Disposition of Fertilizer Nitrate Applied to a Swelling Clay Soil in the Field
1976
Kissel, D. E. | Smith, S. J. | Dillow, D. W.
This 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.
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