Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 1998
Publication Date: N/A
Interpretive Summary: Nitrogen fertilizers have been widely used in American crop production for the last half century. In 1993, 724,940 tons of nitrogen were applied in Iowa. Unfortunately, nitrate-nitrogen moves through the soil readily in water and can degrade the quality of surface and ground water supplies. Subsurface banding of fertilizer nitrogen with a knife applicator is the most commonly used technique in the midwestern states. In Iowa, greater than 50% of the fertilizer nitrogen is knife injected. With knife injection, a small furrow partly filled with loose soil remains above the fertilizer band. This furrow provides an ideal place for rainwater to accumulate, allows rapid infiltration, increases the amount of water flowing through the nitrogen fertilizer band, and therefore, increases the potential for nitrate movement. In this study we tested a new nitrogen fertilizer applicator that reduces the flow of water through the fertilizer band. To divert water flow, the applicator compacts the soil above the fertilizer band, fills the knife furrow with soil, and builds a ridge of soil over the knife furrow. We conducted our field study in lysimeters, which are enclosed blocks of soil that allow us to collect all the drainage water. This study showed that the applicator successfully modified the soil conditions above the fertilizer resulting in reduced water flow and nitrate movement. The potential impact of this applicator would be to reduce losses of nitrate from farming systems and to reduce nitrate contamination of ground and surface water supplies. Additionally, reducing nitrate movement would result in greater uptake of fertilizer by the crop and may allow lower fertilizer inputs while maintaining yield.
Fertilizer nitrogen that moves with water to subsurface drain tubes is no longer available for crop growth and is often transported to surface waters, where it is considered a contaminant. Localized soil compaction and doming (LCD) to divert water around the zone of fertilizer injection may be one way to reduce nitrate leaching from agricultural fields. This study was designed to determine whether anion tracer transport through soil profiles to subsurface drain tubes could be delayed relative to conventional knife application by applying the tracers using an LCD fertilizer applicator. Three fluorobenzoate tracers, which are not adsorbed to soil and move similarly to nitrate through soil, were applied to eight lysimeters using three application treatments. The three anion application treatments were 1) injected using a conventional knife shank; 2) broadcast; and 3) injected and covered using an LCD apparatus. Three different tracers were used for the application treatments, and each lysimeter received all three application treatments. Two rainfall intensity treatments were imposed, with four lysimeters used for each rainfall intensity treatment. There were no statistical differences in cumulative tracer leaching between the different rainfall intensity treatments. At the end of six months, 5% of the tracer applied by a conventional knife shank leached to the subsurface drain tubes, 4% applied as broadcast leached, and 1% applied by the LCD apparatus leached. At 18 months, corresponding values were 25%, 17%, and 13%. These results suggested that soil modification in the fertilizer injection zone was a simple and effective way to reduce fertilizer nitrate leaching.