Everything you need to know about sweet potato cultivation and growth. Learn about:- 1. Description of Sweet Potatoes 2. Habitat of Sweet Potatoes 3. Climate and Soil Suitable 4. Propagation 5. Cultural Operations 6. Diseases and Pests 7. Harvesting and Yield 8. Storage 9. Chemical Composition and Uses.
Contents:
- Description of Sweet Potatoes
- Habitat of Sweet Potatoes
- Climate and Soil Suitable for Sweet Potatoes Cultivation
- Propagation of Sweet Potatoes
- Cultural Operations for Growing Sweet Potatoes
- Diseases and Pests of Sweet Potatoes
- Harvesting and Yield of Sweet Potatoes
- Storage of Sweet Potatoes
- Chemical Composition and Uses of Sweet Potatoes
1. Description of Sweet Potatoes:
A slender, prostrate, trailing or ascending perennial herbaceous vine, producing succulent, tuberous roots. Leaves alternate, variable, ovate-cordate, 2.5-8.75 cm. long, entire, angular or deeply lobed; flowers solitary or cymose, 5 cm. long, funnel-shaped, white or purple; capsules globose or ovoid, brownish in colour; seeds 2-4, small, black, somewhat flattened.
The origin of sweet potato is still a matter of conjecture as most of the known wild species of Ipomoea differ greatly from the cultivated plants. The cytological make-up of sweet potato also suggests the complexity of its origin. It is believed by some workers that sweet potato originated from I. tiliaceae (Willd.) Choisy, a wild species growing in tropical America.
Sweet potato is considered to be a hexaploid with 90 somatic chromosomes. It is of comparatively recent origin derived by amphidiploidy of a tetraploid (2n=60) and a diploid (2n=30) species. The plant does not generally produce flowers; even if it does, seeds are rarely set. The high degree of self-incompatibility and failure to produce seeds make breeding of new strains difficult.
Sweet potato improvement was, until recently, confined to the selection of new types which arose as somatic mutations and a sexual propagation was followed. It has now been found that flowering and seed formation in sweet potato can be induced by various treatments and improvement of the crop by hybridisation has thus been possible.
The treatments include girdling, overwintering, training vines on trellis, controlling exposure to day light, foliar application of 2, 4- D and grafting on root-stocks of closely related ornamental species without tuberous roots.
Under conditions of induced flowering, many of the flowers are sterile, and only a few seeds are produced under special cultural treatments. Marked sterility of flowers is attributed to high polyploidy and the low percentage of seed formation, to cytological abnormality in meiosis of anthers which leads to incompatibility in the process of fertilisation. The types which are self-incompatible are compatible with other types.
Self-sterile types will set seed when grown in close proximity to other types or when artificially pollinated. Thus production of new strains has, in recent years, been effected by means of seedlings raised from seeds which have naturally set or from seeds obtained by controlled cross pollination. In India, breeding work on sweet potato has been carried out in several places, and the technique of training vines to trellis is adopted to induce flowering.
2. Habitat of Sweet Potatoes:
Sweet potato is widely grown throughout the tropical and some parts of temperate regions in Africa, India, China, Japan, Malay Archipelago, the Pacific Islands, tropical America and southern United States. In India, it is grown in practically all the States and ranks third in importance among tuber crops, exceeded by potato (6 Lakh acres) and tapioca or cassava (5 lakh acres).
The area under sweet potato cultivation is about 4 lakh acres, the major areas being Bihar and Uttar Pradesh, followed by Mysore, Kerala, Bombay, Orissa, Madras and Madhya Pradesh. Bihar and Uttar Pradesh together account for nearly 60% of the total area.
Sweet potato is cultivated in most of the districts in Bihar. In other States, the principal areas of cultivation are – Sultanpur, Etah, Pratapgarh, Badaun, Farukhabad, Allahabad, Basti, Shahjahanpur and Gonda districts in U.P.; the sub-montane and central districts in Punjab; Midnapur, Hoogly, Murshidabad, Hwrah, 24-Parganas and Nadia districts in West Bengal; Tiruchirapalli, S. Arcot, N. Arcot, Madurai, Tirunelveli and Chinglepet districts in Madras; Visakhapatnam, Srikakulam, Cuddapah, Chittoor and West Godavari districts in Andhra Pradesh.
3. Climate and Soil Suitable for Sweet Potatoes Cultivation:
Sweet potato requires a warm, moist and long growing season; humidity should be low when the crop approaches maturity. It grows best where the mean temperature is above 72°F. It is sensitive to frost but it can withstand drought. A well-distributed, fairly low to moderate rainfall (30—50 in. annually) is necessary for growth. In areas of high rainfall, the crop is grown towards the close of the wet season, and in areas of low rainfall, irrigation is required.
Sweet potato can be grown on a variety of soils, but it is best adapted to friable well-drained sandy loam (pH, 5.0-6.8) with clayey sub-soil. It does well on light sandy soils, lightly manured. Good drainage is essential. Heavy and rich soils tend to produce excessive vine growth at the expense of tuber formation: tubers of vines growing in rich soils are likely to be rough and irregular in shape. In areas which are not suitable for other vegetable crops or on new land sweet potato can give good yields. It can be grown also in dry river beds.
The land should be repeatedly ploughed and harrowed to a depth of about 6 in. and brought to a fine tilth deeper ploughing is not recommended as it results in long slender tubers. After ploughing, beds are prepared for planting, and where rainfall is heavy or the soils drain poorly, ridges 10-14 in. high are thrown up.
Sweet potato is a heavy feeder and depletes the soil nutrients rapidly and should not be grown too often in the same area; the crop does not follow any particular rotation but it is advisable to raise other crops in the area for three or four years before it is again planted.
Sweet potato responds to good soil management and manurial treatment. In heavy fertile soils, manure is not usually applied. In less fertile or poor soils, farmyard manure is given at the rate of 5- 20 tons per acre; cattle are sometimes penned in the field. In the west coast districts of Malabar and S. Kanara, wood ash, fish guano, tobacco stems and green leaves are applied as manure.
When sweet potato follows a previously manured crop, no other manure need be applied; thus in Madras, it is not usual to apply any manure as the crop generally follows heavily manured ragi (Eleusine coracctna).
Application of 500 lb. of bone meal or 650 lb. of a mixture containing 2 parts bone meal, 4 parts groundnut cake and 1 part of potassium sulphate per acre is reported to have given good results. Top dressing at the rate of 60 lb. of ammonium sulphate gives a higher yield of tubers. On light sandy soils, a dressing of 300 lb. of superphosphate and 100 lb. of ammonium sulphate per acre is given. Fertilizers are worked into the rows on ridges or in flat beds where cuttings are to be set. Green manuring has also proved beneficial.
In America, high yields have been reported from areas receiving application of 1,000-1,500 lb. of fertilizer (N, 2-4%; P2O5, 2%; and K2O, 8-10%) per acre. The quantity of nitrogenous fertilizer is reduced in the case of soils treated with green manure or animal manures. Good response to the application of potash is obtained from light sandy soils.
The fertilizer is applied in two stages- a part of it is worked into ridges about 2 weeks before planting; the rest is applied as a top dressing to the sides 2-3 weeks after planting.
4. Propagation of Sweet Potatoes:
Though a perennial, the crop is treated as an annual under cultivation. It is grown as a summer crop in the sub-tropical plains and on the lower hills of northern India. Two crops are usually raised in parts of Bihar and Bengal, one in August-September and the other in December-January, both under rain fed conditions.
In South India, sweet potato is grown in more than one season; in the districts of Visakhapatnam (Andhra), Malabar and S. Kanara, the crop is grown during the southwest monsoon (planting in June); a second crop is raised in Visakhapatnam district during the northeast monsoon (planting in September). As an irrigated crop, it is grown throughout South India in September-November or even later. In Nilgiris, the crop is fed by pre-monsoon rains (April-May). In areas of heavy rainfall in Western Ghats, it is grown on terraces.
Sweet potato may be propagated by seeds; the seedlings produced are very variable and propagation by seed has been used mostly for selection work. The seeds have a hard coat and germination takes a long time, unless the seeds are pre-treated. The seed coat may be scarified or seeds soaked in conc. sulphuric acid for 20 minutes and washed in warm water before sowing; better results are obtained by increasing the soaking period to 45—60 minutes and notching the coat with an emery grinding wheel.
For raising crops, sweet potato is propagated vegetatively by vine cuttings obtained from the previous year’s crop or by sprouts raised from tubers. Propagation by cuttings is favoured as it is cheaper, free from soil-borne diseases, and the tubers produced are of uniform size and shape. The planting material is obtained from a nursery, in which cuttings from fairly mature portions of stems have been previously planted 9—12 in. apart.
The nursery is irrigated once in 4-7 days in the early stages and at longer intervals afterwards. The cuttings take root readily and the bed is covered with trailing vines in 2-3 months. Cuttings from vines in the primary nursery are replanted once or twice to get sufficient planting material.
For raising planting material from tubers, selected tubers are planted in a nursery. The beds are thoroughly prepared and manured; sometimes ammonium sulphate is applied. Tubers are placed 2-3 in. deep in the soil and beds are lightly watered. Each tuber produces a number of sprouts, also called slips or draws.
The sprouts grow to a length of 9—12 inches in 4—6 weeks, when they are pulled out and transplanted in the field on ridges 3 ft. apart, the distance between plantings being 1—1½ ft. Two or more crops of slips are collected from the nursery for planting. Slip planting is the usual method of propagation adopted in America. In India, the practice is different; slips are allowed to run into vines in the nursery and cuttings are taken from the vines for planting.
The vine cuttings, 1— 1½ ft. long, with 3-4 nodes are planted on ridges or in flat beds in loose moist soil. The ridge method of planting is preferred. The ridges are spaced 2—2½ ft. apart, the distance between plantings on the ridge being 9-12 in. The cuttings are set on both sides of the ridges half way on the slope. The central part of the cuttings is buried deep into the wet soil leaving the two ends, with one node on each side, exposed.
When planted in flat beds, a spacing of 9-12 in. between cuttings is allowed, the distance between rows being 1½ ft. The rate of planting material per acre varies from 15,000 to 30,000 according to type grown, nature of soil, method of planting and the spacing between cuttings.
5. Cultural Operations for Growing Sweet Potatoes:
Once the crop is established, no further cultivation is needed; the field requires weeding in the early stages. During dry weather, 8-20 irrigations are applied between transplanting and harvesting. Trailing vines tend to strike roots at points where nodes come into contact with moist soil, which later develop into small thin tubers, thereby, adversely affecting the development of the main tubers.
To obviate this, vines are periodically turned over to disturb the root development at contact points. Ridges are earthed up during the growing season to maintain their height and shape.
6. Diseases and Pests of Sweet Potatoes:
The fungal diseases affecting sweet potato in India are of minor importance. They include leaf spot caused by Cercospora batatae Zimm., white blister caused by Cystopus ipomoeae- panduratae (Schw.) Steven & Swing., and black rot caused by Rhizoctonia solani Kuhn.
The only serious pest of sweet potato reported in India is the weevil (Cylas formicarius Fb.). The grub bores tunnels into vines and tubers in the field and also tubers in storage. Infested vines die and dirty patches appear on the surface of affected tubers.
Weevil incidence is reduced by adopting plant sanitation methods, such as using only un-infested and disinfected material for planting, practice of crop rotation and destruction of infested vines and tubers; cleaning the field after harvest; earthing up of rows or ridges before tuber formation; application of 2% DDT dust in nursery beds and furrows before planting and also dusting the crop frequently.
Vine cuttings, before planting, may be dipped in tobacco decoction or 2% DDT solution, or exposed to Gammexane fumes; elimination of Convolvulus spp. which act as alternate hosts for the weevils from the field is recommended. Spraying of crop with Parathion and Dieldrin, three times a month, has proved effective in controlling weevils; applications of Gammexane and Hexyclan (BHG group) have also given good results.
Fumigation of storage houses with methyl bromide at the rate of 1 lb. per 1,000 cu. ft. for 4 hr. destroys the pest; infested tubers should be destroyed and the rest fumigated.
Minor insect pests of sweet potato are vine borer (Omphisa anastomosalis D.); leaf beetles (Aspido-morpha miliaris F., Metriona circumdata H., Chiridia sernotata B. and Oncocephala tuberculata O.); and leaf caterpillars (Herse convolvuli L., Catephia inquieta W., Brachmea effera M., Junonia orthyia L., Euchromia polymeria L. and Diacrisia obliqua W.). A scolytid beetle (Stephanoderes sp.) has been recorded from Coimbatore in stored tubers. Field rats, white ants and wild pigs damage the crop.
7. Harvesting and Yield of Sweet Potatoes:
The crop comes into bearing in 4-5½ months from planting. Harvesting begins a fortnight before the crop is fully mature and continues for a fortnight or so after full maturity; where proper storage facilities for tubers are unavailable, the crop may be left in the field a little longer and tubers dug up as needed. The tubers are considered to be mature and ready for harvest when the leaves begin to turn yellow and drop off.
A more reliable indication of maturity of crop is provided by cutting through a tuber dug out from the field; if the juice dries up readily without discolouration, the tuber has attained maturity; if the cut surface however, changes to a dark or greenish colour, the tuber is still immature.
Vines are cut close to the ground and the field cleared of surface growth. An interval of a week or so between the cutting of vines and harvesting of tubers improves the sweetness of the produce. Harvesting should be done when the soil is dry and the weather is fair. The field is dug up and exposed tubers are lifted to the surface. Care must be taken to prevent injury to tubers by cutting bruising, etc. which facilitates the entrance of decay organisms. The tubers are dried in the sun for a week, freed from adhering soil and taken to the market.
The average yield of tubers varies in different areas from 8,000 to 24,000 lb. per acre. There has been considerable improvement in yields since the introduction of selected types. Pusa Suffaid, a high yielding variety is reported to give a yield as high as 32,000 lb. per acre.
8. Storage of Sweet Potatoes:
Tubers intended for storage are harvested a little later than usual and cured to hasten the healing of wounds caused during lifting. The usual practice in India is to spread the tubers in the sun during the day and either transfer them to a room during the night or cover them with a tarpaulin; exposure to sun is continued for about a week or more. The tubers are then stored in ventilated rooms, frequently inspected and tubers which show signs of rotting are periodically eliminated.
Sweet potato tubers may be stored in sand or straw; they may also be stored in pits between layers of wood ash. They may be split longitudinally into slices, sun-dried and stored. In America, the tubers are cured in store houses under controlled temperature and humidity conditions.
Curing is effected at 80—95°F, relative humidity 85—90%, the period of treatment varying from one week to three weeks according to the type to be cured. The temperature in the store house is lowered gradually to 50—55°F. (50°F. in some cases), the humidity being maintained at 80—85%. Under these conditions tubers keep well for 3-7 months.
9. Chemical Composition and Uses of Sweet Potatoes:
The average composition of Indian sweet potato is as follows – moisture, 68.5; protein, 1.2; fat, 0.3; carbohydrates, 31.0; and mineral matter, 1.0%. The composition varies with the type of sweet potato, conditions of growth and duration of storage after harvesting. The usual range of values for different constituents is as follows – moisture, 58-75; protein, 0.5-3.5; fat, 0.2-1.5; N-free extr., 18.0—37.0; sugars, 2.2-5.6; fibre, 0.6—2.5; and ash, 0.6—1.5%. Indian types with white flesh contain little or no carotene, while American types with pink flesh (experimentally grown in India) contain as high as 5.4-7.2 mg./100 g. of carotene.
Vitamins present in the tubers are – thiamine, 0.09—0.14; riboflavin, 0.05—0.10; and vitamin C, 16-22 mg./100 g. Large tubers generally contain more vitamins than small ones. The concentration of vitamins is higher in the inner core than in the outer portion.
Sweet potatoes are rich in starch. Sucrose and a few reducing sugars are present, but maltose, mannose, galactose and pentose occur, if at all, only in traces. During storage, a part of the starch is converted into reducing sugars and subsequently into sucrose. Thus in a sample stored for five months, the starch content was reduced from 19.1% to 14.1% while the percentage of reducing sugars (as dextrose) and sucrose increased from 0.9 to 1.7 and 1.9 to 6.1 respectively.
Cooking is accompanied by a considerable increase in sweetness as a result of the hydrolysis of starch to maltose and dextrines through the action of β- amylase. Analysis of cooked tubers gave the following figures: total solids, 32.55; alcohol insol. solids, 18.29; reducing sugars, 6.45; sucrose, 2.23; maltose, 8.64; dextrins, 0.51; and polysaccharides, 14.13%.
The nutritive value of sweet potato proteins compares well with other vegetable proteins. Experiments on human subjects receiving steamed sweet potato as food gave the following values for the rates of absorption (av. values) of various constituents: protein, 66.97±2.38; fat, 85.66±8.94; carbohydrate, 98.50+0.23; fibre, 9.66 ± 5.14: and ash, 51.81 ±6.91%.
The following proteins have been identified – a globulin soluble in 5% sodium chloride, 68.0; water- sol, protein, 11.3; a protein soluble in 0.2% sodium hydroxide, 5.4; prolkmin, 4.0; and a protein soluble in hot 5% sodium chloride, 1.3%. The distribution of nitrogen in the purified globulin (N, 15.07%) is as follows: humin N, 2.34; amide N, 8.38; histidine, 1.01; arginine, 4.28; lysine, 3.98: tryptophan, 1.89; tyrosine, 4.44; and threonine, 4.13%.
The essential amino acids present in the total proteins are as follows (calculated to 16.0 g. N): arginine, 2.9; histidine, 1.4; lysine, 4.3; tryptophan, 1.8; phenylalanine, 4.3; methionine, 1.7; threonine, 3.8; leucine, 4.8; iso-leucine, 3.6; and valine, 5.6.
The pectic substances (total, 0.78; soluble, 0.43%) present in fresh tubers contain: uronic acid, 60; and methoxyl, 4-5%. Pectin content upto 3% has been reported in certain high-starch types. Other constituents reported in the tubers are: phytin (1.05%), two mono-amino-phosphatides (probably lecithin and cephalin), organic acids (oxalic acid, 0.1%), phytosterolin, phytosterol, resins, tannins and colouring matter.
Sweet potato contains the following mineral constituents: calcium, 30; magnesium, 24r potassium, 373; sodium, 13; phosphorus, 49; chlorine, 85; sulphur, 26; and iron, 0.8 mg./100g.; iodine, 4.5 pg./kg,; manganese, copper.and zinc are present in traces.
The enzymes reported to be present in the tubers are: amylase, protease, invertase, catalase, laccase, arabinase, galactanase, polygalacturonase, peroxidases, monophenolase, catecholase, cytochrome c-oxidase, phosphorylase and phosphatases. The occurrence of two thermolabile trypsin inhibitors and glutathione reductase is also recorded.
The amylolytic activity is pronounced (activity of pressed juice of the tuber, 130° Lintner). The amylase present is mainly β-amylase with optimum activity at pH, 5.5—6.0 and temperature, 50—55°; it is most stable in the pH range corresponding to its optimum activity. The inner portion of the tubers show greater amylolytic activity than the layers near the surface. The pH optimum for phosphatases is 5.3-5.7.
The tubers infected with black rot (Ceratostomella fimbriata) yield an essential oil (0.14%) containing a ketone, ipomoeamarane (C15H22O3; b.p., 140—44°/6 mm.) as the chief constituent. Three substances have been isolated from the acidic fraction of the oil, viz. furan-3-carboxylic acid, trans-cinnanic acid and a furan keto acid, named batatic acid (Cl2H12O4; m.p., 88.5-89.5°). The oil is toxic to animals; oral administration to rats in doses of 0.1—0.2 g. of oil/120—150 g body wt. proved fatal within 27-42 hr. A dosage of 0.3 g. of oil/day had no ill effects on human subjects, Ipomoeamarone and a fraction of the oil boiling below 120° at 4 mm. possess anthelmintic properties.
Starch:
Sweet potatoes have been utilised commercially for the production of starch in Japan and U.S.A. The process consists of grinding the tubers with lime water and separating the starch from the pulp by washing over a series of fine screens. Lime water (pH, 8.8—9.2) flocculates the impurities and dissolves the pigments present. The starch suspension is treated with sodium hypochlorite and centrifuged. It is then dried to c. 12% moisture in a vacuum drier.