Influence of Compost Amendment and Tillage Method On Nutritional Quality of Organically Grown Broccoli and Lettuce

The potential of organic production for enhancing health-promoting antioxidant phytochemicals in fruits and vegetables as compared to their conventional counterparts has been demonstrated in recent studies. Moreover, it is suggested that even within organic systems different management practices may result in distinctly different impacts. An experiment was carried out in fall 2009 to assess the nutritional quality attributes of broccoli and romaine lettuce grown using different organic amendment and tillage methods. Broccoli and lettuce plants were grown in certified organically managed field plots at Puyallup established since 2003. A split plot design with 4 replications was employed involving tillage (conventional plow vs. till with spader) as the whole plot factor and amendment (low C:N composted broiler litter applied preplant at 2 dry tons/acre vs. medium C:N on-farm compost applied preplant at 8 dry tons/acre) as the subplot factor. Lettuce was cultivated as a double crop with the first and second plantings harvested on 6 July and 21August, respectively, while broccoli was harvested on 23 July. Yields of broccoli and lettuce were not significantly influenced by the tillage method, whereas composted broiler litter resulted in a significant increase in yield of broccoli, in contrast to the on-farm compost. Vitamin C, total carotenoids, total phenolics, and antioxidant capacity measured by Oxygen Radical Absorbance Capacity (ORAC) assay were measured in harvested broccoli and lettuce samples that were frozen and sent to Gainesville for analysis. The two organic amendments resulted in similar levels of phytochemicals and hydrophilic ORAC values in both broccoli and lettuce, whereas tillage method showed prominent effects on the levels of total carotenoids and phenolics and hydrophilic ORAC values depending upon the crop and harvest. In contrast to conventional tillage, the spade tillage method reduced significantly the concentrations of carotenoids in broccoli. With respect to lettuce, decrease of total phenolic content and antioxidant capacity in the spade till treatment was observed in samples from the first and second harvest, respectively. With the increasing interest in conservation tillage for organic vegetable production, more in-depth research is warranted to elucidate the impact of tillage methods on crop productivity and produce quality as well as the underlying mechanisms.