Herbicide persistence in soil may injure crops planted in subsequent years. Despite recommendations for re-cropping restrictions, injury to crops is frequently reported, particularly with respect to legume and oilseed crops (Moyer, 1995; Moyer and Esau, 1996). Herbicide persistence and the degree of injury to crops seeded in the next season appears to be influenced by a number of parameters related to soil (Moyer and Hamman, 2001) as well as weather conditions (Moyer and Blackshaw, 1993).
Currently, most producers treat agricultural fields as single homogeneous units, applying the same management practice across the field. However, there is growing recognition of within-field spatial variability and the economic and environmental benefits of applying differential management practices. Site-specific agriculture and variable rate application aims to manage this spatial variability by optimizing the placement of seed, fertilizer, and pesticide inputs. Variable rate herbicide application could also be used to reduce potential persistence problems.
At the field scale, differential sorption of herbicides across the landscape has been reported. Greater absorption of metribuzin (4-amino-3-methylsulfanyl-6-tert-butyl-4H-1,2,4-triazin-5-one), diuron [1-(3,4-dichlorophenyl)amino-N,N-dimethyl-formamide], triallate [N,N di-isopropyl-1-(2,3,3-trichloroprop-2-enylsulfanyl) formamide] and 2,4-D [2-(2,4 dichlorophenoxy)acetic acid] on lower slopes was attributed to their higher organic matter levels (ranged from 1.9 to 4.7) when compared to the upper slopes (ranged from 0.8 to 1.8 percent) (Mallawantri and Mulla, 1992; Farenhorst et al., 2001). Soil organic carbon and pH and to a lesser extent clay content influenced atrazine sorption (Novak et al., 1997).
The persistence of atrazine increases with an increase in soil pH above 6.8 (Holford et al., 1989; Reinhardt et al., 1990) due to a reduction in degradation by chemical hydrolysis (Blumhorst and Weber, 1994; Walker et al., 1997). Chemical hydrolysis is also the major mechanism for sulfonylurea breakdown (Beckie and McKercher, 1989; Moyer, 1995). The primary mechanism for degradation of imidazilinone herbicides is microbial (Mangels, 2000) with pH being considered the most influential soil factor in determining persistence (Bresnahan et al., 2002). Persistence of imidazilinone herbicides increases as the soil pH falls below 6.0 due to binding of the molecular form to soil organic matter and a reduction in the availability of the herbicide for degradation by the microbial population.
The use of landscape analysis, which involves digital elevation models, fuzzy logic, and heuristic rules to delineate landform subunits can provide a meaningful basis for site-specific management (MacMillan et al., 1998; 2000). Good relationships have been reported between landform units derived from landscape analysis and differences in soil properties including organic matter, pH, sand, silt and clay content, and rooting depth (Brubaker et al., 1993; Young and Hammer, 2000; Manning et al., 2001; Machado et al., 2002).
Soil pH is one of the simplest properties to measure and link to the landscape (Oliveira et al., 1999). Oliveira et al. (1999) proposed that fields could be segregated into zones based upon soil pH and that these zones could be used to model, in their case, the potential for herbicide leaching. A similar approach could provide a rationale for variable rate herbicide applications based upon soil pH and the...