Sweet Onion (Allium sativa) Yield as Affected by Potassium Rates

After nitrogen, potassium (K) is the nutrient required in largest amounts by plants. Potassium deficient plants show reduced growth and limited photosynthesis, and, under severe deficiency, can present chlorosis. Objective was to evaluate the effect of potassium rate on sweet onion bulb yield and quality. Experiments were conducted at the Horticulture Farm, Tifton Campus, University of Georgia, in the winter of 2013 and 2014. Soil was a sandy-loam soil with a pH of about 6.5.  Plants were grown on raised beds. Each bed had four rows 23-cm apart, with a plant spacing of 15 cm.  Beds were covered with black plastic film mulch and there were two lines of drip tape per bed, each drip tape being located midway between rows in alternate rows.  Starting eight weeks after transplanting, N (as 28-0-0) and K [as 0-0-25 (potassium thiosulfate)] were applied through the drip tape. Total N applied was 169 kg/ha. Five total K fertilizer levels (67, 134, 202, 269, and 336 kg/ha K₂O) were evaluated in a randomized complete-block design. Experimental plot consisted of a 6.1 m long bed (1.8 m centers) section. Marketable and total number and weight of onion bulbs were not affected by K rate. This indicates that a rate of 67 kg/ha K₂O was sufficient to cover the K needs of onion plants. Incidences of bolting (mean = 5%), sour skin (caused by Burkhordelia cepacia; mean = 11%) and double-bulbs (mean = 1%) were also unaffected by K levels.  Bulb dry matter content, soluble solids content, and pungency were positively correlated among them, but they did not show linear relationships with K rate. Thus, K rates above 67 kg/ha K₂O had minimal effects on bulb yield and quality.