Choosing which cotton varieties to grow

The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311

Effects of Shank Spacings of Dually Injected Nitrogen and Phosphorus on Winter Wheat

Agronom

Cotton yield goal � Nitrogen rate recommendation

The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311

SOILS Ammonium and Nitrate Nitrogen in Soil Profiles of Long-Term Winter Wheat Fertilization Experiments

ABSTRACT Accumulation of NH~ -N and NO3 -N in soils has not been thor. oughly evaluated in long-term continuous winter wheat (Triticum aes. tivum L.) production systems. The objectives of this study were to determine long-term response of winter wheat to N fertilization and to evaluate accumulation of NH~ -N and NO3 -N in the soil profile. Four long-term winter wheat soil fertility experiments on thermic Ustoll soils that received annual applications of N for > 18 yr at selected N rates were sampled. At each location, one soil core 4.4 cm in diameter was taken to a depth of 240 cm from plots receiving variable N rates. Cores were separated into 30-cm increments and analyzed for 2 M KCI-extractable NH~ -N and NO~-N. At all locations, NH~ -N levels were not significantly different from the check (no fertilizer N) when rates were applied at or below yield goal requirements (90 or45 kg N ha -~ vs. 0 N). At N rates >90 kg N ha -~, surface (0-15 cm) NH~ -N increased compared with the check, while subsurface NH~ -N did not. Similarly, when N rates were <90 kg N ha -~, no significant differences in either surface or subsurface NO 3 -N were found. At N rates >90 kg N ha -1, NO5 -N accumulated in the subsurface soil profile (>30 cm). Estimates of N rates determined from simultaneous solutions of NO5 -N accumulation minimums and yield maximums generated from quadratic regression were greater than N rates currently recommended to achieve yield goals at all locations. For these long-term continuous winter wheat experiments, no accumulation of NH~ -N and NO5 -N occurred at recommended N rates where near maximum yields were obtained. p AST AND PRESENT use of N fertilizers for winter wheat production has been related to the potential for NOA--N contamination of surface and subsurface water. Although N fertilizers are essential for economic grain production, long-term N accumulation as a result of excessive N rates has not been monitored closely. Work by Liang et al. (1991) found that residual soil NO~--N did not increase in the soil profile (0-60 cm) over a 4-yr period when comparing N rates of 170 and 400 kg ha -1 applied to corn. MacDonald et al. (1989) indicated that following harvest, unfertilized wheat plots had inorganic N contents equal to those where 234 kg N ha -1 had been applied. This work further suggested that almost all of the NO~--N at risk to leaching over the winter period comes from mineralization of organic N and not from unused fertilizer applied in the spring; therefore, even a drastic reduction in N fertilizer use would have little effect on NO~--N leaching. Lamb et al. (1985) reported that the addition of N fertilizer increased the amount of NO~-N accumulated but did not change the accumulation pattern. Tillage system (no-till, stubble mulch, and plow) did not affect the time at which the NOA--N started to accumulate during the fallow period nor the rate of accumulation (Lamb et al., 1985). Sharpley al. (1991) reported no evidence of N accumulation the soil profile (0--180 cm) after 5 yr for either no-till or reduced-till cultural practices with N fertilizer applied to sorghum at recommended rates (0--146 kg ha-1 yr-1), although annual total N in surface runoff . was 0.76 kg N ha -1 for no-till, 0.99 kg N ha -1 for reduced-till, and 7.28 kg N ha -1 for conventional till. Smika (1990) reported that time must be allowed for the equilibration of soil conditions before evaluating NO~--N accumulation, citing research that showed less NO~--N accumulation to 120 cm for reduced-till methods compared with conventional tillage for short-term studies, but more NO~--N accumulation for reducedtill methods in long-term studies. Tracy et al. (1990) noted that tillage method (conventional, no-till) did not affect NO~--N accumulation below 5 cm; differences in NO~--N in the topsoil were attributed to organic matter incorporation over 16 yr of winter wheat farming. Varvel and Peterson (1990) reported that high N application rates (180 kg N ha-1) resulted in greater residual soil NO~--N to 150 cm for continuous corn and grain sorghum systems than for other cropping systems. This same study found that all systems had similar NO~--N accumulation at lower N application rates. Work by Liang et al. (1991) found that under irrigation, 100 kg NOA--N ha -~ was lost from the rooting zone (0-60 cm) during four growing seasons, with the majority coming from the surface 40 cm. The effects of N fertilizer rate (90 and 180 kg ha -x) and nitrification inhibitors on urea lSN leaching and balance on a irrigated sandy loam were summarized by Waiters and Malzer (1990). The higher N application rate resulted in 3.4 times more N leached over a 3-yr period (206 vs. 88 kg ha -1 to 1.2 m depth). Nitrification inhibitors delayed N losses, but did not decrease the total N lost. Westerman and Tucker (1979) noted that the presence of organic residue can lower denitrification by increased immobilization of inorganic or mineralized N. Immobilization was thus considered to be an N conserving process competing with denitrification for nitrate. Nitrate studies in field microplots showed that 17% of applied 15N (120 kg N ha -~ equivalent) was still in the 45-cm soil profile after 1 yr (Kowalenko, 1989). Webster et al. (1986) evaluated the movement (92 and 102 kg NHnNO 3 ha -~) in clay and sandy loam field microplots and found that < 1% of the fertilizer was leached beyond 130 cm in the first winter following application. Response of wheat grain yields to N fertilization has been documented in numerous soil fertility experiments. However, very few of these experiments have included evaluation for more than 3 to 5 yr that also accounted for accumulation of NOA--N and NH~--N within the soil profile. The objectives of this study were to determine the long-term response of 9