FERTILIZER NITROGEN REQUIREMENTS FOR COTTON PRODUCTION AS AFFECTED BYTILLAGE AND TRAFFIC

Soil compaction and its associated problems have led to interest in in vestigating the interactive effects of traffic and tillage systems on fertilizer N requirement for cotton (Gossypium hirsutum L.). In 1987, a study was initiated on a thermic Typic Hapludult soil complex with a cropping system of wheat (Triticum aestivum L.)-cotton double cropped . The experimental design was a split-split plot with four replication s. Main plots were two traffic treatments (conventional and no-traffic ), subplots were four tillage systems for cotton (surface tillage with out subsoiling [surface-only], surface tillage and annual in-row subso iling [subsoiling], surface tillage with one-time-only complete disrup tion of the tillage pan [complete], or no surface tillage and in-row s ubsoiling [strip-till]), and sub-subplots were four N rates (0, 45, 90 , and 135 kg N ha(-1)). In addition, application of N-15-labeled NH4NO 3 was made to microplots inside each tillage-traffic-90 kg N ha(-1) pl ot. In 1990 and 1991, increasing N application increased cotton biomas s and decreased lint percentage. In the dry year of 1990, no-traffic d ecreased seed cotton yield from 1500 to 1360 kg ha(-1), while tillage had no significant effects on cotton yield components. Above-normal ra infall in 1991 resulted in the strip-till with no-traffic treatment ha ving the highest seed cotton yield (2749 kg ha(-1)) and the greatest f ertilizer N uptake efficiency (35%). Results indicate that the detrime ntal effects of traffic on N uptake efficiency may be reduced with con servation tillage systems and that higher fertilizer N application rat es may not be needed for conservation tillage practices such as strip- till in Coastal Plain soils.