Persistence of herbicides and its impact on soil micro flora in

Experiments were carried out in the field and laboratory during 2001-02 to study the persistence of three herbicides, namely butachlor, pretilachlor (Rifit) and 2,4-D in a rice – rice system and the effect of their residues on soil micro flora. Irrespective of the seasons and treatments, all the three herbicides dissipated quickly from soil and no residues were detected at the time of harvest. Even in the plots where butachlor was sprayed continuously for the four crop seasons without addition of organic matter, more than 96.20 per cent of the applied herbicide disappeared from the soil by 30 days after spraying. Therefore, alternate application of butachlor or pretilachlor between years or seasons could not exert any influence on their persistence in soil. The results also indicated that considerable variation in the microbial population would result from the application of herbicides. However, the change was observed only for a short period. Drastic decline in the population of soil bacteria noticed at one day after the application of herbicides extended up to 30 to 45 days in the first crop and 15 to 30 days in the second crop. In all the treatments, microorganisms attained the initial (normal) level of population by the time of harvest. Inhibitory effect of the herbicides was more on soil bacteria than soil fungi or actinomycetes and the extent of inhibition was more manifested by the application of butachlor than pretilachlor. The treatments with farmyard manure had higher population of micro flora at all the sampling intervals and they registered higher rate of degradation of butachlor, pretilachlor and 2,4-D in the soil.