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. 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. 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. Inorganic soil N concentrations fail to predict wheat response to spring-applied N. Objectives Relate field estimates of crop-available N to laboratory estimates of soil N mineralization potential 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 Predict N fertilizer needs using an N budget that includes laboratory estimates of nitrogen mineralization potential Methods Wheat response to N fertilizer Field research:Nineteen small-plot and on-farm trials, 1994 through 1999 Field research:Nineteen small-plot and on-farm trials, 1994 through 1999 Wheat cultivars:‘Stephens’, ‘Gene’ and ‘Madsen’ Wheat cultivars:‘Stephens’, ‘Gene’ and ‘Madsen’ Soils:Argiaquic Xeric Argialbolls, Aquultic Argixerolls, Cumulic Ultic Argixerolls, and Pachic Ultic Argixerolls Soils:Argiaquic Xeric Argialbolls, Aquultic Argixerolls, Cumulic Ultic Argixerolls, and Pachic Ultic Argixerolls Preceding crops: Clover, corn, grass, and oats Preceding crops: Clover, corn, grass, and oats N rates:0 to 224 kg N ha -1 applied at Feekes 5 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 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 Measured responses:Grain yield, grain protein, and aboveground N removal regressed on N rate Laboratory analyses Plant N:Leco CNS 2000 Plant N:Leco CNS 2000 Soil sample:0 to 30 cm, collected in Jan- Feb Soil sample:0 to 30 cm, collected in Jan- Feb Soil inorganic N:NO 3 -N and NH 4 -N Soil inorganic N:NO 3 -N and NH 4 -N Mineralizable soil N:Anaerobic incubation Mineralizable soil N:Anaerobic incubation  20 g soil sample plus 50 mL H 2 0 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 NH 4 -N  Subtract pre-incubation NH 4 -N Calculations Crop-available N (kg N ha -1 ) = (plant N t2 - plant N t1 ) + (soil N t2 - soil N t1 ) where:plant N = N uptake by unfertilized control where:plant N = N uptake by unfertilized control soil N= NH 4 + NO 3 to 30 cm t 1 = Feekes 5 t 2 = harvest N fertilizer prediction (kg N ha -1 ) = 300 – (N min + soil N t1 + plant N t1 ) where:300 = empirical constant where:300 = empirical constant N min = anaerobic soil test N soil N= NH 4 + NO 3 to 30 cm t 1 = Feekes 5 N rate at Maximum Economic Yield (kg N ha -1 ) = ((P N /P Y ) – b 1 )/2*b 2 where:P N = $ 0.62 kg -1 N ($ 0.28 lb -1 N) where:P N = $ 0.62 kg -1 N ($ 0.28 lb -1 N) P Y = $ 0.13 kg -1 grain ($ 3.60 bu -1 ) b 1 = linear regression coefficient b 2 = quadratic regression coefficient Results Conclusions Nitrogen available to winter wheat can be estimated by measuring mineralizable soil N in the laboratory (Fig. 1). 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). 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). 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 N min soil test ( mg N kg -1 ) Crop-available N ( kg ha -1 ) Available N = 5.71*N min R 2 = 0.78 Fig. 1.N min soil test correlation Fig. 2.N fertilizer prediction N fertilizer recommended ( kg ha -1 ) N fertilizer for MEY ( kg ha -1 ) MEY N = 1.11*Rec N – R 2 = 0.89 Fig. 3.Grain yield and protein at MEY Grain protein ( g kg -1 ) Grain yield ( Mg ha -1 ) Outlier: Wheat after clover unresponsive >56 kg N ha -1