Soil Chemistry, Ecology, and Properties

Posted on Nov 4, 2024 in Geology

Soil Chemistry and Physical-Chemical Properties

pH

Ion Exchange Capacity

Properties related to clay and silt in the mineral fraction, ion exchange capacity is one of the most important soil properties. It refers to the reversible exchange of cations and anions between minerals and high surface area organic particles with specific molecules and ions. The adsorption of ions is of great significance for:

  • Reactions in the soil (pH)
  • Nutrient supply
  • Structure of soil formation processes

Exchangeable ions are those that can be exchanged in a limited period by other ions from a standardized concentration and pH (ammonium acetate at pH 7.0, sodium acetate at pH 8.2, etc.). Exchangeable ions are those that are not locked in inaccessible sites within minerals or chemically linked.

Cation Exchange Capacity (CEC)

Organic and inorganic colloids contribute to the CEC. It is the total change of positions (negative charges) or the total quantity of adsorbed cations.

Origin of the Negative Charges:
  1. Isomorphic Substitution: pH-independent, generates a net negative charge by substituting a cation without compensation. For example, substituting Si4+ for Al3+ in the tetrahedral position, or substituting Mg2+ or Fe2+ for Si4+ in the tetrahedral position.
  2. Broken Edges or Faces Exposed: Depends on the pH; higher pH favors the dissociation of H+ from carboxyl and hydroxyl groups.
  3. Ionization of -OH and -COOH Groups of Organic and Inorganic Colloids: pH-dependent, very important in soils with high organic matter content, such as Histosols.
Exchange Mechanisms:

The exchange occurs on a solid surface (colloid) and liquid (soil solution). Cations are attracted to negatively charged sites of the colloid. The mechanism is reversible, always reaching a balance.

Anionic Exchange Capacity (AEC)

AEC is less significant in soil than CEC. Variable charge soils, such as Histosols, also tend to have a positive charge. There is a positive charge on clay minerals, hydroxides, and organic substances.

Origin of the Positive Charges (Low pH):
  • Protonation of clay minerals, Fe and Al hydroxides, and =NH and -NH2 groups of humic substances.
  • Dissociation and replacement of interchangeable OH ions from Al-OH groups on the broken edges of the minerals.

Percentage of Base Saturation

It expresses the portion of the CEC occupied by the bases of exchange. The bases that predominate in most soils are Ca2+ and Mg2+.

Soil Reaction

A very important property related to CEC, soil reaction is a feature (physiological) of the soil solution. It is measured by the activity of H+ in the solution and is expressed in units of pH. It indicates the acidity or basicity of the soil.

Acidity Causes:

  • Climate
  • Parent material
  • CO2 production by the activity of soil organisms
  • Production of H+ by plant roots
  • Humification of organic matter
  • Oxidation

Importance of pH:

  • Processes involved in soil formation and development
  • Availability and absorption of nutrients
  • Activity of soil organisms
  • Decomposition of organic matter

Soil acidity is caused by the predominance of H+ in solution (active acidity). It should be noted that H+ ions are also found adsorbed to soil colloids.

Buffer or Buffering Power of Soils

The capacity to resist changes in the pH of the soil solution. The buffering power varies between soils. Organic and inorganic colloids are important buffers.

Changes in soil pH are achieved by manipulating the ion exchange mechanism.

Salt-Affected Soils

Characterized in relation to:

  • Content of soluble salts
  • Exchangeable sodium percentage (ESP)
  • Sodium adsorption ratio (SAR)
  • Content of toxic elements (boron, chloride, sodium, etc.)

Salinity is one of the most serious limitations associated with arid and semiarid regions. In soils with salinity problems, crops have different tolerances to salinity.

Effects of Salinity:

  • Poor germination
  • Restricted plant growth, limited by water availability due to excess dissolved ions
  • Possible problems of toxicity (boron, sodium, chloride)

Origin of Salts in the Soil:

  • Accumulation when introduced by irrigation water
  • Salt-rich parent materials
  • Arid climate where leaching is limited

Effects of Na:

  • Cultures differ in their sensitivity to the Na content of the soil
  • Induces undesirable chemical and nutritional conditions
  • Na may be the dominant ion in soil solution due to the nature of the soil

Soil Ecology

A fertile soil is a biological entity where organic processes take place. Organic matter is a mixture of:

  1. Micro and small animals and decomposition debris
  2. Products of decaying plants and animals
  3. Substances synthesized biologically and/or chemically

Organic Matter

Consists of humic and non-humic substances.

General Importance of Soil Fauna

  • Maceration and grinding of plant debris
  • Distribution of these plant residues
  • Contribution to aeration and drainage

Importance of Earthworms

  • Determine the decomposition of organic debris
  • Perform a plowing effect that improves soil aeration and drainage
  • Improve and stabilize soil structure

Microflora of Soil

Never occupy more than 0.0001% of the available volume. Their distribution is heterogeneous.

General Importance of Microflora

  • Decomposition of soil organic remains
  • Humification
  • Mineralization of organic matter
  • Association with higher plants

Synthesis of Humic Substances

The microflora is directly responsible for the process of transforming organic substances.

Elemental Composition of Humic Substances

  • Carbon (C) = 45-65%
  • Oxygen (O) = 30-48%
  • Nitrogen (N) = 2-6%
  • Hydrogen (H) = 5% (approx.)

Organic Matter Content of Soils

Varies according to climate, vegetation, topography, soil parent material, soil age, and human management actions.

Effects on Chemical Properties

  • Increased cation exchange capacity
  • Resistance to pH changes

Effect on Biological Properties

  • Allows the development of soil organisms

Loss of N

  • Immobilization by microorganisms
  • Erosion
  • Burning
  • Biological processes (denitrification)

Association Between Soil Microorganisms and Higher Plants

1. Rhizosphere Association

The rhizosphere is the zone of soil influenced by root exudates of plants. An example is the association of bacteria of the genus Azospirillum with grasses.

2. Mycorrhiza Associations

Mycorrhizae (mykes = fungus, rhiza = root) are associations between fungi and roots. There are ectomycorrhizal and endomycorrhizal associations. Ectoendomycorrhizae have intermediate properties. All three types mainly improve phosphate nutrition.

3. Rhizobium-Legume Symbiosis

This plant association allows legumes to:

  • Be self-sufficient in nitrogen
  • Provide nitrogen to other crops (mixed pastures of grasses and legumes)
  • Leave nitrogen in the soil for subsequent crops in the rotation