Kevin Bronson and James P. Bordovsky
Objective: The objective was to evaluate nitrogen source affects in terms of nitrogen use-efficiency in cotton grown under subsurface drip irrigation.
Methodology: Two nitrogen fertilizers, urea solution (20-0-0) and urea ammonium nitrate (32-0-0) were evaluated for plant use efficiency. Since water use efficiency is high in cotton grown under subsurface drip irrigation (SDI), it was assumed that nitrogen efficiency was also high when applied in low rates with the irrigation water however, Chua et al. (2001) reported a 25 to 50 % loss of N15-labeled urea ammonium nitrate in the plant-soil system of cotton grown under SDI in West Texas. This was probably due to de-nitrification in the wet zone around the 12-inch deep emitter. Using urea solution (20-0-0) as the N source may reduce denitrification losses and improve N fertilizer use-efficiency.
A randomized complete block design of a one-way factorial with three replications was applied to 40 -acres of Paymaster 2326 RR. Each replication was divided into two, 72- row plots (30-in spacing) that were randomly assigned to either the urea treatment (20-0-0) or the urea ammonium nitrate treatment (32-0-0). Each plot had its own irrigation and fertilizer injection station. Nitrogen fertilizer was applied at 150 lb N/ac, the recommendation for 2.5 bale/ac production (Zhang et al., 1998). Nitrogen fertilizer was injected into the SDI system daily, between first square and peak bloom growth stages. Seasonal irrigation totaled 12.4 inches. Rainfall totaled 1.4 inches from 3 May to 28 August. Normal cultural practices were followed.
Leaf samples for N analysis and biomass samples were taken at peak bloom. One hundred chlorophyll meter readings and continuous spectral reflectance readings were taken from each plot at peak bloom.
Results: The source of nitrogen did not affect chlorophyll meter readings, leaf N, biomass, NH4-N, or NO3-N at peak bloom (Table 1). The Green vegetative index (GVI) at peak bloom was greater with urea than with urea ammonium nitrate (UAN). Lint yields were not affected by the N source. Multiple years of similar data need to be collected before conclusions can be made about the importance of N source on subsurface drip irrigated cotton.