Modifying the N Index to Predict Nitrate Leaching Losses From Agricultural Soils.

The Nitrogen Index has been proposed in Ontario to determine the risk for contamination of surface and ground waters by nitrate leached out of the crop rooting zone.  The N Index is based on hydrologic soil group (HSG), with the most permeable soils generally having the greatest leaching risk, and the least permeable soils generally having the lowest leaching risk.  The importance of tile drainage and N transformations on the N index were examined by comparing in situ nitrate loss from the soil profile over time to the corresponding losses of a non-reactive “tracer solute” (chloride). Calcium nitrate fertilizer and potassium chloride were added to 7 field sites and movement of inorganic N and chloride were tracked over the following year. After 2-3 weeks, the majority of the applied nitrogen and chloride was still in the 0-10 cm depth of all soils, although there was some evidence of inorganic N leaching to the 10-20 cm depth. After 8 weeks, there was considerable leaching of inorganic N in the profile of the 3 soils investigated with maximum concentrations in the 20-30 cm depth. Except for the not-tiled clay loam soil, very little of the applied N was present in the soils after 32 weeks, and virtually no inorganic N remained in the soil 1 yr after application.  Inorganic N and chloride were found to rapidly move in some soil profiles (within weeks), and soil information such as site-specific measurements of saturated hydraulic conductivity were required to accurately identify the potential leaching risk.  Tile drainage increased inorganic N and chloride leaching substantially, and it increased the Ksat-derived hydrologic soil group. It consequently appears that site-specific application of the Ontario N Index should consider the measured Ksat of the crop rooting zone and the presence/absence of tile drainage.