Using Mineralization Estimates to Predict Nitrogen Fertilizer Needs

1. 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