Many herbicides are applied directly to the soil with the hope that these will control weeds for a desired period and then dissipate to leave the soil uncontaminated. Herbicides which are applied to the standing weeds also eventually reach the soil as run-off from the foliage as well as during the decomposition of the dead weeds.
In either case, a study of the mode of dissipation of herbicides in soils is very important. Had there been no herbicide dissipation form the soil, their every year use in agriculture would have been impossible since then it would have led to their gradual accumulation, resulting in permanent damage to our precious natural resource – the soil.
There are two ways by which herbicides dissipate from the root zone of the soils. These are:
1. Transfer
2. Decomposition
a. Microbial decomposition
b. Chemical decomposition
c. Photodecomposition
Way # 1. Transfer of Herbicides:
The transfer of herbicides is their induced absence in active form from the root zone of the crop, without change in their molecular moiety. This can happen in four important ways- First, a herbicide may be subjected in soil to irreversible adsorption on the colloidal particles. Paraquat and diquat dissipate from soils in this fashion, almost spontaneously. Second, herbicides may gradually leach below the root zone of crops with downward movement of water.
The intensity of leaching of a herbicide is governed by its molecular structure which determines its solubility in water, soil texture and structure, and the presence and rate of downward movement of free water. A light texture soil permits faster leaching of the herbicide than a heavy texture one.
Similarly, with the same amount of water, its slow rate of downward movement shall induce more herbicide leaching than its fast movement since in the former event there is longer herbicide-water contact time in the latter case. Third, a herbicide may be subjected to run-off losses, and/or voltalisation into the atmosphere.
Voltalisation as a mode of dissipation is a feature of those herbicides which have high vapour pressures like fluchloralin and EPTC. Fourth, the herbicides are invariably absorbed by the weeds and the crop plants which keeps them away, at least temporarily, from the scene.
Way # 2. Decomposition of Herbicides:
Decomposition of herbicides involves changes in their moeity by their molecular disintegration to non-phytotoxic constituents. This is a permanent source of herbicide dissipation and is, therefore, considered more important than the transfer of herbicides.
a. Microbial Decomposition:
Recent findings have shown that microbial decomposition is a major mechanism of herbicide dissipation in soils in nature. Herbicides having polar groups, like-CH, -COO, -NH2, and NO2 succumb easily to their microbial decomposition, e.g. 2,4-D, diuron, EPTC, atrazine, etc. Specific microbes have now been identified which biodegrade particular herbicides.
Repeat applications of a herbicide to the same soil helps in building up their population, which in turn biodegrades the subsequent doses of the herbicide in question, much faster. Biodegradation of herbicides is characterised by its initial slow rate.
During this phase, called the lag phase, the specific microbial population builds up itself. After this, there is a rapid increase in the activity of herbicide degradation till there is lack of the substrate. If a soil is sterilized for experimentation completely to the point of zero microbe, no herbicide biodegradation shall take place in such a medium.
2,4-D, EPTC, diuron, and dalapon are typical examples of biodegradable herbicides. To cite one specific example, 2,4-D is biodegraded in soils by 14 species of bacteria, two of actinomycetes, and one fungus (Aspergillus niger). The end products of its sequential degradation are CO2, Cl, and H2O.
The ester formulations of 2,4-D take longer time for their biodegradation than its amine and sodium salts. Diuron and the like substituted urea herbicides are attacked by several species of bacteria and, at least, two fungi. CO2, NH3, and CI are the end products.
b. Chemical Degradation:
Many herbicides are prone to chemical degradation in soil. In variance with their microbial decomposition, the chemical decomposition of herbicides starts immediately and continues at a steady rate till the availability of the reactant. In other words, it is free of any lag phase.
The extent of chemical degradation of herbicide depends mainly upon temperature, pH, moisture, and aeration status of the soil. In respect of temperature it is governed by the first order reaction, whereby the herbicide degradation is faster in areas of high temperature than in places with cold climate.
Simazine and trifluralin are typical, and well studied examples of chemically decomposed herbicides in soils. In the case of simazine, first the hydroxylation of the benzene ring displaces CI from the moeity making it non-phytotoxic. The reaction is more rapid at pH less than 7.0 than at higher pH.
It is important to point out here that many herbicides are subjected to both biodegradation and chemical decomposition. For instance, simazine, besides its above cited chemical hydroxylation, is dealkylated and deaminated by a fungus A. fumigatus. For many new herbicides such mechanisms are not yet known.
c. Photodecomposition:
Solar energy is able to energize certain herbicide molecules to the extent of their decomposition to specific non-phytotoxic components. The process is called photodecomposition or photolysis. Ultra voilet rays of solar spectrum are particularly active in this respect. Herbicides like trifluralin, nitralin, and Fluchloralin are known to undergo photolysis. Such herbicides are usually incorporated in the soil immediately after their application to save their photolysis losses.