2019 ASHS Annual Conference

Vidalia onions (Allium cepa L.) are sweet, short-day, low pungency, yellow Granex-type bulbs that are popular in the U.S. because of their mild flavor. There are few reports on sweet onion plant growth in response to organic fertilization rate. The objective was to evaluate the effects of organic fertilizer rates on sweet onion plant growth and mineral nutrients concentration in leaves and bulbs. Experiments were conducted at the UGA Horticulture Farm, Tifton, GA in the winters of 2012-2013 and 2013-2014. There were five treatments [organic fertilizer 3-2-3 equivalent to 0, 60, 120, 180 and 240 kg·ha^-1 nitrogen (N)]. Root, stem and bulb biomass of mature plants increased while the root-to-shoot ratio decreased with increasing fertilization rate up to 120 kg·ha^-1 N. Foliar concentrations of N and Ca decreased while Cu concentration increased with increasing organic fertilization rate. Bulb Mg and Mn increased while P and Cu decreased with increasing organic fertilization rate. The N use efficiency decreased with increasing organic fertilization rate; the agronomic efficiency of N decreased quadratically. In conclusion, onion plant growth increased with increasing organic fertilizer rate probably because of augmented soil N levels. Plant nutrient deficiencies late in the season, even at high organic fertilization rates, indicates that preplant application of organic fertilizer was sufficient to cover plant nutritional needs only partially and that applications of N fertilizer later in the season may be necessary. High application rates of organic fertilizer (above those required by the crop) may have resulted in significant N leaching since it is unlikely that the crop used the majority of the N that was mineralized.