Fate of herbicide in soil
When herbicide are applied to the soil, only a small portion of herbicide is absorb by the plant. The rest is being lost through one or more of the following ways, viz. volatilization, leaching, photo – decomposition, chemical decomposition and microbial degradation.
Most of the herbicide as vapour is called volatilization. Volatilization is more in warm condition. It is also more from wet soils as adsorption by the clay complex is less. Herbicide like EPTC, Trifluralin, Fluchloralin are losses by this process. About 90% of Trifluralin applied to the soil surface in summer is losses within two to three days after application. The losses are reduced by incorporation of herbicide immediately after its application. When Trifluralin is incorporated in top 2.5 cm in the soil, the losses are about 22% in 120 days.
Adsorption is attachment of herbicide molecule to soil colloids and organic matter. It is more in soil and organic matter. Triazines and Ureas are more adsorb so that higher dose of herbicide should be applied in heavy and organic soil.
Leaching was downward movement of herbicide along with water through the soil. The extent of loss depends on solubility of herbicides. Leaching loss is more in light soil. Highly soluble herbicide like 2, 4 – D, Sodium salt, Dalaphon etc. have to be avoided in sandy soil.
4. Photo – decomposition:
Photodecomposition is breakdown of herbicides in the presence of light. It occur rapid rate when solar radiation is high. Photodecomposition herbicides are Trifluralin, Monuron, PCP etc. The herbicide should be applied in evening and incorporated immediately.
5. Chemical decomposition:
Herbicide are breakdown into simple non – toxic compound by several reaction namely hydrolysis, oxidation, hydroxylation, De – halozination etc.
6. 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. Micro – organism involved in herbicide detoxification includes bacteria, fungi, algae etc.
7. Soil residues:
The herbicide that is not affected by none of above process is left in the soil as soil residues. When herbicides are applied at recommended rates there is no possibility substantial accumulation in the soil. When heavy doses are used the accumulated in the soil.