Nitrogen Fertility In Semi-Arid Dryland Organic Winter Wheat Production Is Challenging.

Organic farming systems use green and animal manures to supply N to their soils for crop production. The objective of this study was to evaluate the effect of green manure and composted cattle manure on the subsequent winter wheat (Triticum aestivum L.) crop in a semiarid environment. Dry pea (Pisum sativum L.) was seeded in early April and terminated at first flower in late June. Composted cattle manure was applied at 0, 11.2, or 22.5 Mg ha^-1 just prior to pea termination. Winter wheat was planted in mid September following the green manure or tilled summer fallow. No positive wheat response to green manure or composted cattle manure was observed in any of the three years of the study. In two of the three years, wheat yields and grain test weight were reduced following green manure. Green manure reduced grain yields compared to black fallow by 220 and 1190 kg ha^-1 in 2009 and 2010, respectively. This may partially be explained by 40 and 47 mm less soil water at wheat planting following peas compared to tilled summer fallow in 2008 and 2009, respectively. Also in 2008 and 2009, soil nitrate level averaged 45 kg ha^-1 greater for black fallow compared to green manure fallow when no compost was added. Organic growers in the semiarid Central Great Plains will be challenged to supply N fertility to their winter wheat crop in a rapid and consistent manner as a result of the inherently variable precipitation. Growers may need to allow several years to pass before they see the benefits of their fertility practices in their winter wheat cropping systems.