Responses of Wheat Genotypes to Phosphorus In Organic and Conventional Cropping Systems In the Pacific Northwest.

Phosphorus (P) is arguably the most limiting macronutrient in the world. Dwindling resources of easily mineable P, along with water quality concerns, have lead to a renewed interest in improving phosphorus use efficiency in crops.  Field trials were conducted in five locations across Washington State over two years to test the responsiveness of five spring wheat (Triticum aestivum) cultivars to phosphorus fertilizer and inoculation with arbuscular mycorrhizal (AM) fungi. These sites, representing conventional, organic and no-till management, are located within the major dryland production region of the Pacific Northwest (PNW). Each site consisted of two fertilizer treatments, two inoculation treatments, and five cultivars arranged in split-split plot design with four blocks. The fertilizer treatment consisted of no additional fertilizer and 20 pounds per acre of P₂O₅, applied as either inorganic ammonium polyphosphate (11-37-0) or OMRI-certified fertilizers from Perfect Blend® and Nature Safe®. Multiple agronomic traits were measured over each field season during tillering, heading, grain fill and senescence. These traits include leaf phosphorus concentration over time, phosphorus and micronutrient concentration in seeds, grain yield, tillers per square meter, thousand kernel weight, seed protein concentration and other traits. Despite high soil test values for P, yield responses due to P fertilizer were observed in all three cropping systems. Phosphorus concentration in the leaves and seeds were significantly effected by P fertilizer, inoculation and cultivar. In a multivariate analysis of variance, phosphorus fertilizer resulted in significantly decreased concentrations of the micronutrients copper, iron, manganese and zinc in seeds. The phosphorus by inoculation interaction was significant for many traits measured within the high rainfall sites including thousand kernel weight, seed P, Cu and Zn concentration. Together, these results indicate that (1) there is a response to added phosphorus in PNW soils; (2) the observed response to phosphorus is cultivar-specific; and (3) in organic and no-till sites, inoculation with AM fungi would amplify the response from phosphorus.