Disposition of Fertilizer Nitrate Applied to a Swelling Clay Soil in the Field

Abstract

AbstractThis study was prompted by the present controversy over the role that N‐fertilizer use may have in reducing water quality. Our objective was to determine the disposition of N fertilizer (enriched with 15N) applied to level (< 2% slope) Houston Black clay near the economic optimum application rate (112 kg N/ha) for grain sorghum (Sorghum bicolor [L.] Moench). Particular emphasis was placed on determining the amount of applied N which leached below the root zone at different times during and after the growing season. A large, undisturbed, field‐drainage lysimeter was used to measure leaching of NO3−‐N below the root zone.During spring 1973 94 mm of drainage water containing a mean concentration of 2.4 ppm fertilizer‐derived NO3−‐N percolated through the soil profile. At crop maturity, only 55% of the N applied the previous spring was recovered by the crop or was present in drainage water. Large amounts of N not recovered by the crop were either measured as immobilized N (20% of the applied N) or were unrecovered and assumed denitrified (17%). During fall and winter approximately 120 mm of drainage water containing 0.5 ppm or less fertilizer‐derived NO3−‐N percolated through the soil profile. These results indicate that for rainfall conditions observed in this study (minimal crop water deficit), the application N fertilizer to grain sorghum at the near‐optimum economic rate probably will not seriously reduce ground‐water quality on a swelling clay soil, even though crop recovery of applied N may be low.