Sweet Onion Yield as Affected by Organic Fertilization Rate

Organic production of short-day sweet onion (Allium cepa L.) is gaining popularity in Georgia. The objective was to determine the effects of organic fertilizer rate on onion yield and quality. The experiment was conducted in Tifton, GA in the winter of 2012-2013. Conventionally grown onion seedlings were planted in raised beds (1.8m centers), using four rows of plants per bed. Beds were covered with black plastic mulch and had two lines of drip tape per bed. Experimental design was a randomized complete block with five treatments (rates of organic fertilizer).  The organic fertilizer (3N-0.9P-2.5K) was applied at 0, 2, 4, 6, and 8 t/ha to the beds and incorporated with rototiller before laying the plastic mulch. No further fertilizer was applied after planting. Yield differences were determined by differences in bulb weight, rather than bulb number. Onion marketable yield and individual bulb wt. increased with increasing fertilization rate. Highest marketable yield (13.1 t/ha) and mean marketable bulb weight (148 g/bulb) were obtained with the highest fertilization rate [8 t/ha fertilizer (ca. 240 kg/ha N)]. Yield responses indicate that the highest fertilization rate was likely insufficient to meet the nutrient needs of onion plants.