Submitted to: Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/1997
Publication Date: N/A
Interpretive Summary: The spatial and temporal variability of residual herbicides present in the soil during the growing season were studied on a field-size claypan soil watershed. Atrazine and alachlor concentrations showed spatial patterns within the study area starting 1 wk following application. The study showed that very little herbicide moved from the soil layer where it was applied due to runoff or leaching, and these processes did not explain the measure decreases in herbicide concentration over time. Herbicide concentrations in the watershed were significantly correlated with soil pH and organic matter content. This study can benefit scientists and producers who are trying to develop and implement more environmentally friendly methods of herbicide management by providing them with important knowledge of the within-field variations in herbicide concentrations in the soil. Knowledge of the spatial variability of herbicide concentrations, in combination with hinformation on weed growth patterns, could serve as the basis for variable rate herbicide application. Also, areas with low organic matter and appreciable slope are most vulnerable to herbicide losses by surface runoff immediately following herbicide application. These areas, once identified, can be targeted for conservation efforts aimed at reducing surface runoff velocity and transport of herbicides. Overall, understanding the processes controlling spatial and temporal variability of herbicides at the field scale is important for improved herbicide management in row crop settings.
Technical Abstract: The spatial and temporal variability of herbicides were studied on a 35-ha watershed located in the claypan soil region of north-central Missouri. Soil samples were collected from 0-5 cm soil depth in 1993, and 0-5, 5-10, and 10-15 cm soil depths in 1995 and analyzed for atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and alachlor (2-chloro-N-(2, 6-diethl-N-(methoxymethyl) acetanilide) concentrations. The effects of rainfall, topography, soil pH, cation exchange capacity (CEC), and organic matter (OM) content on the spatial distribution of herbicides were evaluated. There was no spatial dependency between samples taken immediately after herbicide application, however, spatial dependency was observed in the following sampling periods. During the year, less than 2.5% of atrazine and 2.0% of alachlor applied to the soil was lost in runoff, and the movement of herbicides below the layer of application was very low. Thus, surface runoff and leaching were not significant enough processes to explain temporal decreases in herbicide concentration. Atrazine and alachlor concentrations in the soil decreased rapidly during the growing season. Concentrations measured 4 and 8 wks after application were less than 20 and 5% for atrazine and less than 10 and 3% for alachlor, respectively, of those measured immediately after application. The spatial and temporal variability of herbicide concentration was a function of the interaction between soil pH and the sorptive capacity of the soil These findings are useful for site-specific crop management to increase the efficiency of herbicide application and also to reduce the loss of excess herbicides to surface runoff.