Improving bio-physical and economic water productivity of menthol mint (Mentha arvensis L.) through drip fertigation

Two field experiments were conducted to determine the optimum irrigation method and drip fertigation rate in menthol mint during the spring of 2015 and 2016. Experiment I comprised combinations of four drip irrigation rates [60, 80, 100, and 120% of crop evapotranspiration (ETC)] and three fertigation rates [60, 80, and 100% of the recommended dose of fertilizers (RDF), i.e. 75 kg N and 40 kg P2O5 ha(-1)] with an additional control (furrow irrigation and soil application of RDF). Experiment II had a split-plot design, with three irrigation methods [sprinkler, drip, and furrow irrigated raised bed (FIRB)] used in main plots and four irrigation regimes [(120, 100, 80, and 60% of cumulative pan evaporation (CPE)] for drip and sprinkler methods while maintaining irrigation water (IW): CPE of 1.2, 1.0, 0.8, and 0.6 for FIRB in subplots. Drip irrigation at 120% ETc resulted in significantly higher fresh herb and oil yield than at 100, 80, and 60% ETc. While being statistically similar with each other, fertigation with 100 and 80% RDF recorded significantly higher oil yield than that with 60% RDF. Drip irrigation at 100 or 120% ETc and fertigation with 60, 80, and 100% RDF recorded significantly higher oil yield than the control treatment. Drip irrigation led to 51.4 and 135.0% higher oil yield than FIRB and sprinkler irrigation, respectively. Drip irrigation at 120% ETc with fertigation at 80% RDF resulted in 25.4% higher oil yield, 10.1% higher biophysical water productivity, 27.1% higher economic water productivity, and saving of 34.4% IW and 20% fertilisers than the control treatment.