A Review on the Current State of Knowledge of Growing Conditions, Agronomic Soil Health Practices and Utilities of Hemp in the United States

Abstract

Hemp (Cannabis sativa L.) is an emerging high-value specialty crop that can be cultivated for either fiber, seed, or cannabidiol (CBD). The demand for hemp and its products has been consistently on the rise in the 21st century. The United States of America (USA) has reintroduced hemp and legalized its production as an agricultural commodity through the 2018 Federal Farm Bill. Although there is a renewed interest in the adoption of hemp due to the emerging market, its production in the United States (US) remains limited partly because of unclear agronomic guidance and fertilization recommendations. This review article provides information on the current agronomic management practices that are available in the literature and identifies the future research needs for cultivating this multipurpose crop to address the growing market demands. Hemp production could be beneficial if managed properly. Hemp fertilizer requirements vary in accordance with the type of hemp grown (seed, fiber, or CBD), soil, environmental conditions and requires a wide range of macro- and micronutrients. Integrating management practices in hemp cultivation intended to build soil health is promising since the hemp cropping system is suitable for crop rotation, cover cropping, and livestock integration through animal waste applications. Hemp also has significant environmental benefits since it has the potential to remediate contaminated soils through phytoremediation, convert high amounts of atmospheric CO2 to biomass through bio-sequestration, and hemp biomass for bioenergy production. This review identifies that most of the agronomic research in the past has been limited to hemp fiber and, to some extent, hemp seed but not CBD hemp. With the increase in the global markets for hemp products, more research needs to be conducted to provide agronomic guidelines for sustainable hemp production.

Industrial Hemp (Cannabis sativa subsp. sativa) as an Emerging Source for Value-Added Functional Food Ingredients and Nutraceuticals

“…The cultivation of industrial hemp is more efficient and less environmentally degrading than that of many other crops [ 5 ]. Hemp can be grown under a variety of agro-ecological conditions and has a capacity to grow quickly, especially after the first 4–5 weeks after emergence, making it an excellent candidate for carbon sequestration [ 4 , 5 , 23 ]. Hemp grows best in sandy loam with good water retention and drainage at temperatures between 16–27 °C, in nutrient balanced soil (especially nitrogen, phosphorus, potassium, magnesium, copper, and others).…”

“…The planting density depends on the type of crop. Fiber hemp does well in high density to encourage stalk growth, but oilseed and CBD hemp should be planted farther apart to encourage greater branching and flower yields [ 23 ]. Densely seeded fiber varieties may reach 5–6 m tall, while some recent grain varieties may only reach 1–1.2 m tall.…”

“…Many multiple-use or resin cultivars are intermediate in height. Industrial hemp is either harvested for the stalk or seeds, whereas the flowering buds are collected from the narcotic type cultivars [ 18 , 23 , 24 , 25 ]. Selection for a specific final product (fiber, seeds, or products from the inflorescences) is reflected in the plant architecture of available varieties and clones [ 14 ].…”

“…As a fiber crop, hemp provides a high yield; it produces 250% more fiber than cotton and 600% more fiber than flax, from the same acreage [ 5 , 21 ]. Due to the fast-growing, dense canopy, fiber hemp is a natural weed suppressor and could be grown without herbicides; it also suppresses levels of fungi and nematodes in the soil and can be grown without fungicides or pesticides [ 5 , 21 , 23 , 24 ]. Hemp contributes to the maintenance of soil quality by its anchored roots, which prevent soil erosion and nutrient leaching, may extract nutrients from deeper soil layers, and are effective for phytoremediation by absorbing heavy metal contaminants from the soil and storing them within the plant.…”

“…The continual shedding of leaves through the growing season adds moist organic matter to the soil [ 1 , 4 , 21 ]. Because of the functions in improving the soil quality, hemp is a prime candidate to be used for crop rotation programs to improve the yield of the main crop [ 23 ]. Despite the historical functionality of this multi-purpose crop, global hemp production declined in the 19th century, and still only comprises about 0.5% of the total production of natural fibers [ 27 ].…”