In addition to deep soils and adequate moisture, hemp requires substantial available nutrients to produce high biomass yields. Even the earliest investigators in the United States and Europe noted that only soils maintained in a high state of fertility produced good crops of hemp (Anon.,1890; Dewey, 1901 and 1913; Dempsey, 1975; Van der Werf, 1991). When rapid expansion of hemp production became necessary in the United States during World War II, very little information existed on the use of fertilizers on the crop, and several researchers began trials to determine the optimum fertilizer management in the Midwest. Hemp showed a good response to nitrogen, some response to phosphorus, and minimal or negative response to potassium in Iowa (Black and Vessel, 1945). Moderate yield increases from increased nitrogen levels were reported in Illinois and Wisconsin, but fiber quality was poor and increased nitrogen rates resulted in decreased fiber strength (Howard et al., 1946). In Illinois, nitrogen fertilizers increased stem and retted fiber yield, but produced coarse, weak fiber (Hessler, 1947).
These trials have led to the popular belief that no fertilizer is required for hemp production, however, these trials were carried out on highly productive soils in the U.S. corn belt and did not account for nutrients available in the soil. Adequate nitrogen should be supplied to allow rapid growth of hemp while avoiding excessive levels which seriously affect fiber quality.
Research on fertilizer requirements of hemp have continued for years in many countries, and a summary of fertilization rates used in various countries is presented in Table 3 (Dempsey, 1975; Van der Werf, 1991 and 1992; Kozlowski et al., 1995; Low, 1995a).
Over a wide range of soil and environmental conditions, hemp has been shown to require liberal fertilization for maximum production. Although nutrient uptake by hemp is high, a substantial portion of withdrawn nutrients are returned to the soil as leaves and roots, since only the stems are removed from the field. If the crop is retted in the field, nearly all soluble nutrients are washed into the soil during retting (Dewey, 1913).
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