Pubdate: 1969
Source: The World's Major Fibre Crops - their Cultivation and Manuring
Author: Prepared by Josef Berger, Agronomist with the Centre d'Etude de l'Azote
Pages: 216-222

Hemp (Cannabis sativa)

Hemp (Cannabis sativa) is a member of the family Cannabinaceae, a small group of herbaceous plants. In all probability, the original home of the hemp plant was Central Asia, whence it spread to China. It is one of the oldest cultivated plants and is thought to have been growing in China for more than 4,500 years. The genus Cannabis includes many varieties, strains, types or lines, which differ from one another in height, colour, extent of branching of the stalk, size of leaves and other characteristics.

There are at the present time three main groups of varieties under cultivation:

1. Varieties cultivated primarily for their fibre. These have very long stalks and branch very little.
2. Varieties grown for the seed, which is used as a source of oil. They are usually short in stature and mature early. The seeds contain up to 32 to 35 per cent of a greenish-coloured drying oil.
3. Varieties grown throughout India, North Africa and the Middle East for their medicinal and narcotic properties. They are short in stature, much-branched and have small, dark-green leaves. The crushed, dried inflorescences, stalks and leaves produce a resinous juice from which the drug is obtained. The drug is used in medicine for the treatment of various complaints and as a narcotic, being known in many forms such as "hashish", "ganja" and "bhang".

The oldest use of the hemp plant seems to have been for fibre, but it is known to have been cultivated for medicinal purposes as early as 900 to 800 B.C.

At the present time, hemp is grown for its fibre in temperate regions, whereas in tropical regions it is grown for drug production.

The economic importance of hemp fibre production has fallen off sharply since the Second World War. However, hemp still holds a certain position today as a valuable fibre crop. It is a fibre that is strong and durable, stiffer and more rigid than flax, lacking flexibility and elasticity, but unaffected by water. The fibre will not bleach easily and is not used for fine textiles, but is employed mainly in the manufacture of ropes, twines, sacking, carpets, nets, tarpaulins and webbing. In addition, hemp also serves today as a raw material for the paper industry. Hemp has a cellulose content of about 67 per cent and contains about 16 per cent hemicellulose.

The Principal Hemp-Producing Countries

By far the most important producer is the Soviet Union, which accounts for approximately one-third of world production. The three main producing areas are the region between Gorki, Penza and the Oka River, the region between Orel, Kursk, Kiev and Gomel, and the region near the Polish border. Nearly all the hemp produced in the U.S.S.R. is used locally. The Soviet Union is, therefore, not normally a major exporting country.

Yugoslavia, Hungary, Poland and Rumania come next in importance. Hemp production in Italy and France has been falling consistently since the Second World War, due to the rise in the cost of production, which has meant that the relatively high price of hemp has made it more profitable for manufactures to use alternative fibres.

Outside Europe, reference should be made to Turkey and Korea as producers of hemp. In China hemp has been grown for hundreds of years. The distribution of hemp fibre production is shown in Table 16.

West Germany and France are by far the most important buyers of hemp fibre.

The yield obtained per hectare varies according to climatic conditions, soils and agricultural techniques employed.

The weight of the dried stalks of hemp usually amounts to between 2 and 5 tons per hectare, and yield of fibre is about 25 per cent that of the dried stalks. The average world yield is about 500 kg per hectare.

The Hemp Plant and Its Cultivation

Botany
Hemp is an annual, herbaceous plant with a slender stem, ranging in height from 1.2 to 5 m and with a diameter from 4 to 20 mm. The stem is more or less branched, depending on the conditions of cultivation. When the plants are sown closely together, the stems do not branch. For fibre production, therefore, the seed should always be sown thickly. The leaves of the hemp plant are of a compound palmate type and each leaf has 7 to 11 leaflets, pointed at both ends, with serrate margins, dark green on the upper surface but lighter underneath. The root formation of the hemp plant varies considerably according to local conditions. The strong tap-root can penetrate far down into deep soil; if the soil conditions are unfavourable, however, the main root remains short, while the lateral roots become more developed. Although this root system is in itself strong, it must be regarded as relatively weak in comparison with the extent and speed of growth of the hemp plant above ground.

The hemp plant is naturally dioecious, the staminate or pollen-bearing flowers and the pistillate or seed-bearing flowers being borne on separate plants. In any one field, male and female plants occur in roughly equal numbers, the tendency being towards a somewhat higher proportion of females. The male and female plants are much alike, except for their flowers and the presence of seeds on the female plant only. There is, moreover, no apparent difference in the fibre, provided the crop is harvested at the right time. Dioecism considerably complicates the cultivation of hemp, since the male plants grow more quickly and are thus too mature when harvested, which results in harsh and brittle fibres. The female plants do not overtake the male plants in height until after flowering, and the seed then takes two or three weeks more to ripen. When hemp is grown for its fibre, the male and female plants are harvested together. Where both fibre and seed are to be used, however, the male plants are collected first (being pulled out by hand), and the female plants are left for a further two to three weeks or more to enable the seed to ripen.

Hemp breeders have, therefore, been at great pains to eliminate this inconvenient characteristic in hemp plants, viz. their dioecism. Since the Second World War in particular, new, monoecious varieties have been developed, possessing both male and female blossoms which flower at the same time.

The hemp fibre is a bast or phloem fibre obtained from the stems of the plants. The fibre bundles which lie under the epidermis and the cortex, occur as a ring in the phloem parenchyma. The number of fibre bundles varies from stem to stem and in different parts of the stem; there may be as few as 15 bundles or as many as 35. Each fibre bundle contains between 10 and 40 individual fibre cells. The number of cells in the bundle and the size of the individual fibre cells depend on the position of the bundle in the stem. The cambium is the next layer, which separates the bast (i.e. the phloem / or food-conducting tissue containing the sieve cells) from the woody core of the stem. The cambium layer is followed, first, by the woody core, consisting of thick and short woody cells which support the plant during its growth, and finally the innermost layer or pith: This has at its centre the pith cavity.

The taller the stem of the plant, the longer the fibre will be, and the greater will be the yield of fibre per plant. Anything that increases the length of the stem is advantageous from the point of view of commercial production.

Cultivation

Climate and soils. Hemp varieties planted in the temperate zones fall into two groups, namely the northern and the southern varieties. The latter require high temperatures and a long vegetative period, and consequently grow taller and yield more fibre.

In general, the hemp plant requires a mild, temperate climate, a humid atmosphere and an annual rainfall of at least 700 mm. Droughts are damaging, especially while the seed is germinating and during flowering. Although the plant can endure considerable changes in temperature, frost for any long period will destroy young plants.

The hemp plant makes heavy demands on the soil. Because its root system is relatively weak and sensitive, the supply of suitable nutrients must be readily accessible. This means that in addition to a large quantity of easily assimilable nutrients, the hemp plant needs a sufficiently deep, well aerated soil with a favourable, almost neutral reaction, as well as a regular water supply. It grows best on rich and fertile, neutral or slightly alkaline, well drained clay-loam or silt-loam soils in which the subsoil is fairly retentive of moisture. Acid, sandy soils, heavy clays and soils that dry out quickly are not suitable.

Rotation. Although the hemp plant can be grown on the same land for several years in succession, rotation with other crops is considered desirable. In Italy, for example, it is grown on the same field year after year, apparently without any harmful effects on the yield. None the less, hemp responds well to a good preceding crop such as wheat or potatoes or to temporary pasture.

Preparation of seedbed. Proper preparation of the ground before sowing is essential if a good crop is to be obtained. In the temperate regions of Europe, the land is usually ploughed in the autumn to a depth of about 20 to 25 cm, and repeatedly harrowed. In the following spring, the land is harrowed again and rolled to make a fine, uniform tilth over the whole field. In the southern part of Europe, green-manure plants are very often sown immediately after the first ploughing in the autumn. In January or February, a second ploughing is then carried out to turn these grasses or legumes under as green manure.

Time and method of sowing. Due to the risk of late frosts, sowing in the northern regions of Europe does not usually take place until the end of April or the beginning of May, i.e. only in sufficiently warmed soils. The seed will germinate at low temperatures, but not below 1°C. In Italy, on the other hand, hemp is sown as early as March. After sowing, the seed is covered by means of a light harrow, with not more than 2 to 3 cm of soil. In many growing areas, rolling the land after the seed has been sown is claimed to be beneficial. Today most of the seed is sown mechanically (by means of drills), the distance between the rows varying from 12 to 23 cm. If the distance between the plants is too great, excessive branching of the stems results, whereas if sowing is too dense there is over-much competition among the young plants and their development is retarded.

The quantity of seed per hectare varies with the type of fibre it is desired to produce. When fibre and seed are to be produced together, 60 to 100 kg of seed per hectare is usually sown in Central Europe. In France, 80 kg of seed per hectare is recommended for producing textile fibre, but only 65 kg per hectare for paper production. The seed weighs between about 1.5 and 2.5 grams per 100 seeds.

After sowing, hemp requires little cultivation, except for thinning if the seedlings come up too thickly. It grows rapidly and soon covers the ground, choking out the weeds. Weeding is generally only needed in the early stage of growth.

Harvesting. The time of harvesting depends very largely on climate and variety (i.e. whether the crop is being grown for fibre or for seed). In the temperate regions, hemp is normally ready for harvesting from four to five months after planting. In France, for example, it is harvested from the end of August to the beginning of October. In Italy, harvesting begins between the second half of July and the first week in August, according to the area and to the climatic conditions. As a general rule, hemp should be harvested between the time of flowering and the ripening of the seed, which gives a harvesting period of at least three weeks (the female plants take about three weeks to mature). In certain growing areas and particularly on small holdings, cutting is carried out by hand with a hemp knife, which resembles a long handled-sickle. The plants are cut off about 2 to 3 cm above ground level. After cutting, the stems are spread on the ground to dry. Although manual harvesting involves less risk of breaking the stalks, it takes up a great deal of time and is increasingly being replaced by mechanical harvesting. The main methods of mechanical harvesting are as follows :

1. The hemp is harvested with a specially modified cutter-binder. The sheaves are built up into shocks of 15 to 20 each. Once the leaves are dry, the sheaves are stored in barns and then taken to the factories where the seed is separated and the fibre extracted. This method of harvesting can only be used if the textile factories are equipped to take sheaves and not pressed bales. A further disadvantage of the method is that a large amount of seed is lost as a result of the harvesting
operations.

2. The hemp is cut by machine and laid in swathes, where it is left to dry for two to four days. The seed is then gathered by a combine harvester, and the straw is laid back in swathes where it is retted by moulds and bacterial action, induced by frequent rains or dews. Retting is usually completed in one to three weeks, depending on the weather. In moist, warm weather, the process is more rapid and may require only one week; in cooler or drier weather it is slower and may take a month. In this method of retting, often called dew retting, it is important to spread the straw evenly and thinly on the ground, as the dew should reach all the straw in order to ensure uniformity. To facilitate uniform retting, the straw is sometimes turned during the process.

The main disadvantage of dew retting is that it demands a great deal of labour and is very dependent on the weather. If continued wet weather prevents the straw in the field from being lifted at the proper time, it becomes over-retted and is of little value. In nearly all cases the product is not uniform, and the fibre must be sold at a price lower than that of water-retted hemp.

THE MANURING OF HEMP

Nutrient Uptake
The hemp plant makes relatively heavy demands on the available nutrient resources and tends to exhaust the soil, but much of the nutrient material, taken from the soil is put back into it after the plants are cut.

Hemp needs particularly large supplies of nitrogen. Its calcium and potassium requirements are however also very high. Nutrient uptake has been found to vary widely, depending upon the fertility level of the soil and environmental conditions.

Bredemann (1945) investigated the nutrients removed from the soil by a hemp harvest of 6,000 kg of leaf-free stalks and 700 kg of seed per hectare. His results are summarized in Table 17.

Another source (Scheel, 1936) gives the amounts of nutrients removed by hemp according to yield as follows:

N 85 to 145 kg per hectare
P2O5 32 to 55 kg per hectare
K2O 70 to 120 kg per hectare
CaO 130 to 215 kg per hectare
MgO 14 to 25 kg per hectare

These sets of figures give a general indication of the needs of the hemp crop that have to be met from the soil and from fertilizers.

Considerable research has been devoted to recording the course of nutrient uptake over the life of the plant (Becker-Dilligen, 1934; Scheel, 1936; Bredemann, 1945). Beginning early, the uptake is most intense during the first two months of growth, especially in the case of nitrogen and potassium. The great need for N continues until the end of the development. During flowering and fruit formation, the demand for potassium and in particular for phosphorus is very high. The volume of nutrients assimilated reaches its peak at the beginning of maturity. After this, considerable quantities of nutrients are returned to the soil by the plant, in particular with the large numbers of falling leaves, so that the removal of nutrients as a result of the harvesting of the largely leaf-free stalks and fruits involves only about half of the maximum uptake.

To achieve an optimum hemp yield, therefore, at least twice as much nutrient must be available in an easily assimilable form as will finally be removed from the soil by the leaf-free harvest. This is an important factor both in relation to the manuring of the hemp itself and in connection with the subsequent harvest.

Nitrogen plays a predominant part in the nutrition of the very bulky hemp plant. Hemp accordingly needs large applications of N, to which it responds well. The length and thickness of the stalks can be increased by high N rates, and, although it is true that the percentage of fibre in the stalk drops somewhat with the additional growth, the yield and length of the fibre both increase. On the other hand, unbalanced N manuring can have a negative effect on the tensile strength and the flexibility of the hemp fibre (Corodni, 1957; Corschkov, 1959). Heavy nitrogen rates should therefore be balanced by equivalent large supplies of other nutrients and in particular of potassium and phosphorus.

The main contribution of phosphorus to the achievement of an optimum yield lies in assisting ripening and in reducing any negative effect from high N rates.

Experiments by Scheel (1936) have shown that phosphate considerably affects not only yield but also quality, by reducing the diameter of the fibre cells.

Potassium is important both for increasing yield and for its influence on the quality of the fibre. Its main function is to thicken the cell walls, producing a greater tensile strength and a general improvement in the fibre. It has also been established that potassium promotes the formation of a strong stem and increases the ability of the roots to resist the dreaded parasitic plant Orobanche ramosa.

As has already been indicated, large quantities of calcium are essential for hemp (Neuer, 1953; Schropp and Arenz, 1938; Koch, 1935; Becker-Dilligen, 1934). Soils that are not already well supplied with calcium therefore usually require liming.

The hemp plant also needs a relatively large amount of magnesium. It is very sensitive to Mg deficiency. In typical cases of such deficiency, shoot and root development is retarded and the young leaves turn dark green, while in the older leaves a loss of chlorophyll results in greyish-white patches of discoloration.

Zhukov and Repyakh (1966) have established in experiments that stalk and fibre yields and fibre strength are reduced by large amounts of chloride and improved by increased applications of sulphate. When both SO4 and Cl were applied, the yield and strength of the fibre improved as the ratio of SO4 to Cl increased. The harmful effect of high rates of Cl was due to the accumulation of chlorine in the young leaves. This factor can have an influence on the forms of fertilizer chosen, especially in the case of potassium fertilizers (Baev, 1964).

The need for micronutrients is very closely linked with the type of soil. Copper deficiency is often to be found in peat soils, which in the case of hemp shows itself among other things in the breaking of stems. Manganese and boron deficiencies have also been reported (Gorschkov, 1959; Olofsson, 1956). According to Russian sources, the boron requirement of hemp is 270 grams per hectare (Zhukov and Bedek, 1963). Russian experiments on peat-humus soils (Getmanov, 1967) have further shown that where hemp is already fertilized with sufficient amounts of P and K, both yields and quality of fibre and seeds can be significantly increased by the additional application of 1 kg boron (as boric acid, H3BO3), 1 kg of copper (as copper sulphate, CuSO4 and 10 kg of manganese (as manganese sulphate, MnSO4) per hectare.

The Use of Mineral Fertilizers
In manuring, it must above all be remembered that hemp has to produce a very large bulk of plant material in a very short period of vegetative growth. The nutrients must accordingly be applied in good time and they must be available in an easily assimilable form. The following rates can be recommended for soils that are generally well supplied with P and K:

60 to 100 kg N per hectare
40 to 60 kg P2O5 per hectare
75 to 120 kg K2O per hectare

In addition, 20 to 40 tons of farmyard manure per hectare should be ploughed under in the autumn.

The recommendation for mineral fertilizers given above should be adjusted according to the amount of farmyard manure used, as well as any green manuring. Whether this should be supplemented by additional applications of calcium and possibly copper, manganese and boron will depend on local conditions. According to the situation (soil and climate) and the preceding crop, the application of nitrogen fertilizer takes the form of a single dressing prior to sowing or, with larger rates, is also partly applied as a top dressing three to four weeks after sowing. When the initial stage of growth of the hemp plant is slow, 20 kg N per hectare in the form of urea may also be applied as a foliar spray (dissolved in 400 litres of water).

The other nutrients, especially P and K, and possibly also calcium, are ploughed under in the autumn. Splitting these, i.e. with an initial application before ploughing and a second small dressing before or at sowing, is only effective with fairly large rates and on light soils.

It has already been mentioned that hemp is very sensitive to chlorine. The use of large quantities of fertilizers containing chlorine can accordingly have a bad effect on fibre yield and quality. Sulphur, on the other hand, even when fairly heavy rates are applied, has a good influence on the yield and quality of the fibre. The use of potassium sulphate is therefore strongly recommended in place of potassium chloride. Alternatively, the chloride may be dug in during autumn ploughing; it is then carried into the deeper layers of the soil by the autumn and winter rains.

References.

Bredemann, G. 1945. Untersuchungen uber die Nahrstoffaufnahme und den Nahrstoffbedarf des Hanfes. Bodenkde. und Pflanzenernahr. 36, 167-204,

Becker-Dillingen, J. 1934. Handbuch der Ernahrung der landw. Nutzpflanzen. Parey Verlag, Berlin.

Scheel, R. 1936. Einfluss der Dungung auf Ertrag und Faserausbildung des Hanfes. Ernahr. der Pflanze 32, 322-327.

Gorodni, N. 1957. Grundungung zu Hanf. Hanf und Flachs 2, No. 5, 28 - 30.

Gorschkov, P. A. 1960. Pecularities of the influences of ammonia and nitrate forms of nitrogen on hemp. Visnyk. sil'skogo-spod. Nauky 3, No. 10, 35-40.

Neuer, H. 1953. Hanf (Cannabis sativa L.) Handbuch der Landwirtschaft 2, 551-561. Parey Verlag, Berlin.

Schropp, W. and Arenz, B. 1938. Uber die Wirkung des Bors auf das Wachstum einiger Ol- und Gespinstpflanzen. Forschungsdienst
6, 564-574.

Schropp, W. and Arenz, B. 1938. Uber den Calcium- und Magnesiummangel bei einigen Ol- und Gespinstpflanzen. Bodenkde. und Pflanzenernahr. 12, 32-45.

Olofsson, S. 1956. Tillforsel av koppar och mangan till kalkrika organogend jorda. Kungl. Lantbrukshogskolan Statens Lantbruksforsok, Statens Jordbruksforsok, Medd No. 64. 175-220.

Zhukov, M.S. and Bedak, G.R. 1963. Boron fertilizer and its application on hemp. Len Konop. 8, 24-25.

Zhukov, M. S. and Repyakh, I. I. 1966. Reaction of hemp to chlorides and sulphates. Agrokhimiya No. 11, 86-90.

Gorschkov, P. A. 1959. Der Einfluss von Dungemitteln auf die Qualitat der Hanffaser. Arb. Unions-Forschungs-Inst. Faserpflanzenanbau 24, 77-95.

Getmanov, P.I. 1967. Effect of trace fertilizers during hemp cultivation of peat-humus soils. Khim. Sel. Khoz. 5, 412-413.

Koch, H. 1935. Der Anbau von O1- und Spinnpflanzen. Reichsnahrstand Verlag-GmbH, Berlin.

Repyakh, I, I. 1966. Effect of various forms of potassium fertilizers on productivity of a hemp rotation. Agrokhimiya No. 10, 53-59.

Lukashevich, E.S. 1966. Fertilizers for hemp. Agrokhimiya, No.9. 144-152.

Baev, K. H. 1964. Effect of some nitrogen fertilizers on yield and quality of hemp. Rast. Nauki 1, No. 2, 37-40. Opit. Stants. Poliv. Zemedel., Pazardzhik.

Federation Nationale des Producteurs de Chanvre, Le Mans. 1968. La culture du chanvre monoique non battu pour l'industrie du papier.