Validation and Recalibration of a Soil Test for Mineralizable Nitrogen

Abstract

A soil test for mineralizable soil N had been calibrated for winter wheat in the Willamette Valley of western Oregon. Seventy‐eight percent of the variation in spring N uptake by unfertilized wheat was explained by N mineralized from mid‐winter soil samples incubated anaerobically for 7 days at 40°C. Mineralizable N (Nmin) ranged from 10 to 30 mg N kg⁻¹ and was used to predict N fertilizer needs. Recommended rates of N were correlated (R²=0.87) with maximum economic rates of N fertilizer. Subsequent farmer adoption of no‐till sowing and a high frequency of soil tests>30 mg N kg⁻¹ prompted reevaluation of the soil test. Four N fertilizer rates [0, 56, G, and G+56 kg N ha⁻¹] were compared in 12 m×150 m farmer‐managed plots. Grower's N rates (G) ranged from 90 to 180 kg N ha⁻¹ and were based on N_min and NH₄‐N plus NO₃‐N soil tests. Averaged across ten no‐till and five conventionally tilled sites, grain yield and crop N uptake were maximized at the recommended rate of N. Results demonstrate that N fertilizer needs for winter wheat can be predicted over a wide range of mineralizable soil N (10 to 75 mg N kg⁻¹) and that the same soil test calibration can be used for conventionally sown and direct‐seeded winter wheat.

Keywords:

• Anaerobic incubation
• N fertilization
• Triticum aestivum L.
• winter wheat