Phosphorus Fertilization of Late-Seeded Winter Wheat in a Chemical Fallow System.

Winter wheat (Triticum aestivum L.) is produced in the low precipitation (200 to 300 mm) zone of north-central Oregon and east-central Washington using a conventional, tillage-based, summer fallow system.  The no-till, chemical fallow (CF) system is an alternative to the traditional method of farming.  Optimism about CF is tempered by an understanding that yield reductions often are a consequence of delayed (late) seeding.  Late seeding of winter wheat in CF is necessary because seed-zone moisture during optimum (early) planting dates is frequently less than that required for uniform germination and emergence.  Yield reductions from late seeding may be offset, to some extent, by P fertilization.  Effects of fertilization were evaluated for three years in field experiments conducted at nine locations.  Phosphorus applied at rates of 5 and 15 kg P ha^-1 increased grain yield by an average of 119 and 211 kg ha^-1, respectively.  Yield responses among sites ranged from 0 to 336 kg ha^-1 and were most pronounced on slightly acidic soils where initial soil test P levels were less than or equal to 12 mg kg^-1.  Yield increases were correlated to improvements in early-season dry matter accumulation, early-season P uptake, and the number of spikes per unit area (SPU).