Soil-herbicide chemical degradation

The soil-herbicde chemical decomposition are those processes that are not influenced by microbes.  Chemical breakdown processes are

• Oxidation
• Reduction
• Hydrolysis

Herbicides that are applied in wet soils with low oxygen will not be degraded through oxidation since those soils have low oxygen levels.

Physical processes of herbicide degradation

Soil-herbicide physical decomposition are processes such as:

• Volatilization:  Many herbicides are so volatile that that will quickly evaporate if they are not incorporated into the soil.
• Photo-degradation: Light coming from the sun has large amounts of energy which in turn excites the electrons of the herbicide and that causes the breakage of chemical bonds. Herbicides that are sensitive to light must be incorporated into the soil directly after application.
• Degradation through water:  Water provides H⁺ and OH^– ions which are used during the degradation process.
• Soil clay colloids have a significant effect on the degradation of herbicides.  Not only do they degrade the active ingredient but they can also absorb certain chemicals and release them at a later stage.  This has advantages and disadvantages.  The advantage is long term control of a weed but it can also affect the commercial crop at a later stage when the active ingredient is released.
• Leaching of herbicides:  Herbicides that have a negative charge may not be bound to the negatively charged clay colloids.  These herbicides are leached into the underground with irrigation water or rain and will have no effect on the land.  The disadvantage are:
  •  Chemicals might influence other life forms such as fish, birds and larger farm animals which come in contact with drinking water.
  • Poor weed control due to partial removal of the herbicide
  • Commercial crop injury to the following crop

However there are some advantages in leaching of herbicides.

• Leaching improves the spread of the herbicide
• It increases the chance of reaching other plants
• It reduces herbicide residues from the soil which will affect the following commercial crop.

Leaching is determined by

• Soil texture
• The permeability of the soil
• Volume of the water applied
• The water solubility of the herbicide
• The strength with which the herbicide is bound to the clay particles.

Biological degradation of herbicides in the soil

Biological degradation of herbicides is becoming more and more important method of preventing the spread and accumulation of herbicides in ground waters and soil systems.  Biological degradation consist of microbial degradation and degradation by plants.

The soil microbe system consist of many different types of algae, fungi, actinomycetes and bacteria.  These microscopic organisms have to feed on organic material found in the soil.  They cannot distinguish between natural organic material and the organic compounds which came from herbicides.

If a herbicide is applied to the soil, the microorganisms immediately start to absorb the chemicals which is the first process in the degradation.  This implies that if a herbicide is applied to a soil, the a high concentration of a specific organism which is attracted to it, the herbicide will no longer be effective for long periods.

There are various factors that determine the rate of decomposition of a herbicide in the soil, these are:

• The amount of water in the soil
• Temperature of the soil (also determined by the amount of water in the soil and the colour and texture of the soil).  Low temperatures decrease microbial activity and slows down degradation of the herbicide.
• Oxygen levels in the soil.  Soil micro-organisms are sensitive to the oxygen levels in the soil.  Badly aerated soils tend to have less micro-organisms than well aerated soils.
• The amount of minerals, nutrients and other organic compounds on which the organisms can feed on.
• The pH of the soil has a significant influence on the amount of micro-organisms.  Most bacteria and fungi flourish between pH 5.5 and 6.8.

Some of the microbes that help in the degradation of herbicides are:

• Agrobacterium
• Arthrobacter spp.
• Aspergillus spp.
• Bacillus spp.
• Clonostachys spp.
• Fusarium spp.
• Geotrichum spp.
• Nocardia spp.
• Paecilomyces spp.
• Penicillium spp.
• Pseudomonas spp.
• Trichoderma spp.

Biochemical mechanisms of herbicide degradation

Many of the soil-herbicides are degraded in plants through the following processes:

• Oxidation-reduction:  Release of energy through electron removal and addition of oxygen or the removal of hydrogen.  Reduction process is the opposite of oxidation.
• Beta-oxidation:  The process of cleaving long-chain alkyl group.  The process removes two-carbon fragments from alkyl groups until only acetate or formate residue remains
• Hydrolysis:  The process of adding a hydrogen (H⁺) and hydroxyl (OH^–) group to either of the two molecules taking part.
• Hydroxylation: The process of substituting a hydroxyl (OH) group from on molecule for another atom or group of atoms on another molecule or compound.
• Dehalogenation:  The removal of halogens such as F, Cl, Br and I, from a molecule.
• Dealkylation:  The process of cleaving alkyl groups from larger compounds.
• Conjugation:  The process of combining two compounds in to one thus forming a total different compound.