From 1990 through 1992, we studied water and nitrate‐nitrogen (NO3‐N) present in the soil and flowing into a subsurface drainage system. Tillages were chisel plow (CP), moldboard plow (MB), no‐till (NT), and ridge‐till (RT). Crops were continuous corn (Zea mays L.) and a corn‐soybean [Glycine max (L.) Merr.] rotation. Tillage and crop only slightly influenced soil NO3‐N and water. Drainage water flows were highest for treatments that minimized soil disturbance and maximized crop residues produced during the previous year. The 3‐yr total flows by crop were 38 cm water for rotation‐corn plots, 56 for rotation soybean, and 59 for continuous corn. Flows by tillage were 41 cm water for MB plots, 52 for RT, and 55 for both CP and NT. In continuous corn, NT plots had more monthly water drainage than MB plots for most of 1990. Ridge‐till and CP plots had more drainage than MB plots for part of 1991. Crop rotation had the greatest effect on NO3‐N drainage loss. The 3‐yr total NO3‐N losses were 77 kg ha−1 for rotation corn plots, 84 for rotation soybean, and 164 for continuous corn. Tillage losses were 95 kg ha−1 for RT, 102 for MB, 106 for NT, and 131 for CP. No‐till and RT plots always had the lowest NO3‐N concentrations in drainage water; their yearly NO3‐N losses were usually smallest. Leaching can be best minimized by applying fertilizer in amounts to just meet crop N demand and at times closest to peak uptake.

The distribution and dissipation of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide], atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5 triazine), and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] in soil were studied in 1990, 1991, and 1992. Crop management practices included four tillage methods-chisel plow, moldboard plow, no-till, and ridge-till-and two crop rotations-continuous corn (Zea mays L.) and a corn-soybean [Glycine max (L.) Merr.] rotation. All herbicides were broadcast-spray applied with no incorporation. No-till plots had the smallest amounts of alachlor and metribuzin, whereas ridge-till plots had the smallest amounts of atrazine. Moldboard-plow plots usually contained the highest amounts of all three herbicides, although ridge-till plots had the highest metribuzin levels in 1992. These differences were seldom significant at the 0.05 level of probability, however. Throughout the growing season, 50 to 84% of the alachlor and metribuzin were retained in the top 10-cm layer of soil, and at least 68% of the atrazine was retained in the top 20 cm. From 84 to 98% of the herbicide applied was lost each year, probably by microbial degradation and, for alachlor, by volatilization after application. First-order half-lives were 36 d for alachlor, 55 d for atrazine, and 32 d for metribuzin. A two-compartment model better fitting the alachlor data returned a half-life of 24 d for that herbicide.

A soil-column laboratory experiment and a 2-yr field-sampling study evaluated the overall dissipation of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide]. Theses studies also measured the effect of no-till and chisel-plow tillages on alachlor leaching and dissipation. In the top 30 cm layer of soil, the overall half-life was 3 d or less, and the time to 90% dissipation ranged from 17 to 30 d. In no-fill soil, alachlor dissipated slightly faster, and more was transported into the 10 to 30 cm soil layer. Weather conditions promoting the movement of alachlor into the soil, however, weakened the effect of tillage on the dissipation rate. Most of the alachlor present in the soil, regardless of tillage, was found in the top 10 cm at all times. Of the alachlor applied to 30-cm tall soil columns, only 0.4% was removed by water flowing from chisel-plow columns and 1.6% from no-till columns. The results show that tillage was not a key factor in alachlor dissipation and leaching. Alachlor leaching was also a minor component in overall dissipation. [2-chloro-N-(2,6-dietbylphenyl)-N-(methoxymethyl) acetamide]. Theses studies also measured the effect of no-till and chisel-plow tillages on alachlor leaching and dissipation. In the top 30 cm layer of soil, the overall half-life was 3 d or less, and the time to 90% dissipation ranged from 17 to 30 d. In no-fill soil, alachlor dissipated slightly faster, and more was transported into the 10 to 30 cm soil layer. Weather conditions promoting the movement of alachlor into the soil, however, weakened the effect of tillage on the dissipation rate. Most of the alachlor present in the soil, regardless of tillage, was found in the top 10 cm at all times. Of the alachlor applied to 30-cm tall soil columns, only 0.4% was removed by water flowing from chisel-plow columns and 1.6% from no-till columns. The results show that tillage was not a key factor in alachlor dissipation and leaching. Alachlor leaching was also a minor component in overall dissipation.

Alachlor [2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl) acetamide] dissipation in the field shows two characteristics: (i) rapid, initial loss followed by slower degradation and (ii) sensitivity to environmental conditions including precipitation patterns, soil temperature, and soil‐water content. Empirical models, such as a first‐order equation, are simple and easy to use, but they do not accurately predict alachlor dissipation in the field. Complex mechanistic models provide theoretical mechanisms for dissipation, but their usefulness can be limited because they typically require data that is difficult to acquire under field conditions. We developed a hybrid model (M2CM) for alachlor dissipation based on a two‐compartment model (2CM). Our model requires only easily‐collected weather, soil, and pesticide information. It predicts the daily amount of alachlor in the soil and calculates times for 50 and 90% dissipation. We calibrated it with data from 30‐cm‐deep field‐soil samples and weather records from the 1993 growing season. Using 1992 and 1994 environmental data, the model calculated dissipation values that fit field‐sample data accurately (0.95 ≤ r2 ≤ 0.99). Results from the 2CM were not as consistently satisfactory (0.64 ≤ r2 ≤ 0.97). According to the M2CM, the alachlor half‐life (time to 50% dissipation) was 1 d for all 3 yr. Time to 90% dissipation was 18 d for 1992, 23 d for 1993, and 15 d for 1994.

Alachlor c 2.2 Atrazine c 2.8 Alachlor 2.2 Atrazine 2.8 Alachlor 2.2 Cyanazine c 2.8 Alachlor 2.2 Cyanazine 2.8 Alachlor 2.2 Metribuzin c 0.45 a Active ingredient .