Effect of Organic Nitrogen Fertilizer on Leaf Biomass and Quality of Peppermint (Mentha x piperita) in Eastern Washington.

Peppermint, (Mentha x piperita), is an herbal medicinal species often used in dietary supplements.  Trout Lake Farm East located in Ephrata, WA grows peppermint organically and supplies leaf feedstock for extraction and processing.   In 2015, a nitrogen benchmarking study was conducted to investigate the peppermint leaf production and phytochemical content in response to nitrogen rates.  Nitrogen feathermeal (13-0-0) was applied to peppermint (Variety: Black Mitcham) twice a year in a randomized 5x3 factorial design.  Factor A represents the spring application of 5 levels of nitrogen (0, 100, 150, 200, or 250 pounds Nitrogen/Acre (lbs. N/Ac)) and factor B represents the 3 levels of nitrogen (0, 100, or 200 lbs. N/Ac) applied in the summer after the first harvest.  Results showed that organic nitrogen fertilizer increased peppermint leaf biomass in both the spring and the summer application.  In the spring application, plants that received 200 and 250 lbs. N/Ac application reached the highest leaf biomass.  Nitrogen fertilizer also increased peppermint leaf nitrogen content after both the spring and summer applications.  In the spring application, plants that received 200 and 250 lbs. N/Ac application had the highest leaf nitrogen content.  Results indicated that 200 lbs. N/Ac is the optimum nitrogen fertilizer rate for peppermint growth.  There were no interactions between the two nitrogen fertilizer applications on leaf biomass and leaf nitrogen content, suggesting that nitrogen fertilizer should be applied after each harvest.  Spring or summer nitrogen fertilizer application did not affect peppermint leaf eriocitrin content based on leaf dry weight. 

Key words:     Peppermint, organic farming, leaf biomass, Nitrogen content, Eriocitrin content