic matter to generate an AEC, the soil pH must be 2.0.

Management

Without additions of organic matter, tillage practices will greatly reduce organic matter content in the soil. And so, no-till and minimum tillage systems with the return of organic matter to the soil are gaining favor by farmers to improve and conserve soil quality.

Volcanic materials

Volcanic soils are developed from volcanic materials, which have a glassy, or non-crystalline, structure. As a result, volcanic soils largely consist of amorphous materials that lack a crystalline structure.

Three types of identifies amorphous minerals in volcanic soils:

• Allophone
• Imogolite
• Ferrihydride

On Maui, most volcanic soils consist of materials that are indistinguishable.

Amorphous materials generally have a great surface area; and so, they may absorb a lot of water. However, due to its high AEC, amorphous materials have a low ability to retain and supply nutrients under acidic conditions.

Cation Exchange Capacity or Anion Exchange Capacity?

Amorphous materials have a high AEC under acidic conditions.

Management

The ability to supply nutrients can be improved in soils containing amorphous materials through the addition of organic matter. Since organic matter has a large CEC under most soil conditions, additions of organic matter enhances the CEC of the soil.

Oxides

There are many different types of oxides in highly weathered tropical soils.

Three basic types of oxides in Hawaii:

• Aluminum (e.g. Gibbsite)
• Iron (e.g. Hematite, Goethite, Ferrihydrite)
• Titanium (e.g. Anatase)

Oxides generate a greater AEC than all other soil constituents. Therefore, soils which contain a lot of oxides will often have an ‘anion exchange capacity.’

Cation Exchange Capacity or Anion Exchange Capacity?

• Oxides have a very great AEC. The AEC may possibly develop even under neutral and slightly alkaline conditions.

Management

Organic matter may be added to increase CEC of oxidic soils. The following web site from North Carolina State University is a simple animation which shows how primary minerals eventually weather into oxide materials:
http://courses.soil.ncsu.edu/resources/soil_classification_genesis/mineral_weathering/mineral_change.swf

Soil Type and Mineralogy

In Hawaii, there are a variety of soils with very diverse mineralogy. Moderately weathered shrink-swell soils (Vertisols), highly fertile grassland soils (Mollisols), volcanic soils (Andisols), organic soils (Histosols), and highly weathered soils (Ultisols and Oxisols) are types of soils in Hawaii that behave differently due to differences in mineralogy.

• A soil that is moderately weathered and has a shrink-swell potential contains montmorillonite. As a result, these soils crack when dry, and swell when wet. These soils are generally fertile, but carefully managed irrigation may be required.

• Moderately weathered, grassland soils are generally highly fertile soils. Naturally high in organic matter, these soils are well suited for agricultural use.

• A highly weathered soil, which may be found in the Lahaina area, is contains kaolinite and oxides. This soil forms very stable aggregates and drains well. This makes the soil workable, although nutrient additions may be necessary.

• Organic soils, located in the uplands of Maui, are organic soils that primarily consist of partially decomposed organic matter. Since organic matter has a tremendous water holding capacity, these soils generally hold a lot of water. However, these soils have poor drainage and poor aeration. Organic soils are not widely distributed on Maui, and most are not under cultivation.

• Volcanic soils have developed from volcanic ash or cinders. These soils contain amorphous minerals, such as allophone and imogolite, which bind strongly with organic matter and iron and aluminum oxides. Although volcanic soils have a great water holding capacity like organic soils, they are well-aggregated, resist erosion and have good drainage like well aggregated Oxisols.