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Disposition of Fertilizer Nitrate Applied to Sorghum-Sudangrass in the Southern Plains¹

ABSTRACT

Results are reported for a 3-year field-plot study on the fate of fertilizer ¹⁵N associated with sorghum-sudangrass (Sorghum sudanense) production. The fertilizer was applied under dryland conditions at a 112-kg N/ha annual rate to eight area soils representative of five soil orders. On a seasonal average, slightly more than half the fertilizer was taken up by the plants (tops + roots), and about a third was incorporated in the soil organic matter. Very little (<5%) remained an inorganic leachable form, and about 7% was unaccounted. The unaccounted is considered to represent denitrification, since soil profile sampling during and after the cropping season showed no evidence of fertilizer-N leaching below the root zone. Preplant soil nitrate was a better indicator of total soil N availability the first 2 years, whereas the mineralization potential was better the third year. From an environmental standpoint, use of the N fertilizer at the described rate and prevailing farming conditions appeared to pose no potential pollution problems.

Key Words: nitrogen transformations • nitrogen balance • nitrogen-15 • mineralization potential • dryland farming

¹ Contribution of the USDA-ARS, Southern Region.

² Soil Scientist and Technicians, respectively, Southern Plains Watershed and Water Quality Lab., P.O. Box 1430, Durant, OK 74701.

Received for publication June 22, 1981.