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Spatial and Temporal Variability of Herbicides in a Claypan Soil Watershed

Dep. of Biological and Agric. Engineering, Univ. of Missouri, Columbia, MO 65211;

E. E. Alberts and R. N. Lerch

USDA-ARS, Cropping Systems and Water Quality Research Unit, Columbia, MO 65211.

^* Corresponding author (fessehaie_ghidey{at}muccmail.missouri.edu).

ABSTRACT

The spatial and temporal variability of herbicides during the growing season were studied on a 35-ha watershed located in the claypan soil region of north-central Missouri. Soil samples were collected from the 0- to 5-cm soil depth in 1993, and 0- to 5-, 5- to 10-, and 10- to 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, <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 also very low throughout the sampling period. Atrazinc and alachlor concentrations in the soil decreased rapidly during the growing season. Concentrations measured 4 and 8 wk after application were <20 and 5% for atrazine, and <10 and 3% for alachlor, respectively, of those measured immediately after application. The study showed that the spatial and temporal variability of herbicide concentration was a function of the interaction between soil pH and the sorptive capacity of the soil (CEC and OM content). 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.