MSc.

Experiments were carried out to study the influence of organic matter and soil moisture on the adsorption of chloroacetanilide herbicides, viz., butachlor and pretilachlor, in laterite soil. The technical programme consisted of an incubation experiment and a pot culture study. In the incubation experiment, two moisture levels viz., air dry and field capacity were included along with two herbicides, butachlor and pretilachlor, and three organic matter sources, FYM, vermicompost and soil. The pot culture experiment consisted of six treatments with two herbicides and three organic matter sources. The herbicides, butachlor and pretilachlor were applied @ 1.25 kg ha-1 and 0.45 kg ha-1 respectively and the manures were applied @ 5.0 t ha -1.

The soil sample collected for the study belongs to Ultisol order, the major characteristics of which are low pH, low CEC (4.7 and 4.67 C mol (+) kg-1 respectively ) and high sand content (73.13 % sand, 16.88 % silt and 10.00 % clay). Organic carbon content was in the medium range (0.61 %).

Dissolved organic carbon (DOC) is the most active fraction of organic matter and hence the procedure for DOC estimation in soil, FYM and vermicompost was standardized. Quantity of herbicides adsorbed by soil was determined by gas chromatography using standard protocols developed at AICRP on Weed Control, College of Horticulture, Vellanikkara.

In the incubation experiment, the degree of adsorption of butachlor and pretilachlor was evaluated using the Kd (distribution coefficient) values. The Kd values followed the order of organic carbon content in the soil samples taken from the treatments. The Kd values were higher for pretilachlor with the order being FYM > vermicompost > soil. Only slight variations were observed between air dry and field capacity levels indicating that moisture levels had no significant influence on the Kd values. This could be explained by hydrophobic character of the herbicides under study.

The pot culture study showed that adsorption as well as dissipation of herbicides were more in vermicompost treatment. The results also indicated that initial adsorption of herbicides was lessened by dissolved organic carbon released from the farmyard manure. However, the half life of herbicides was more in FYM treatment when compared to vermicompost and control (soil alone). The half life of butachlor was 17.00 days and that of pretilachlor was 19.00 days in FYM treatment. The results indicated the superiority of FYM in retaining the herbicides for the desired period of weed control in rice.

Phytotoxicity due to herbicide application was more with vermicompost treatment than that of FYM and soil alone treatments. This was due to the higher initial adsorption of herbicides by the soil matrix. Biometric observations on plant height, productive tillers, yield of grain and straw showed that FYM is superior to vermicompost in improving the growth and yield of rice.

From the above study, it could be concluded that butachlor and pretilachlor are strongly adsorbed on soil organic matter and their adsorption onto soils is affected by the presence of soluble organic matter. Presence of high levels of dissolved organic carbon in the FYM treatment at the time of application of herbicides resulted in a reduction in the magnitude of adsorption. On comparing two organic matter sources, it was noticed that total as well as dissolved organic carbon were higher in the FYM applied soil. The dissolved organic carbon would have facilitated movement of chemicals while the particulate organic carbon improved their retention. Therefore, it is essential to consider both these factors when pre -emergence herbicides are applied to rice.