The herbicides after application undergo various reactions in the soil and environment. Of the total herbicide applied to a soil, only a small quantity is actually utilized in controlling weeds and the remaining quantity is diluted by soil and is subjected to transfer and decomposition processes.
The fate of herbicides in soils determines (i) their availability in soils for immediate as well as sustained weed control, (ii) selectivity to crops, and (iii) herbicide carry over from one season to another. Rapid loss of herbicides from soil will cause ineffective weed control. The fate of toxicants in soils is influenced by herbicide structure, soil conditions, and herbicide application methods.
The fate of herbicides in soils pertains to their interaction with the environment. It is governed by the following factors:
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Microbial degradation
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Chemical decomposition
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Adsorption
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Leaching
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Volatility
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Photodecomposition and
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Plant uptake
1.Microbial degradation
Organic compounds of the soil nourish microbes for their growth, except for few, which feed on inorganic sources. The microorganisms can decompose organic herbicides rapidly under a warm, moist, well aerated and fertile soil conditions. Some of the applied herbicides are utilized by microbes, whereas the others resist decomposition. The herbicides are beneficial for one group of microbes and injure another group. Grossbard (l972) observed that several herbicides of the concentrations equivalent to field doses had resulted in a severe depression of microbial activities.
2.Chemical decomposition
This involves oxidation, reduction, decarboxylation, hydroxylation, hydrolysis, dealkylation, conjugation and ring cleavage and these reactions may destroy some herbicides or activate others.
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Phenoxy, benzoic and triazine groups of herbicides are decomposed by hydroxylation.
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Phenoxy esters, carbamates, triazines and ureas groups of herbicides are made inactive by hydrolysis.
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The toxicity of herbicides is altered by dealkylation for dinitroanilines, carbamates, ureas and triazines groups of herbicides.
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Majority of the herbicides is decomposed by conjugation in which the herbicides conjugate with indigenous plant components such as sugars, amino acids or soil minerals.
3.Adsorption by clay colloids
Herbicides are made unavailable for uptake by plants and microorganisms due to adsorption, which determines the activity of herbicides. The adsorption of herbicides occurs on the clay and humus fractions of soil and the adsorption is more in humus than clay. The herbicides, which are adsorbed on clay, are readily displaced by ions, similar molecules or stronger charge and the herbicides adsorbed on humus are not readily displaced.
4.Leaching
The movement of herbicides in soil under the influence of water flow is called as leaching, which is influenced by soil texture, soil permeability, volume of water flow, adsorption of herbicides and solubility of herbicides. This is desirable in case of soil incorporation of herbicides and reduction and removal of herbicide residues from soils.
5.Volatility
Herbicides in the soil solution are carried to the soil surface by water movement and are lost to the atmosphere by volatilization. Eg. Chlorpropham, diallate, EPTC and trifluralin. The loss of herbicides through volatilization may be reduced by soil incorporation.
6.Photo decomposition
Some organic herbicides on exposure to sunlight undergo molecular alterations leading to deactivation which is due to photochemical reaction. Trifluralin, substituted ureas and S-triazines group of herbicides are susceptible to photodecomposition. Incorporation of herbicides in the soil prevents photodecomposition.
7.Plant uptake
Herbicides are absorbed and metabolized by plants and they return to soil, either in the original form or as metabolites, through crop and weed residues and root exudation.
Management of herbicide residues in soil
1.Use of recommended dose of herbicide
The germination, growth and development of succeeding crops are not affected by the herbicides applied to the preceding crop when they are applied at the recommended level. The total quantity of herbicides required for application to a particular crop can be reduced by the selection of proper application technology. The band application of herbicides requires lesser quantity of herbicides when compared to broadcast application which help minimizing the hazards of herbicide residues.
2.Application of farm yard manure
The activity of microbes is enhanced by the application of organic sources of nutrients viz., FYM, compost, crop residues etc. as they provide sufficient food materials for their growth and physiological activity. The herbicides are adsorbed by colloidal fraction of FYM and made unavailable for crops and weeds on application. The applied herbicides are also degraded by the enhanced microbial activity, which is favoured by the application of FYM.
3.Tillage
The concentration of applied herbicides is diluted to a greater extent when mixed with large volume of soil. This can be achieved by proper tillage operations using disc or mould board plough thus reducing the toxicity of herbicides. Dinker et al., (1986) reported that the damage caused by herbicide residues was reduced by ploughing.
4.Crop rotation
Crop rotations limit the buildup of weed populations and prevent major weed species shifts. Application of herbicides to a particular cropping system favours the accumulation of residues in the soil over the years. This can be prevented through the adoption of crop rotation. .Sankaran et al., (1990) reported that under irrigated field conditions of Coimbatore district, integrating hand weeding at 35 DAT along with fluchloralin at 0.9 kg or butachlor at 0.75 kg/ha is recommended for ragi + sunflower (border crop), pendimethalin at 1.0 kg/ha + hand weeding on 35 DAS for cotton inter cropped with onion and two manual weedings at 15 and 35 DAS for sorghum inter cropped with cowpea. The above weed management schedule did not show any residual effect in the cropping system.
5.Use of non phytotoxic oil
Use of non phytotoxic oil improves the weed control efficiency and reduces the harmful effect of herbicide residues. This helps to obtain desirable yield of crops with better quality.
6.Use of activated carbon
The toxicity of applied herbicides can be reduced by the application of charcoal,which has high adsorptive capacity influenced by large surface area(600-1200m2/g ).Jayakumar et at., (1990 b) reported that application of charcoal at 5.0 kg/ha along the seed line reduced the residual toxicity of atrazine .
8.Use of safeners and antidotes
The use of safeners and antidotes protects the crops from herbicide injury. This favours the use of certain herbicides on crops that would normally be affected by herbicides.
Safeners
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R27788 (N, N- dialIyl-2, 2-dichioroacetamide) is used along with EPTC and butylate.
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1, 8- naphthalic anhydride (NA) is used along with molinate and alachlor.
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Cyometrinil is used along with metolachlor
8. Leaching
The water soluble herbicides are leached down to the lower layers of soil by frequent
irrigations which improve the germination, growth and development of crops.
Authors
A.Suganthi, A.P.Sivamurugan and V.Manivannan
Tamil Nadu Agricultural University, Coimbatore