Introduction
There has been increasing interest in cannabinoid hemp (Cannabis sativa <0.30% total THC) production since hemp was reclassified as part of the 2018 Farm Bill. Cannabinoid or floral hemp is grown for a high concentration of cannabinoids found in female flowers, with the main cannabinoid of interest being cannabidiol (CBD). There is interest in floral hemp production in North Carolina, especially in an organic production setting.
In this study we compared different organic nitrogen fertility sources and bedding systems to understand their effect on plant-available nitrogen throughout the season, cannabinoid composition, and floral hemp biomass.
Field Studies
Field trials were conducted in Clinton and Kinston, North Carolina, in 2021, and in Clinton, Kinston, and Salisbury, North Carolina, in 2022. We evaluated three different species of leguminous fall-planted cover crops and two fertilizer control treatments as nitrogen sources. The three leguminous cover crop species used were crimson clover, hairy vetch, and Austrian winter pea. Cover crops were planted in early November 2020 and mid-October 2021 and terminated in early- to mid-May each year. We included two nitrogen fertilizer controls: 150 lb N/ac (15-0-2, micropelletized Allganic Nitrogen Plus, SQM Specialty Plant Nutrition, Atlanta, GA) and a control of 0 lb N/ac. The 150 lb N/ac control was based on prior floral hemp research (AG-914, Establishing Nitrogen and Potassium Fertilizer Rates for Floral Hemp Production) that found this amount to be adequate for cannabinoid hemp production. Each of these nitrogen treatments was compared in two bedding systems: bare ground with drip irrigation and plasticulture (white polyethylene mulch) with drip irrigation. We used asexually propagated clones of the high-CBD cultivar BaOx in all locations and years.
In early May, cover crop biomass was sampled prior to termination to quantify total potential nitrogen contribution for hemp production. Cover crop species differed in their maturity at the time of termination: crimson clover was at 90% floral senescence and hairy vetch and Austrian winter pea at 50% floral senescence. The cover crops were terminated using a rotary tiller and left to partially decompose for two weeks before bedding and transplanting. We also applied potassium (90 lb/ac) and boron (1 lb/ac) prior to bedding.
We collected soil samples throughout the season to measure plant-available nitrogen. Plants were harvested at five weeks after floral initiation. Plant height and width were measured at harvest then the plants were dried in a tobacco bulk barn and bucked to quantify extractable biomass. Finally, we submitted representative biomass samples from each plot for cannabinoid analysis.
The available nitrogen results were analyzed by year and location because these data were collected at different times. Harvest measurements, yield, and cannabinoid data were pooled and analyzed with environment (the unique location × year factor) as a random effect to broaden our inference space.
Results
Cover crop species differed in their nitrogen content and C:N ratio. Austrian winter pea and hairy vetch had higher nitrogen content and lower C:N ratios than crimson clover (Table 1). Average biomass for the cover crops was similar among species at 2,953 lb/ac for crimson clover, 2,468 lb/ac for hairy vetch, and 2,522 lb/ac for Austrian winter pea (Table 1). The biomass for all cover crop treatments was lower than what can be produced for the region. For example, Parr et al. (2011) reported biomass yields of 5,085 and 4,817 lb/ac for hairy vetch and crimson clover, respectively, in North Carolina. We believe that yields could be improved with better synchrony of cover crop harvest date and cover crop maturity.
Crimson clover is likely not a good candidate for a cannabinoid hemp system as this cover crop reached seed production and senescence by the time we started field preparation. Leguminous cover crops should ideally be at 75% full bloom at the time of termination to maximize biomass production and avoid seed production. As cover crops continue to mature, the C:N ratio increases and, importantly, the amount of available nitrogen decreases as it is remobilized to seed production. Despite lower-than-expected biomass, cover crops still contributed meaningful quantities of nitrogen to the soil in this study.
| Cover Crop |
Biomass [w] (lb/ac) |
Nitrogen Content (%) |
Total N lb/ac |
Carbon Content (%) |
C:N Ratio |
|---|---|---|---|---|---|
| Crimson Clover | 2953 | 1.7 b [x] | 49.8 | 42.1 | 32.9 a |
| Hairy Vetch | 2468 | 2.9 a | 62.2 | 43.2 | 17.7 b |
| Austrian Winter Pea | 2522 | 2.7 a | 58.7 | 42.1 | 17.9 b |
| p-value | 0.5201 | <0.0001 | 0.4910 | 0.0756 | <0.0001 |
[w] Prior to cover crop termination, biomass from each fertility treatment was sampled. Crimson clover was sampled at 90% floral senescence; hairy vetch and Austrian winter pea were taken at 50% floral senescence in 2021 and 2022. ↲
[x] Means followed by the same letter within a column are not significantly different (p > 0.05) and represent one sample × four replicants × five environments (n = 20 data points per mean). ↲
Total available nitrogen (TAN) is an important measure of how much nitrogen in the soil is available to crops and is impacted by many environmental factors, including weather, soil type, and nitrogen source. Throughout the study TAN was impacted by fertility treatment (cover crop, controls of sufficiency N, or 0 N), bedding treatment (plasticulture or bare ground), and time. We observed similar trends in TAN across years and locations and will thus focus only on Clinton and Kinston 2021 results for the sake of simplicity. Plasticulture in Clinton and Kinston extended nitrogen availability later into the season and the 150 lb N/ac provided the highest soil available nitrogen content (Figure 1).
The 2021 season was marked with heavy rainfall events, which likely resulted in nitrogen leaching. Plastic mulch can help maintain nitrogen in the root zone by shedding rain water. When extreme rain events are absent, less nitrogen is lost to leaching, and consequently differences between plastic mulch and bare ground are minimized.
Differences in plant height and width were observed at the end of the season (Table 2). No height differences were observed among fertility treatments within the plasticulture treatment. In the bare-ground treatment, the 150 lb N/ac treatment resulted in the shortest plants compared to the hairy vetch and 0 lb N/ac treatments. In the bare-ground system, plant width treatment did not differ, but in the plasticulture system, the 0 lb N/ac and 150 lb N/ac treatments had the largest difference in width (Table 2). There were few consistent trends in plant growth with treatment, but plasticulture tended to produce wider plants.
| Bedding Treatment | Fertility Treatments | Response [y] | |
|---|---|---|---|
| Height (cm) | Width (cm) | ||
| Bare ground | 0 lb N/ac | 86.7 abz | 92.5 e |
| 150 lb N/ac | 76.7 c | 92.9 de | |
| Crimson Clover | 84.5 bc | 93.3 cde | |
| Hairy Vetch | 91.5 ab | 108.2 bcde | |
| Austrian Winter Pea | 85.3 bc | 94.7 cde | |
| Plastic | 0 lb N/ac | 90.0 ab | 108.0 bcde |
| 150 lb N/ac | 95.5 ab | 136.2 a | |
| Crimson Clover | 90.2 ab | 115.0 bcd | |
| Hairy Vetch | 97.8 a | 124.8 ab | |
| Austrian Winter Pea | 93.5 ab | 115.0 bc | |
| p-value | <0.0001 | 0.0001 | |
[y] Height and width were measured on three random plant subsamples per plot approximately five weeks after floral initiation. Height was measured from the base of the plant to the apical meristem. Two widths were taken; the first was measured from the widest apices of the plant and the second measurement was perpendicular to the first. These widths were averaged together to give the numbers seen in the table. ↲
Z Means followed by the same letter within a column are not significantly different (p > 0.05) and represent one sample × four replicants × five environments (n = 20 data points per mean). ↲
The highest floral hemp yield was in the plasticulture control treatment of 150 lb N/ac at a dry weight of 464.2 g/plant and the plasticulture hairy vetch treatment at a dry weight of 367.5 g (Figure 2). These two fertility treatments were not statistically different. In bare-ground treatments, there was no difference in yields among fertility treatments, which were similar to the 0 lb N/ac treatment in the plasticulture bedding treatment.
Finally, bedding and fertility treatments had no impact on CBD and THC concentrations, and these concentrations were in line with legal limits and established expected cultivar ratios of CBD:THC. Average total CBD content was 5.23%, total THC content was 0.23%, and CBD:THC ratio was 20.5:1. These results are in line with other studies demonstrating that cannabinoid synthesis is largely driven by genetics, while time, environment, and cultural practices have little to no impact. Please see Extension publication AG-937, Avoiding a "Hot" Crop: Minimizing the Risk of Non-compliant THC Tests through Proper Harvest Timing, which details hemp’s unique biochemistry as it pertains to cannabinoid synthesis.
Figure 1. The influence of bedding × fertility treatments × time interaction on soil total available nitrogen (TAN) at Clinton (p < 0.0001; A, B) and Kinston (p < 0.0001; C, D), 2021. TAN was calculated as the sum of soil nitrate and ammonium nitrogen. Means followed by the same letter are not significantly different (p > 0.05).
Figure 2. The influence of bedding and fertility treatments on the dry biomass yield of floral hemp. Dry biomass was calculated by averaging end-of-season bucked samples. Results include combined data; years × location.
Means followed by the same letter are not significantly different (p > 0.05) and represent the average of three subsamples × four replicants × five environments (n =60).
Key Takeaways
Nitrogen is dynamic in its cycling and availability. Several different environmental factors can influence its availability to plants and plant uptake. In the two years of this study, we saw variability in how bedding system and fertility treatment impacted nitrogen availability in soil.
The fertilizer treatment of 150 lb N/ac consistently provided the most TAN in both bedding systems throughout our study. The impact of plasticulture versus bare ground seemed to be more variable in terms of impact on nitrogen availability. We did observe that in some instances, however, plasticulture extended the availability of nitrogen. In this study we also observed that depending on the year and location, the in-season effects of bedding treatment and fertility source on TAN were variable.
The highest biomass yields were produced under the 150 lb N/ac control in the plasticulture. Despite showing a less clear advantage in in-season nitrogen availability, this treatment had significantly higher yields than all other treatments, except for the hairy vetch in a plasticulture system. The lowest yields were in the 0 lb N/ac plots and crimson clover in the bare-ground system. Overall, crimson clover did not perform as well as Austrian winter pea or hairy vetch as a nitrogen source. As mentioned earlier, the timing of crimson clover’s maturity may not be ideal for a cannabinoid hemp production system.
Hairy vetch showed promise in its ability to contribute to in-season nitrogen in a plasticulture system. Biomass yields from hairy vetch were 79% that of the 150 lb N/ac control. It is important to emphasize that though they are numerically different, we cannot confidently say these yields were statistically different. Ultimately, these results indicate that hairy vetch in combination with plasticulture may be a viable fit for cannabinoid hemp production as a full or partial external nitrogen replacement.
References
Parr, M., Grossman, J. M., Reberg-Horton, S. C., Brinton, C., and Crozier, C. 2011. Nitrogen Delivery from Legume Cover Crops in No-Till Organic Corn Production. Agronomy Journal, 103(6), 1578–1590. ↲
Publication date: Aug. 19, 2024
AG-971
N.C. Cooperative Extension prohibits discrimination and harassment regardless of age, color, disability, family and marital status, gender identity, national origin, political beliefs, race, religion, sex (including pregnancy), sexual orientation and veteran status.