Date of this Version
Fluid Fertilizer Foundation, Volume 28, February 20-22, 2011, pp. 132-37
Water and nitrogen are the first and second constraints to cotton production in the arid southwestern U.S, respectively (Morrow and Krieg, 1990). Subsurface drip irrigation (SDI) area in cottonland is currently estimated at 300,000 ac, and is growing (Jim Bordovsky, personal communication). Efficiency of water application to cotton in SDI systems is about 90% (Bordovsky and Lyle, 1998). However, N management research for cotton in SDI has not kept up with the water management research. Improving N fertilizer use efficiency would allow lower rates of N fertilizer to be used by producers without hurting lint yields. The reduced costs of improving efficiency of inputs such as fertilizer would help keep cotton farmers competitive in the world market place. Additionally, residual nitrate (NO3) can be leached to groundwater and impact water quality. The environment of the West Texas Region is thereby protected when N fertilizer use efficiency is improved.
Timing of N application is an important management tool that can result in improved N use efficiency in cotton. Norton and Silvertooth (1998) reported reduction in N fertilizer needed and increased N use efficiency if pre-plant N was avoided in irrigated cotton in Arizona. Based on that research, the Cooperative Extension of the University of Arizona states that the main window for N applications to cotton is centered at peak bloom or about 2200 heat units (base 60°F). The rate of N uptake at peak bloom is apparently maximum in cotton (Silvertooth, 2001). Previous research conducted in this area has indicated that improving the timing of N fertilizer injections in SDI cotton systems based on canopy reflectance assessments of in-season N status can save up to 90 lb N/ac, without hurting yields (Bronson et al., 2003; Chua et al., 2003). We also observed in earlier work that modifying the timing of in-season N applications by applying N when chlorophyll meter readings were low, resulted in reduced N fertilizer applications and reduced residual soil NO3 --N (Chua et al., 2003). However, more research is needed on basing the timing and rates of N fertilizer injections to SDI cotton on spectral reflectance. In the previous work (Chua et al., 2003), our SDI system was not set up for fertigation treatments, but our existing, present SDI system is. In addition to reflectance treatments and their associate reference treatments (i.e. 1.5 * soil test treatment), we added a low, 0.5 * soil test treatment N rate to provide more information on a wide range of N fertilizer inputs.