Responses of Alfalfa Growth and Nitrogen Utilization to Foliar Fertilization with Different Urea Concentrations

Foliar fertilization is increasingly used to apply nitrogen (N) to alfalfa ( Medicago sativa L.) at late growing season. Urea concentration is an important factor that affects fertilizer use efficiency in foliar fertilization. The purpose of this study was to evaluate the effects of urea concentration on alfalfa growth and nitrogen utilization. Furthermore, difference in N utilization between foliar and soil fertilization of alfalfa was also quantified. Urea solutions of eight concentrations ranging from 0.1% to 3% were applied in alfalfa fields through foliar fertilization in 2016. Based on the results obtained in 2016, urea- 15 N solutions of five concentrations ranging from 0.05% to 0.4% were used to fertilize alfalfa grown in pots through foliar and soil application in 2017. Non-fertilized alfalfa was selected as the control treatment in the two years. The leaf damage, total N content, percentage of N in alfalfa derived from labelled fertilizer ( Ndff %), 15 N fertilizer use efficiency ( NFUE ), yield and quality of alfalfa were measured. The results showed that leaf damage appeared with urea concentrations exceeding 0.4%. Increasing the urea concentration improved the Ndff % in all alfalfa organs but generally reduced the NFUE of alfalfa. The NFUE of alfalfa under foliar application had no significant difference with that under soil application in general, and reached the high values with urea concentrations of 0.05%–0.2%. Compared to the control treatment, urea application did not significantly improve the yield, total N content and quality of alfalfa. Overall, the maximum urea concentration with foliar fertilization is recommended as 0.4% for alfalfa.