The merits of hemp as an industrial crop in America are frequently argued on the internet. Advocates tend to paint hemp as a wonder-plant, capable of feeding and clothing the masses for a fraction of the cost and environmental impact of existing technologies. While hemp has its virtues, other plants with similar properties occupy most of the commercial niches it could fill, and the fiber industry in place today has infrastructure that is not cheaply compatible with hemp fiber.
The superiority of hemp fiber in both quantity and quality is often touted. Figures on the length of hemp primary fibers vary, ranging from an average of 8 to 80 inches (20-200cm) in length. Fiber length varies across strains, but figures nearer the shorter end of the spectrum predominate. Individual cellular fibers, which are bundled to form the long primary fiber, vary from 0.19 to 2.16 inches (0.5-5.5cm) in length and from 16 to 50 microns in diameter. For comparison, the similar flax primary fiber ranges from 6 to 40 inches (15-100cm), with cells from 0.43 to 1.49 inches (1-3.75cm) in length and 11 to 20 microns in diameter.
Hemp fiber bundles are longer than those of flax, but flax fiber generally contains less lignin and is therefore more flexible and makes finer fabric. The characteristics of these two fibers overlap and the best hemp can be superior to flax for fine fabric. Although botanically unrelated, flax and hemp have many characteristics in common. Without microscopic or chemical examination, their fibers can only be distinguished by the direction in which they twist upon wetting: hemp will rotate counterclockwise; flax, clockwise. The yield, strength and quality of either fiber are highly dependent on the seed variety, the conditions of growth, time of harvest and manner of retting and other post-harvest handling. Both plants produce very similar oils in their seed, oils with a high percentage of linolenic acid, used until mid-century in paints.
Hemp's primary fibers are produced in the bark, known as the bast, and makes up 25% of the dried stem of the plant. The remaining 75%, the hurd, is composed of short fibers and is unsuitable for paper production. It is however about twice as absorbent as wood shavings and can make good animal bedding.
A 1938 Popular Mechanics article erroneously stated that the woody core of hemp was 77% cellulose. Scientific and technical literature indicates that the cellulose content of hemp's core ranges from 30-40%. The difference is substantial when pulp efficiency is evaluated. This incorrect claim has been repeated and reprinted widely.
In order to be used in existing US papermaking equipment, hemp fiber, which is longer and stronger than tree fiber, must be preprocessed in special fiber shortening machines, which would be a prohibitively expensive retrofit for papermills. However, recent research indicates that non-retting processing of hemp and kenaf can provide fiber that can be processed in current paper making equipment.
Hemp advocates have pushed hemp as the solution to the American fiber shortage, a shortage which does not exist. The wood products industry recognizes that eliminating forests would be a bad business practice, and practices sustainable forestry, ensuring that their wood fiber supply will continue. Indeed, since the 1940's forest growth has exceeded harvest by an average of 33%.
Before hemp was banned in the US, it produced around 900 pounds per acre. High yield plants in Italy have produced up to 1.2 tons of paper quality fiber per acre per year. Pine plantations in the US easily produce two tons per acre per year, and some fast growing hardwoods produce up to 6 tons per acre per year.
While hemp demands much from the soil while growing, it is estimated that more than two-thirds of the nutrients are returned to the soil during dew-retting. However, hemp fields are necessarily laid bare for part of the year, resulting in erosion rates as high as 3.5 tons per year. Pine forests producing more fiber per acre loose between one quarter to one half ton per year (both figures depending on local conditions and cultivation practices).
Hemp advocates suggest that hemp fibers make superior archival quality paper. However, the quality of paper is mostly dependent on the pulping process used in its production. Wood, hemp, kenaf, cotton, flax and many other natural fibers can be used to produce high quality paper, with or without the use of chlorine bleaching agents, which have in recent years come under attach from environmental groups.
Data gathered from numerous non-hemp advocacy sites around the internet. The figures on hemp fiber length varied widely. The fiber per acre and erosion figures came mostly from large pulp producers with vested interests in tree fiber, but also in high profits. Multiple documents provided similar values for the figures.