The fate of herbicide in soil: When herbicides are applied to the soil, only a small portion of herbicide is absorbed by the plant. The rest is being lost through one or more of the following ways, viz. volatilization, leaching, photodecomposition, chemical decomposition, and microbial degradation.

The fate of herbicide in soil

1. Volatilization
2. Adsorption
3. Leaching
4. Photo – decomposition
5. Chemical decomposition
6. Microbial degradation
7. Soil residues

Volatilization:
 
Most of the herbicide as vapor is called volatilization. Volatilization is more in warm condition. It is also more from wet soils as adsorption by the clay complex is less. Herbicides like EPTC, Trifluralin, Fluchloralin are lost by this process. About 90% of Trifluralin applied to the soil surface in summer is lost within two to three days after application. The losses are reduced by the incorporation of herbicide immediately after its application. When Trifluralin is incorporated in the top 2.5 cm in the soil, the losses are about 22% in 120 days.

Adsorption:
 
Adsorption is the attachment of the herbicide molecule to soil colloids and organic matter. It is more in soil and organic matter. Triazines and Ureas are more adsorb so a higher dose of herbicide should be applied in heavy and organic soil.

Leaching:

Leaching was the downward movement of herbicide along with water through the soil. The extent of loss depends on the solubility of herbicides. Leaching loss is more in light soil. Highly soluble herbicides like 2, 4 – D, Sodium salt, Dalaphon, etc. have to be avoided in sandy soil.

Photo – decomposition:
 
Photodecomposition is the breakdown of herbicides in the presence of light. It occurs at a rapid rate when solar radiation is high. Photodecomposition herbicides are Trifluralin, Monuron, PCP, etc. The herbicide should be applied in the evening and incorporated immediately.

Chemical decomposition:
 
Herbicides are breakdown into simple non – toxic compounds by several reactions namely hydrolysis, oxidation, hydroxylation, De – halozination, etc.

Microbial degradation:
 
Soil-applied herbicides are attached by the microbes and used as a source of energy. Herbicides are broken down into simple products by this process. Microbial degradation depends on the type of microbes present in the soil and their population. Microorganism involved in herbicide detoxification includes bacteria, fungi, algae, etc.

Soil residues:
 
The herbicide that is not affected by any of the above processes is left in the soil as soil residues. When herbicides are applied at recommended rates there is no possibility of substantial accumulation in the soil. When heavy doses are used they accumulated in the soil.