Can foliar urea reduce nitrogen losses from potato production in Atlantic Canada?

In recent years it has become increasingly popular for potato producers in Atlantic Canada to include urea in the foliar applications of fungicides. Producers believe this practice sustains their potato crop through periods of water stress and allows for more rapid recovery following a precipitation event. This urea application is not considered as part of their N fertility program and is applied in addition to the recommended rates of N fertilizer at planting. This project examined the potential to reduce the amount of N fertilizer added at planting in situations where foliar urea will be applied to the crop. The project evaluated whether using in-season foliar urea in combination with reduced N fertilizer rates at planting is an effective practice for sustaining potato yields, improving N use efficiency, and reducing nitrous oxide emissions and nitrate leaching.

An experiment at the research station in Harrington, Prince Edward Island, was conducted from 2013 through 2016. The treatments included five rates of N fertilizer application at planting (0, 120, 150, 180, 240 kg N/ha) as well as two treatments with 30 kg N/ha of foliar-applied urea with potatoes fertilized at planting with 120 and 150 kg N/ha. In 2015 and 2016, an additional treatment was added with foliar urea applied to potatoes fertilized at planting with 60 kg N/ha. Marketable yield, growing season nitrous oxide emissions, soil nitrate concentration, and nitrate flux (ion exchange membranes) were measured during the growing season.

Monitoring of nitrous oxide emissions, from field-scale side-by-side demonstration plots comparing 4R and grower practices in 2015 and 2016, revealed high variability, with 4R practices increasing emissions in 2015 and decreasing them in 2016.

Over the four years of the research station study, there was no yield benefit to N application rates higher than 120 kg/ha, and application of foliar urea did not further increase yields. Nitrous oxide emissions were much higher in 2014 than in the other three years. In 2014, fertilizer-induced emissions of nitrous oxide averaged 1.7% of the N applied. The N fertilizer treatments (other than the check) did not differ in nitrous oxide emissions. Use of ion exchange membranes (IEMS) as soil test probes for nitrate explained about 60% of the variation in nitrous oxide emissions. Therefore the use of IEMs might serve as a useful proxy metric for nitrous oxide emission, in addition to a management guide of N application.

This study is scheduled to complete in 2018. It shows promise of contributing toward development of a 4R metric for nitrous oxide emission.