A comprehensive study of planting density and nitrogen fertilization effect on dual-purpose hemp (Cannabis sativa L.) cultivation

Harvesting hemp (Cannabis sativa L.) for both stems and seeds is now a common practice in Europe while crop management strategies for dual-purpose hemp cultivation have not been properly addressed so far. In the present study, the effects of planting density and nitrogen fertilization on hemp stem and seed yields were tested with the cultivars Futura 75 and/or Bialobrzeskie in eight contrasting environments (Italy in 2013; Italy and Latvia in 2014; Italy (two sites), Latvia, the Czech Republic, and France in 2015). Stem yield ranged between 1.3 and 22.3 Mg ha−1. The effects of planting density and nitrogen fertilization on stem yield did not interact significantly with each other, or with cultivar and harvest time. Increasing planting density from 30 to 120 plants m−2 and increasing nitrogen fertilization rate from 0 to 60 kg N ha−1 increased stem yield by 29% and 32%, respectively. Further increase in planting density and nitrogen fertilization did not result in a significant increase in stem yield. Seed yield ranged from 0.3 to 2.1 Mg ha−1. The seed yield was not affected significantly by planting density between 30 and 240 plants m−2. Although the seed yield showed an increasing trend with increasing nitrogen fertilization, the effects of nitrogen fertilization on seed yield were not statistically significant. To grow hemp as a dual-purpose crop it is recommended to plant 90–150 plants m−2 across all tested environments. Nitrogen fertilization rate at 60 kg N ha−1 was generally sufficient in the tested environments whereas further optimization of nitrogen fertilization requires accurate assessment of plant nitrogen status. To facilitate assessing plant nutritional status, a critical nitrogen dilution curve was determined for hemp and a practical method to determine nitrogen nutritional status was discussed.