Graduate Thesis Or Dissertation

Evaluation and use of a soil mineralizable nitrogen test to determine the fertilizer nitrogen needs of winter wheat grown in western Oregon

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  • The assessment of optimum nitrogen (N) fertilizer need for winter wheat (Triticum aestivum L.) is important for economic and environmental sustainability. A comprehensive understanding of fertilizer N requirement depends on estimation of the quantity of N needed by the crop versus that supplied by soil. The objectives of this study were: to assess the potential of using short-term anaerobic incubation test values in developing nitrogen fertilizer recommendations for the region; to evaluate the suitability of a nitrogen balance model for predicting N fertilizer needs of winter wheat; and to document the N uptake patterns of spring and winter wheat. Field experiments were conducted in the Willamette Valley of western Oregon in 1994-1997 and included both on-farm trials and small research plot trials. An array of rotations, crop types and nitrogen fertilizer rates were evaluated. Mineralizable nitrogen in soil samples taken prior to spring fertilization was estimated by a 7-day anaerobic incubation method and results were compared with estimations of soil supplied nitrogen from field experiments. Lab and field estimations were well correlated. A more than four fold increase in soil supplied N values, 20 to 110 kg N h⁻¹, was observed when mineralizable N test values increased from 14 to 29 mg N kg⁻¹. Results indicate that soil mineralizable N values satisfactorily predict approximate soil N availability and that results can be used to adjust fertilizer N requirements in the region. A Feekes growth stage 5 spring soil and tissue test based model was developed and evaluated for predicting the need for additional N fertilizer on winter wheat. Optimum N rates predicted by the model were closely related to N rates required to obtain maximum economic yield. Model validation experiments also gave promising results. Nitrogen uptake patterns for spring and winter wheat were similar. Maximum N uptake for spring wheat was at approximately 1100 accumulated growing GDD, before Feekes 10. The maximum N uptake rate, 0.038 kg N GDD⁻¹, occurred at 750 GDD. Nitrogen uptake in winter wheat was significantly affected by rotations. Maximum N uptake for fertilized winter wheat was at approximately 1400 GDD, also before Feekes 10. The maximum N uptake rate, 0.5 and 0.2 kg N GDD⁻¹, for wheat following clover and oat occurred at 1100 and 1300 GDD, respectively.
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