Determining Nitrogen Fertilization Rates and Time of Application of Primocane-fruiting Blackberries, Poster Board #299

Increasing production of blackberry (Rubus L. subgenus Rubus Watson.) and the development of new worldwide markets allows for important new opportunities in production. New genotypes such as primocane fruiting (PF) blackberries can extend the growing season and create benefits for both farmers and consumers. As new cultivars, there are some important cultural practices, including plants nutrition parameters, which need to be researched in order to optimize yield and financial returns. The objective of this research project was to determine the optimum rate and time of nitrogen (N) application. This study was conducted in 2011 at the University of Arkansas, Fayetteville. Prime-Ark® 45 blackberry plants were cultivated under high tunnel conditions. There were four N treatments:  Trt. 1 (30 kg·ha^–1); Trt. 2 (40 kg·ha^–1); Trt. 3 (40 kg·ha^–1 - 50% split application); and Trt. 4 (50 kg·ha^–1). Single applications of N were completed on mid May and the split application of 50% on mid May and 50% after bloom (last week of July). Ammonium sulfate (NH₄)₂SO₄ was utilized as source of nitrogen. A CRB design was used with four blocks and five plants per experimental unit. The following variables were measured: total and marketable yield, fresh weight of plant above ground, cane diameter; soil and foliar analysis for N content. Leaves for foliar analysis were collected on 11 July. Total fruit yields for Trt. 2 and Trt. 3 (2.52 and 2.49 kg, respectively)  were the higher and significantly different than the other treatments. Although not significantly different, marketable yield had a similar trend as total fruit yield. Cane diameter and plant fresh weight were not significantly affected by the fertilizer treatments. Although there are not significant differences of N content in leaves among treatments, Trt. 3 had the highest concentration of N at the end of the season while the others showed a downward trend. No significant differences among treatments were found for the following elements: P, K, Ca, Mg, S, Na, Fe, Zn, and Cu while Mn and B were significantly different. Preliminary results indicate that either a single or split N application at rate of 40 kg·ha^–1 can result in acceptable yields.