Optimum soil‐applied nitrogen levels for cotton on a high pH alluvial soil

Nitrogen (N) is often applied to cotton (Gossypium spp.) in lower Mississippi Delta soils at rates ranging from 47 to 90 kg ha‐1. This study was conducted to determine optimum N rates for cotton production on a high pH alluvial soil in the Red River valley of central Louisiana, where higher rates are often recommended to growers. Cotton was planted on a Norwood silt loam soil [fine‐silty, mixed (calcareous), thermic Typic Udifluvent] over a 4‐year period and received seven rates of N ranging from 0 to 168 kg ha‐1 in 28‐kg increments. When N was applied at planting, yield response fit a quadratic curve where optimum rates could be estimated. The optimum rates of N was 123 kg N ha‐1 in 1990 and 140 kg N ha‐1 in 1992. When planting was delayed after N application due to weather or other causes, yield fit to a linear model and increased across the range of treatments. These results indicate that higher amounts of N are needed on the high pH Red River alluvial soils of Louisiana, versus the normal pH Mississippi alluvial soils. Recommended rates should be in the 112 to 134 kg ha‐1 range, unless carryover nitrogen may be expected from a previous crop. These results also strongly suggest that N should not be applied far in advance of planting.