Performance of Poly-coated Urea for the Production of Vidalia Onions

There has been an increased interest in the use of controlled release fertilizers in vegetable crops. To determine the suitability of two commercially available PCU products Duration 120 (Koch Fertilizer; Wichita, KS) and ESN smart nitrogen (N) (Agrium Inc.; Denver, CO) for Vidalia Onion production. The varieties Golden Eye and Sapelo Sweet were used. Fertilizer treatments were 180 kg·ha^-N and 146 kg·ha^-1 N applied preplant using both PCU products.  The 146 kg·ha^-1 N treatment received an additional 34 kg·ha^-1 N prior to bulbing using calcium nitrate. Other treatments consisted of a preplant complete fertilizer (10N-4.4P-8.3K) applied at 56 kg·ha^-1 N with an additional 34 and 90 kg·ha^-1 N applied prior to bulbing using calcium nitrate and ESN, respectively. A control treatment using current recommendations for Vidalia onions (160 kg·ha^-1 N for the season) was included. Plants were managed according to standard practices with the exception of fertilizer treatments.  Mesh bags containing both PCU products were placed at depths of 15 cm and excavated throughout the season to determine N release. Approximately 58% N was released from ESN within the first month of growth and then release rates decreased. The Duration product had a linear release pattern with a slower initial N release than ESN. At 117 days after placement the ESN product released approximately 90% of N, while the Duration product had released approximately 85% of N. Total marketable yields were higher in ‘Sapelo Sweet’ than ‘Golden Eye’ due to a higher percentage of colossal bulbs produced by ‘Sapelo Sweet’.  For ‘Sapelo Sweet’, applications of Duration and ESN at planting (146 kg·ha^-1 N) followed by an application of calcium nitrate later in the season yielded no differently than the control or treatments receiving the preplant complete fertilizer followed by an application of ESN and calcium nitrate.  In ‘Golden Eye’, the results were similar. This suggests PCU fertilizers can be used for production of Vidalia Onion, but they should be partnered with a soluble N-source or applications split over the season.