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This study explores how laboratory estimates on soil nitrogen mineralization potential can predict nitrogen fertilizer requirements for rainfed soft white winter wheat in the Willamette Valley of Oregon. By correlating field estimates of crop-available nitrogen with soil nitrogen mineralization potential, a reliable method for predicting nitrogen fertilizer needs has been developed. The research findings also indicate the influence of preceding crops on nitrogen availability and highlight the efficiency of nitrogen uptake at Maximum Economic Yield (MEY) levels.
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Using Mineralization Estimates to Predict Nitrogen Fertilizer Needs N.W. CHRISTENSEN*, R.S. KAROW, D.M. BALOCH, and M.H. QURESHI Department of Crop and Soil Science, Oregon State University • Situation • Rainfed soft white winter wheat is grown in rotation with a number of crops in the Willamette Valley of western Oregon. • Preceding crops affect wheat response to N fertilizer in an unpredictable manner. • Inorganic soil N concentrations fail to predict wheat response to spring-applied N. • Laboratory analyses • Plant N: Leco CNS 2000 • Soil sample: 0 to 30 cm, collected in Jan-Feb • Soil inorganic N: NO3-N and NH4-N • Mineralizable soil N: Anaerobic incubation 20 g soil sample plus 50 mL H20 in 250 mL bottle Incubate air-tight bottle at 40° C for 7 d Extract with 50 mL 2 M KCl and filter Analyze for NH4-N Subtract pre-incubation NH4-N Results Grain yield (Mg ha-1) Available N = 5.71*Nmin - 50.73 R2 = 0.78 Crop-available N (kg ha-1) • Objectives • Relate field estimates of crop-available N to laboratory estimates of soil N mineralization potential • Predict N fertilizer needs using an N budget that includes laboratory estimates of nitrogen mineralization potential Grain protein (g kg-1) Calculations Crop-available N (kg N ha-1) = (plant Nt2 - plant Nt1) + (soil Nt2 - soil Nt1) where: plant N = N uptake by unfertilized control soil N = NH4 + NO3 to 30 cm t1 = Feekes 5 t2 = harvest N fertilizer prediction (kg N ha-1) = 300 – (Nmin + soil Nt1 + plant Nt1) where: 300 = empirical constant Nmin = anaerobic soil test N soil N = NH4 + NO3 to 30 cm t1 = Feekes 5 N rate at Maximum Economic Yield (kg N ha-1) = ((PN/PY) – b1)/2*b2 where: PN = $ 0.62 kg-1 N ($ 0.28 lb-1 N) PY = $ 0.13 kg-1 grain ($ 3.60 bu-1) b1 = linear regression coefficient b2 = quadratic regression coefficient Fig. 3. Grain yield and protein at MEY Nmin soil test (mg N kg-1) • Conclusions • Nitrogen available to winter wheat can be estimated by measuring mineralizable soil N in the laboratory (Fig. 1). • Preceding crops influence crop-available N • Differences exist within and among crops • Nitrogen fertilizer requirements can be calculated from laboratory estimates (Fig. 2). • Mean N fertilizer uptake efficiency at MEY = 0.66 • Yield and protein of soft white winter wheat fertilized for MEY fall within acceptable ranges (Fig. 3). • Mean yield = 8.25 Mg ha-1; range 5.62 to 10.8 • Mean protein = 92 g kg-1; range 81 to 104 g kg-1 Fig. 1. Nmin soil test correlation MEY N = 1.11*Rec N – 13.10 R2 = 0.89 • Methods • Wheat response to N fertilizer • Field research: Nineteen small-plot and on-farm trials, 1994 through 1999 • Wheat cultivars: ‘Stephens’, ‘Gene’ and ‘Madsen’ • Soils: Argiaquic Xeric Argialbolls, Aquultic Argixerolls, Cumulic Ultic Argixerolls, and Pachic Ultic Argixerolls • Preceding crops: Clover, corn, grass, and oats • N rates: 0 to 224 kg N ha-1 applied at Feekes 5 • Design: RCB with 4 or 5 N rates in 3 or 4 blocks • Measured responses: Grain yield, grain protein, and aboveground N removal regressed on N rate N fertilizer for MEY (kg ha-1) Outlier: Wheat after clover unresponsive >56 kg N ha-1 N fertilizer recommended (kg ha-1) Fig. 2. N fertilizer prediction