Grass Pea (Lathgyrus sativum L.) as Pre-Plant Organic Fertlizer for Conventionally Tilled Winter Whest: Effects On Yield and Quality.

Sources of organic nitrogen (N) for the southern Great Plains (SGP) - and methods of their use – need testing to find ways to counter the rising cost of N fertilizer. This study investigated the cool-season pulse grass pea (Lathyrus sativum L.) (GP) as a pre-plant N source for continuous, conventionally tilled winter wheat (Triticum aestivum L.). The study was conducted in central Oklahoma during 2004 to 2008, on 3 replicate blocks of 4 experimental plots (6 by 10m). Inoculated grass pea (cv. AC-greenfix) seed was sown (75 kg/ha, 60 cm rows) during late summer (early-September) in one randomly chosen plot per block, and the 3 other plots mimicked traditional summer fallow under 0(control), 40, or 80 kg applied N/ha. All treatments were repeated on the same plots throughout the study. Aboveground (AG) biomass produced by grass pea was measured 50 days after planting (mid-October), and analyzed for amounts of N. Grass pea plots were then shredded and residues incorporated into the soil at tillage of all plots, and wheat was sown (100 kg/ha; 20 cm rows). Aboveground wheat biomass was collected at 3 growth stages (elongation, flowering, grain fill) and analyzed for N. Grass pea produced enough biomass and captured enough N to meet the needs of wheat at planting (40 kg N/ha) in 2 of 4 years (89 and 65 kg/ha in 2004 and 2006). Total AG biomass and N content of wheat under GP was similar to the control at elongation, and intermediate between no and 40 kg N/ha at grain fill in 2 years. Grain yield and N content under GP were intermediate between no and 40 kg/ha applied N in the first 2 years, and similar to the control thereafter. Precipitation had greater effects on yield and N content of wheat than the applied treatments. Grass pea was therefore only marginally effective and will have limited value as a pre-plant source of N for wheat.