Nitrate-N is present in drainage water whenever there is subsurface
drainage flow. Pesticides are present only during the first month or
two after pesticide application (April or May).
Small amounts of pesticide move quickly through the soil into subsurface drainage water during the first several rainstorms after application. The amounts are usually very small on non-sandy soils (less than 1% of what was applied and often less than 1/10%), while the majority of the pesticide is sorbed and/or degraded in the topsoil. The rapid movement of small amounts of chemicals is called "preferential flow" and is due to flow through natural cracks and channels in the soil. Preferential flow of pesticides occurs during the first month or two after pesticide application in the spring (see diagram).
Nitrate-N is present in subsurface drain water at all times that drainage occurs (see diagram). Since most of the drainflow occurs during the "off-season" (November -May) in Indiana, most of the nitrate-N losses also occur then. Nitrate-N losses in drainage water vary with drain spacing as well as crop rotation and fertilizer treatment. In preliminary data from the WQFS, crop rotation appears to be the most significant factor for nitrate flux. In systems where roots are present year-round, such as in the bluestem prairie grass plots, the nitrate-N flux is very low. Summer annual crops like corn and soybean have more limited time periods with active roots, and thus nitrate-N flux increases.
Pesticide and nitrate-N leaching losses occur at different times and by different mechanisms, and therefore management practices to reduce these losses will be different. Although it may appear to be obvious, the difference in the major loss period for pesticides and nitrate-N means that practices developed to minimize pesticide leaching, for example, will have little impact on nitrate-N leaching, and vice versa. For non-sandy soils, pesticide management research should focus on the time period immediately after application, since almost no losses are detected in drainage water in the off-season. For nitrate-N, research is needed on ways to keep N in the soil or return it to the air. Winter cover crops could retain N during the winter season so it will be available during the growing season. Water table management may facilitate denitrification (conversion of nitrate to N-containing gases) in the off-season.
Nitrate-N is present in drainage water whenever there is subsurface
drainage flow. Pesticides are present only during the first month or
two after pesticide application (April or May).