No research has been conducted on the nutritional needs of hemp grown in Wisconsin in 75 years. In order to develop nutrient application guidelines for hemp, available peer reviewed literature as well as conference abstracts/proceedings/posters, research station reports, and extension bulletins from the United States, Canada, and Europe were reviewed and evaluated for their applicability to Wisconsin soils and climate.
Hemp can be grown on soils that are productive for corn. Poorly drained or poorly structured soils are not ideal because it may be difficult to establish a stand. Sandy soils may require irrigation to maximize yield (Purdue Univ.).
None of the research reviewed evaluated the effect of soil pH on yield. Because hemp is a non-leguminous oilseed, assume that the minimum soil pH for good growth is 6.0, similar to flax.
Phosphorus and K fertilizer application should be based on soil test results. Application rates are provided in Table 1. Research in Manitoba demonstrates that relatively small quantities of K are removed in the seed or with field retted fiber, but more than 185 lb K2O/a is taken up by the crop (Heard et al., 2007).
In past North American research, hemp N need has varied from 0 to 175 lb N/a (Aubin et al., 2015; Purdue Univ., 2018; Vera et al., 2010). Various research reports have noted that too much N may delay seed maturation or negatively affect fiber quality. With this in mind, growers should consider using 100 to 140 lb N/a on soils with less than 10% organic matter (OM); 75 lb N/a on soils with 10 to 20% OM; and no N is needed on soils with more than 20% OM.
Hemp is sensitive to fertilizer placed with the seed. If starter fertilizer is used, be sure to place fertilizer at least 2 inches to the side and 2 inches below the seed. Nitrogen fertilizer timing and placement studies have not been conducted. Research in Manitoba demonstrates that N uptake is relatively low until late June then increases substantially through the end of July (Heard et al., 2007).
In Saskatchewan, Vera et al. (2010) did not find any biomass or seed yield response to S applications on fields where canola had been deficient in S in previous years. The authors suggest that hemp may have either a low S need or is efficient at taking up S from the soil. Sulfur applications to hemp grown in Wisconsin may not be profitable. Only consider applying S if growing hemp on a field with a past history of S deficiency. Fields with a recent history of manure application are less likely to be deficient in S. If needed, 10 to 15 lb S/a should be adequate.
Growers should reduce fertilizer application rates to adequately account for manure applications or a previous forage legume crop. At this time, it is unknown if there is a N credit rotation benefit to growing hemp following soybean versus corn.
Thanks to Laura Good for her assistance with the literature review.
Aubin, M-P, P. Seguin, A. Vanasse, G.F. Tremblay, A.F. Mustafa, and J-B. Charron. 2015. Industrial hemp response to nitrogen, phosphorus, and potassium fertilization. Crop Forage Turfgrass Management. DOI: 10.2134/cftm2015.0159 p. 10.
Mooleki, S.P., R. McVicar, C. Brenzil, K. Panchuk, P. Pearse, D. Hartley, A. Hanks, and K. Friesen.2006. Revised by D. Risula, 2017. Hemp production in Saskatchewan. Government of Saskatchewan. http://publications.gov.sk.ca/documents/20/84152-Hemp%20Production.pdf
Heard, J. K. Watson, and J. Kostiuk. 2007. Nutrient uptake and partitioning by industrial hemp. Manitoba Agronomists Conference Proceedings 2007. http://www.umanitoba.ca/faculties/afs/MAC_proceedings/proceedings/2007/Heard_poster.pdf
Purdue University. 2018. Hemp project. https://dev.purduehemp.org
Vera, C.L., S.S. Malhi, S.M. Phelps, W.E. May, and E.N. Johnson. 2010. N, P, and S fertilization effects on industrial hemp in Saskatchewan. Can. J. Plant Sci. 90: 179-184.