Cassava cultivation

The plant
The cassava plant is a perennial that grows under cultivation to a height of about 2 4 m. The large, palmate leaves ordinarily have five to seven lobes borne on a long slender petiole. They grow only toward the end of the branches. As the plant grows, the main stem forks, usually into three branches which then divide similarly. The roots or tubers radiate from the stem just below the surface of the ground. Feeder roots growing vertically from the stem and from the storage roots penetrate the soil to a depth of 50-100 cm. This capacity of the cassava plant to obtain nourishment from some distance below the surface may help to explain its growth on inferior soils.
Male and female flowers arranged in loose plumes are produced on the same plant. The triangular-shaped fruit contains three seeds which are viable and can be used for the propagation of the plant. The number of tuberous roots and their dimensions vary greatly among the different varieties. The roots may reach a size of 30-120 cm long and 4-15 cm in diameter, and a weight of 1-8 kg or more. The plant, its flowering shoot and its various parts are shown in Figures I and 2.
Clusters of root of the Bogor variety, ripe for harvesting, are shown in Figure 3. A cross section of the root is given in Figure 4. The peel consists of an outer and an inner part, the former comprising a layer of cork cells and the phellogen. The cork layer, generally dark-coloured, can be removed by brushing in water, as is done in the washers of large factories. The inner part of the peel contains the phelloderm and the phloem, which separates the peel from the body of the root. The texture of the transition layer makes possible an easy loosening of the whole peel from the central part, thus facilitating the peeling of the roots.
The cork layer varies between 0.5 and 2 percent of the weight of the whole root, whereas the inner part of the peel accounts for about 8-15 percent. Generally in ripe roots this is about 2-3 mm thick. The starch content of the peel is only about half that of the core. The peel is much firmer in structure, hindering a smooth rasping by primitive raspers; small factories prefer to peel the roots before working them up. The loss of starch incurred by rejecting the peel. however' is not acceptable to the larger factories. which remove only the cork layer.
Agricultural practices
PREPARATION OF THE LAND
When cassava is grown as the first crop in forest land. no further preparation is required than the clearing of the forest growth. When cassava is grown after other crops. it often can be planted without further preparation of the soil, once the preceding crop has been harvested or the soil has been ploughed two or three times until free from grass and other plants.
Clearing of forest land is done to let in more sunlight to the ground and to remove weeds and undergrowth which might otherwise compete with economic plants. The practice in tropical southeast Asia is to clear the forest soil completely, including the removal of all roots and other obstructions beneath the soil, by cutting and burning the forest cover; the land is then deeply ploughed. African practice is to burn the land cover only. Burning removes only small branches and underbush but does not consume all of the trunks and branches. It also destroys soil parasites, and the layer of ashes increases the amount of potassium salts available to the growing plants. However, some reports have indicated that complete clearing of the soil in certain parts of Africa caused deterioration due to the leaching out of nutrients.
PLANTING
Cassava culture varies with the purposes for which it is grown.
Cassava is either planted as a single crop or intercropped with maize, legumes, vegetables, rubber, oil palm or other plants. Mixed planting reduces the danger of loss caused by unfavourable weather and pests by spreading the risk over plants with different susceptibilities.
For agricultural purposes, cassava is propagated exclusively from cuttings. It is raised from seed only for the purpose of selection Seeds produce plants with fewer and smaller roots than those of the parents and as many as half of the seeds may fail to germinate. On the other hand, cuttings taken from the stalks of the plant take root rapidly and easily, producing plants identical in character with the parent plants.
FIGURE 2. Manihot utilissima Pohl.
FIGURE 4. Cross section of cassava root. Drawing by R. Soemarsono Slate Botanical Gardens. Bogor. Indonesia.
Cuttings are obtained from the stems of plants at least ten months old and 2.53.5 cm thick. After harvesting, these stems are stored in a dry place until the next planting (Fig. 5). Cuttings about 25 cm long should be taken from the lower 75150 cm of the stem after the first 20 em have been discarded. Cuttings from the upper part of the stem will grow faster, but their final yield is less. The best practice is to saw a bundle of stalks supported by a girder and then to point the cuttings thus obtained at the lower end (Fig. 6), taking care not to bruise the buds or otherwise damage the stem.
Experiments in the Philippines on the relation between the age of cuttings and yields showed that cuttings taken 75 cm or more below the apex of the stem gave the best starch yields. Other experiments concluded that older wood from the basal areas to the midpoint of the stems outyielded apical propagating material.
Key to Figure 4:
Left upper quadrant. after staining with iodine:
1. Peel
1a. Outer cork layer 
1b. Inner layer
2. Cambium 
3. Centre 
4. Pith and primary xylem
(Both peel and pith contain comparatively little starch.)
Right upper quadrant, showing structural elements of the root:
5. Cork 
6. Sclerenchymatous fibres
7. Latex vessels
8. Cambium
9. Xylem vessels
Inset A - Enlarged cross section of peel:
10. Cork tissue
11. Sclerenchymatous fibres
12. Starch (small grains)
13. Parenchyma cell
Inset B - Enlarged cross section of centre:
14. Cell wall (larger cells than in peel)
15. Starch ( big grains)
Cuttings are planted by hand or by planting machines. Hand planting is done in one of three ways: vertical, flat below the soil surface or tilted 45" from ground level. Under low rainfall conditions. vertical planting may result in the desiccation of the cuttings, while in areas of higher rainfall, flat-planted cuttings may rot. In general, flat planting 5-10 cm below the soil surface is recommended in dry climates and when mechanical planting is used. Germination seems to be higher; tubers tend to originate from a great number of points and grow closer to the surface of the soil, making better use of fertilizers applied on the surface and also making harvesting easier. On the other hand' vertical planting is used in rainy areas and tilted planting in semi-rainy areas.
The cuttings are planted on flat soil or on ridges or hills. Some experiments have shown ridging to produce somewhat lower yields than flat cultivation; but the work of weeding and harvesting is greatly reduced by ridge planting. As machine planting would be impossible with furrows or on ridges, flat fields are the most desirable. Spacing between rows is about 80100 cm, and the plants are spaced along the rows according to local conditions. The number of plants per hectare varies in different regions between 10000 and 15000.
FIGURE 6. Scheme of cassava stalk for planting
Time of planting is influenced by weather conditions and the availability of planting material. Cassava is usually planted at the beginning of the rainy season. In order to reduce risk and to distribute the hard work of cultivation more evenly, planting is sometimes divided between the two rainy seasons. It is usually carried out throughout the year in regions with year-round rainfall. If the stalks are saved for a long time after harvest, they are not apt to root and grow well. It is therefore desirable to plant and harvest at the same time.
Experience has shown that, from the standpoint of starch production' the development of the cassava plant is most profitable when planting takes place at the beginning of a humid period (i.e., in tropical regions at the beginning of the monsoon).
CULTIVATION
Cassava is frequently cultivated as a temporary shade plant in young plantations of cocoa, coffee, rubber or oil palm. In Thailand, however, it is grown mostly as a sole crop and the farmer may for ten years or more grow cassava on the same land. If the price of cassava roots drops, the farmer may shift to another crop (e.g., sugarcane, maize or sorghum) until cassava again becomes the more profitable crop.
Water is essential until the plant is well established. In moist soil, sprouting takes place within the first week after planting. Generally about 5 percent of the cuttings will not come to development, so a corresponding surplus has to be provided for. Within a month of the beginning of planting, the substitution of new cuttings is still possible.
When cultivated as a temporary shade plant, no special attention is given to the cassava plant. When grown alone, the plants require little maintenance after planting. Irrigation may be required if there is no rain, and hoeing of the earth helps preserve the subsoil humidity, especially in dry sandy soils. The chief problem is weed control. It may be desirable to weed the crop two or three times until the plants are well developed and their shade prevents the growth of weeds.
CLIMATE
Cassava is a typical tropical plant. The approximate boundaries for its culture may be accepted as from 30ºN to 30ºS latitudes; however, most cassava growing is located between 20ºN and 20°S. In general, the crop requires a warm humid climate. Temperature is important, as all growth stops at about 10ºC. Typically' the crop is grown in areas that are frost free the year round. The highest root production can be expected in the tropical lowlands, below 150 m altitude, where temperatures average 25-27°C, but some varieties grow at altitudes of up to 1 500 m.
The plant produces best when rainfall is fairly abundant, but it can be grown where annual rainfall is as low as 500 mm or where it is as high as 5 000 mm. The plant can stand prolonged periods of drought in which most other food crops would perish. This makes it valuable in regions where annual rainfall is low or where seasonal distribution is irregular. In tropical climates the dry season has about the same effect on Cassava as low temperature has on deciduous perennials in other parts of the world. The period of dormancy lasts two to three months and growth resumes when the rains begin again.
As a tropical crop, cassava is a short-day plant. Experiments conducted in hothouses show that the optimum light period is about 12 hours and that longer light periods inhibit starch storage.
SOIL
Cassava grows best on light sandy loams or on loamy sands which are moist, fertile and deep, but it also does well on soils ranging in texture from the sands to the clays and on soils of relatively low fertility. In practice, it is grown on a wide range of soils, provided the soil texture is friable enough to allow the development of the tubers.
Cassava can produce an economic crop on soils so depleted by repeated cultivation that they have become unsuitable for other crops. On very rich soils the plant may produce stems and leaves at the expense of roots. In some parts of Africa freshly cleared forest soils are regarded as highly suitable after they have borne a cereal crop.
FERTILIZATION
No fertilization is required when the land is freshly cleared or when there is enough land to enable the cultivator to substitute new land for old when yields fall. Like all rapidly growing plants yielding carbohydrates, cassava has high nutrient requirements and exhausts the soil very rapidly. When cassava is grown on the land for a number of years in succession or in rotation. the soil store of certain nutrients will be reduced and must therefore be returned to the soil by fertilization.
Various experiments in Brazil, India and many regions of Africa and the Far East showed significant increases in yield, of roots as well as starch content, obtained by the application of fertilizers. Potassium salts favour the formation of starch, and nitrogen and phosphorus are essential for growth. However, if the soil contains large quantities of assimilated nitrogen, the result will be heavy development of vegetative growth without a corresponding increase in root production.
Generally speaking, fertilization is practiced at present in most parts of Africa and South America only on commercial plantations. In Thailand, only a few farmers apply artificial fertilizers, as they are usually too costly for the small farmer. Most farmers use different kinds of organic manures, such as cattle or duck manure or garbage.
The kinds and quantities of fertilizers required by a cassava crop depend on the nature of the soil.
DISEASES AND PESTS
In many regions, the cassava plant is not normally affected by diseases or pests. However, in others it may be attacked by the following:
(a) Virus diseases. Mosaic, the brown streak and leaf curl of tobacco may attack leaves, stems and branches. Many parts of Africa harbour these diseases and attempts are being made to select resistant varieties.
(b) Bacterial disease. Bacteria such as Phytomonas manihotis (in Brazil), Bacterium cassava (in Africa) and Bacterium solanacearum (in Indonesia) may attack roots, stems or leaves of cassava plants.
(c) Mycoses. There are kinds which attack roots, stems, or leaves of cassava plants and cause various diseases.
(d) Insects. Some insects affect the plant directly (locusts, beetles and ants); others affect the plant indirectly by the transfer of virus (aphids).
(e) Animals. Rats, goats and wild pigs are probably the most troublesome; they feed on the roots, especially in areas adjacent to forests.
TOXICITY
The toxic principle in cassava is hydrocyanic, or prussic, acid, found in the roots, branches and leaves of the plant in both free and chemically bound forms. The plant contains a cyanogenetic glucoside called phaseolunatin begins to break down upon harvest into hydrocyanic acid, acetone and glucose by the action of the enzyme linase. The presence of hydrocyanic acid is easily recognized by a bitter taste. At the harvest of cassava roots, the amount of the acid in the plant varies from harmless to lethal - from a few milligrams to 250 milligrams or more per kilogram of fresh root. Investigations show that the glucoside content in the cassava plant is markedly increased by drought and by potassium defciency.
Hydrolysis of the glucoside by the enzyme can be accelerated by soaking the roots in water, by crushing or cutting them or by heating. It was found that the hydrocyanic acid content varied little in different tubers of one plant but varied considerably in tubers obtained from different locations. The distribution of the acid in roots varied in different varieties. In sweet varieties, the major part of the acid is located in the skin and in the exterior cortical layer, while in bitter varieties the acid is uniformly distributed in all parts of the roots.
In choosing a strain, the hydrocyanic acid content should be taken into account. Highly poisonous strains are preferred for plantings with the object of starch manufacture, thereby minimizing thefts by both animals and men.
VARIETIES
Although cassava is an established commercial crop in many tropical countries and hundreds of varieties are in existence, little is generally known of the nomenclature and identification of varieties. Various varieties are usually differentiated from one another by their morphological characteristics such as colour of stems, petioles, leaves and tubers. Moreover, in many instances the same variety is known in various places by a number of names.
The numerous varieties of cassava are usually grouped in two main categories: Manihot palmata and Manihot aipi, or bitter and sweet cassava. This grouping is a matter of economic convenience, as it is difficult to distinguish the two groups by botanical characteristics. However, the distinction between them rests upon the content of hydrocyanic acid, which causes toxicity in the roots. This toxicity is not a variety constant but varies from place to place; all cassavas are now regarded as varieties of Manihot utilissima, and in certain circumstances a "bitter" variety may become "sweet" and vice versa. Hydrocyanic acid content tends to be higher on poor soils and in dry conditions. According to the recognized classification, sweet. or nontoxic. roots contain less than 50 milligrams of hydrocyanic acid per kilogram of fresh matter.
At one time it was thought that the toxicity of a cassava root was associated with species or variety, but the hydrocyanic acid content was found to vary markedly with growing conditions, soil, moisture, temperature and age of the plant. Certain varieties in Africa, for instance, were found to be innocuous in Dahomey and poisonous when grown in forest soils in Nigeria; the so-called bitter type from Jamaica failed to produce the toxic substance when grown in Costa Rica.
The chemical composition of cassava roots differs considerably. Studies of 30 varieties in Mexico gave the following results: the dry-matter content of the roots varied between 24 and 52 percent, with a medium of 35 percent; protein content varied between I and 6 percent, with a medium of 3.5 percent.
Table 1 is based on an analysis made in Madagascar comparing the cassava root with the potato
For industrial development, many efforts are being made to organize research and experiments in various geographical regions for the selection of new varieties with high yields of roots and higher starch content. For purposes of nutritional improvement. strains with a high protein content are being sought.
TABLE 1. - AVERAGE COMPOSITION OF THE CASSAVA ROOT AND THE POTATO (COMMON VARIETIES AT HARVEST TIME)
  Cassava Potato
  Percent
Moisture 70.25 75.80
Starch 121.45 19.90
Sugars 5.13 0.40
Protein 1.12 2.80
Fats 0.41 0.20
Fibre 1.11 1.10
Ash 0.54 0.92
1 Bitter varieties usually average about 30 percent starch content.
HARVESTING
Harvesting of cassava can be done throughout the year when the roots reach maturity. In regions with seasonal rains, like Madagascar, harvesting is usually done in the dry season, during the dormant period of the plant; where rain prevails all year round, as in Malaysia, cassava is harvested throughout the year.
Maturity differs from one variety to another, but for food the tubers can be harvested at almost any age below 12 months.
From the standpoint of starch production, cassava should be considered ripe when the yield of starch per hectare is highest. An optimum age of 18-20 months was found in experiments with certain strains of the variety "São Pedro Preto" in a tropical climate (Java). The graph in Figure 7 shows the influence of the age at harvesting on the starch yield as the percentage loss of yield in relation to the yield at the optimum age as found in an experiment with a definite strain.
It is seen that both root and starch production increase rapidly to their maximum value, after which root production decreases slowly and starch production much more rapidly on account of the declining starch content of the tubers.
If the roots are left in the ground, starch content increases with age until, at a certain point, lignification takes place, causing the roots to become tough and woody, so that they are harder to prepare for consumption and other uses.
Once the roots are harvested, they begin to deteriorate within about 48 hours, initially owing to enzymatic changes in the roots and then to rot and decay. The roots may be kept refrigerated for up to a week. They may be stored in the ground for longer periods if they are not detached from the plant.
Harvesting is still generally a manual operation, although equipment to facilitate this operation is being considered. The day before harvest, the plants are "topped" - the stalks being cut off 40-60 cm above ground by hand, machete or machine and piled at the side of the field. This length of stalk is left as a handle for pulling. Material required for the next planting is selected and the rest is burned. In light soils the roots are slowly drawn from the soil simply by pulling the stems or with the help of a kind of crowbar and the tubers are cut off the stock. In heavier soils a hoe may be required to dig up the roots before the plant is pulled out. It must be noted that once the plants have been topped, lifting of the roots must not be delayed, as sprouting and a drastic fall in the starch content of the tubers will result.
FIGURE 7. Percentage loss in yield of whole tubers and starch relative to yield at optimum age. The solid line refers to whole tubers and the dotted line to starch.
YIELD
Cassava is not usually grown on soils where it would be most productive - that is, the light sandy loams, fertile and deep, which are reserved for other crops less tolerant of poor soils. When cassava is grown by traditional tropical methods, yields lie between 5 and 20 tons per hectare, varying with the region, the variety, the soil and other factors. However, when the crop is given more attention, yields of 30 40 tons per hectare are obtained. It has been reported that it is normal for some varieties, under appropriate cultivation methods, to yield over 60 tons per hectare.
The high yields frequently achieved at agricultural experiment stations and occasionally by some active farmers show what might be accomplished with improved varieties and better cultural practices.
Nevertheless, cassava yields in total calories per hectare compare very favourably with those of other starchy staples, as shown in Table 2.
TABLE 2. - AVERAGE YIELDS OF TROPICAL STARCHY STAPLES, 1948-52
  Brazil Java India China (prov. Of Taiwan)
  Millions of calories per hectare
Maize 4.4 2.4 2.3 5.0
Rice 3.9 3.9 2.8 5.5
Yams and sweet potato 7.5 5.4 5.6 8.6
Cassava 14.2 7.1 5.8 11.6
SOURCE: FAO Yearbook of Food and Agricultural Statistics (1955).
Mechanization
In most of the tropical world cassava is grown on small plots; however, in some countries (e.g., Mexico. Brazil and Nigeria). large plantations have been started and interest in mechanization is growing. The degree of mechanization depends on the amount of land, available labour in the area and general policy regarding the use of manual labour.
The use of machinery for land preparation is preferable to manual labour to ensure the best possible seed bed for tuber development. Subsequent operations of planting. weeding, topping and harvesting can be done by hand as well as by machinery.
Labour input for the production of a hectare of cassava varies widely in different parts of the tropics. It has been estimated that in Zaire 778-830 manhours are required per hectare of cassava, including preparation of cuttings, planting, weeding and harvesting. as compared with 1 868-2206 man-hours in Uganda. In South America and the Caribbean a maximum of 494 man-hours has been estimated for the same operations. A possible reason for the higher requirements in Africa is the growing of cassava as an intercrop, with more timeconsuming operations.
The peculiar nature of the cassava crop presents a number of problems as regards mechanization, hut it has been successfully mechanized to a degree in some countries.
The following is an outline of the present use of machinery in cassava cultivation:
(a) The hoe remains the principal implement for cultivating, weeding and harvesting.
(b) Basic operations. such as ploughing and harrowing, may be done by tractor.
(c) A mechanical planter made in Brazil is in use there and in Mexico. It is a tworow planter using a tractor driver and two men on the machine to feed cuttings from the reserve bins into the rotating planting turntable. In operation, the cuttings fall in succession through a hole into a furrow opened by a simple furrower. A pair of disks throw dirt into the furrow and floats pulled by chains pack the soil over the cuttings. The planter is able to cover about 5 hectares per day.
(d) A unit for ridging and planting has been developed by modifying a ridger to work as a ridger-cum-planter and a cultivator to work as a six-row planter after the area has been ridged.
(e) A simple machine used in Mexico is a gasoline-powered table saw to prepare the cuttings for planting. The machine has the advantage of speed and regularity of produced cuttings. As compared with manual work the time saving is 3:1.
(f) It has been found to be virtually impossible to carry out the first weeding operation between the ridges with a cultivator. Mechanical weeding of the top of the ridges presents a number of difficulties.
(g) A topping machine consisting of a heavy screen mounted on the front of a tractor has been developed to push down the tops: then a rotary mower on the back of the same tractor can cut the downed top to make harvesting by hand possible. The height at which the tops are cut back can be easily regulated with any rotary mower.
(h) Cassava is not a crop that lends itself readily to mechanical harvesting because of the way the tubers grow. They may spread over I m and penetrate 50 60 cm. Careless use of machinery for harvesting can damage tubers, resulting in a darkening due to oxidation that will lower the value of the flour. However, in Mexico and Thailand, mould-board ploughs have been used to make hand harvesting less tedious. Stalks can be cut successfully by a mid-mounted mower or a topping machine, and the roots are lifted mechanically with a mid-mounted disk terracer. In Ghana, about 2000 m2 could be harvested in 21/2 hours by a tractor' whereas ordinarily 5 man-days were required A modified beet or potato harvester has been suggested for use behind the tractor, with a pulling mechanism in place of the digging shares to raise the tubers by pulling at the cut stems left after topping.