Cultivation harvesting and storage of sweet potato products
by G.Paneque Ramirez

INTRODUCTION

The sweet potato (Ipomoea batata L.) is one of the twelve principal plant species utilized as a human feed throughout the world.

It can be cultivated in many different climatic conditions, and as a result large areas of sweet potato are cultivated in Asia, Africa, Europe, America and Oceania (Table 2). The various ways in which it can be used are shown in Table 1.

ECOLOGY

Sweet potato is a tropical and subtropical plant which can adapt to more temperate climates providing the average temperature does not drop below 20°C and minimum temperatures stay above 15°C. In other words it can be cultivated between the 30 and 40° latitudes in both hemispheres.

Temperature

For the cultivation of sweet potatoes a range of temperature between 15 to 33°C is required during the vegetative cycle, with the optimum temperature being between 20 to 25°C. The highest yields are obtained when temperatures are high during the day (25 to 30°C) and low by night (15 to 20°C); low temperatures during the night favour the formation of tubers, and high temperatures by day favour vegetative development. (Note: tubers development only occurs within a temperature range of 20 to 30°C, optimum 25°C and generally stops below 10°C).

Light

Sweet potato is a short day plant, that needs light for maximum development. However, the growth of the tubers appears not to be influenced by photoperiod alone. It is probable that temperature and fluctuations in temperature, together with short days favour the growth of tubers and limit the growth of foliage (Youg, 1962).

Altitude

In tropical regions it is possible to cultivate sweet potato from sea level to 2500 m; for example in Bolivia, Peru and Colombia it is cultivated from sea level to 2300 m. (Del Carpio, 1969).

Moisture

Moisture has a decisive influence on sweet potato growth and production. In this context it is relevant to note the water content of the leaf is (86%), stem (88.4%) and tuber (70.6%).

At planting it is important to have moist soils in order to achieve good germination. The soil must also be kept moist during the growth period (60–120 days), though at harvesting the humidity must be low in order to prevent the tubers rotting (Carballo, 1979).

Conditions that favour the development of the vegetative part of the plant include an 80% relative humidity and moist soils.

Soils

Sweet potato can be cultivated in a wide range of soils, with the best results obtained in ferralitic, brown humic and calcimorphologic soils. Ideally the soil should be friable, have a depth of more than 25 cm. and have good superficial and internal drainage.

The chemical properties of the soil are less limiting than structural properties in obtaining good yields. For example, in sandy soils poor in nutrients good yields can be obtained whereas in rich soils the vegetation often becomes luxuriant and the roots large and irregular (in the sandy soils of Manacas Villa Clara, Cuba, yields of 28 t/ha. have been obtained).

Other problems include the difficulty of using machinery on hilly land and drainage on flat land.

The sweet potato also prefers lightly acid or neutral soils, with the optimum PH being between 5.5 and 6.5. Soils which are excessively acid or alkaline often encourage bacterial infections and negatively influence yields (Cairo, 1980).

PHYSIOLOGY

Generally the growth cycle of the sweet potato is from 3.5 to 7 months and takes place in three phases; these are:

1. From planting to formation of tubers (40 to 60 days)

2. From formation of tubers to the time of maximum leaf development (60–120 days)

3. From maximum leaf development to the total development of tubers (45 to 90 days)

Normally the cycle is completed within 100 to 150 days at which time the plant can be harvested.

MULTIPLICATION

Sweet potatoes are multiplied by both sexual and asexual means, though the former is only of interest to geneticists and plant breeders.

Asexual reproduction using tubers and stems is the form of production most commonly used.

Propagation by stem

Of the two systems of multiplication of sweet potato, this form is the quickest and most economic and therefore the most commonly used. The material to be used must be selected to avoid the transportation of eggs or larvae of Cylas formicarius elegantus S. to the new plantation and to avoid the use of unwanted crossbred varieties. In order to avoid this it is necessary to establish “seed banks”.

To maintain standards in Cuba it is necessary to obtain a certificate of quality from the CEMSA (Experimental Centre for Improvement of Agamic Seeds), before seed material can be used.

Requirements for the production of “seed material” are:

1. The seed bank must not be planted in a field that has been used for sweet potato during the previous two years.

2. The irrigation system must be reliable.

3. The seed (stem) must be Cylas formicarius elegantus Summers and virus free and less than 0.5% must be infested with the nematode Rotylenchulus remiformis Linfod y Olivera.

Other aspects to be considered are:

• The cut stems must be planted within four days of cutting:

• The fertilisation applied to the soil at planting and after each cut will be as follows:

Formula:

45 days after planting and after every cut, a 2% foliar urea spray (20 g/l with a final solution of 500 l/ha) must be applied (Lopez and Alvarez, 1971).

The first seed cutting to take place between 60 and 80 days after planting.

After each seed cutting, the following actions should be carried out:

• Weeding

• Fertilization

• Hilling

• Irrigation

Further cuts should be carried out every 60 days.

VARIETIES

Table 3 demonstrates the yields of 6 of the best varieties, grown in three different soil types and harvested at different times. Two of these varieties have only recently been produced. Note: the recently developed variety, CENSA, 85–48 which is a short cycle variety (90 to 100 days) has given excellent results in experimental conditions in terms of both tops and tuber yield (40 ton/ha).

TIME OF PLANTING

In Cuba sweet potato can be planted all the year round if you have irrigation and appropriate varieties.

Where the soils are sandy or clay and irrigation is not available it is advisable to plant at the beginning of the spring, (April to May) though good yields can usually be obtained from July to January.

As indicated, time of planting depends upon the variety used; for example, the variety Cuba 3 gives better yields when planted during the dry season, with irrigation, than when it is planted during the rainy season with the same soil moisture content. Other varieties, like Haiti, give the same yields either when planted during the dry season with irrigation or during the rainy season with or without irrigation. The yield of this variety depends upon the soil moisture content which is the principal factor limiting the growth and development of tubers, and is independent of the time of planting.

DENSITY OF PLANTING

The distance between plants in any crop is a factor which can affect yields, however, in the case of sweet potato no significant responses have been obtained with the different distances studied.

At CENSA the effect of planting distance between plants (22.5 to 30 cm.) and between rows (70 to 90 m.) under irrigation were studied; the best results (i.e. 30 ton/ha) were obtained planting at 90 × 22 cm and 70 × 30 cm.

In experimental work carried out at the Central University of Las Villas with different distances between plants and location of stems, i.e. perpendicular or across the hill, no significant yield differences were obtained. The distances studied were 90 × 30 cm and 90 × 15 cm.

On the basis of these studies, the planting density recommended by CENSA is 37000 plants/ha with the following distances of planting: Sept.-Feb. 0.90 × 0.23 m and March-August 0.90 × 0.30 m (Ministry of Agriculture, 1990).

FORMS OF PLANTING

1. In the furrow

2. In the upper part of the hill

3. On one side or on both sides of the hill.

METHODS OF PLANTING

Manual

In Cuba planting is generally done by hand, putting the stem on the hill or in the furrow and covering it with earth using a spade.

Mechanical

In Cuba planting is done with a TR4 planter made in Bulgaria or another similar Trakia planter made in the USSR.

In order to obtain the best results the following planting procedure should be taken into account:

1. - stems should be 25 to 30 cm in length

2. - the depth of planting should be between 7 to 10 cm

3. - planting should be on the hill

4. - 2/3 of the stems should be buried

Cultivation

Cultivation activities depend on a series of factors including: variety, time of planting, type of soil, planting distance, etc. The objective should be to carry out as many operations as necessary to optimise vegetative development of the plants (Rodriguez Nodal and Morales Tejon, 1990).

CONTROL OF WEEDS

In this crop, weeds can be controlled by manual, mechanical-manual or chemical means.

Manual control alone is used only for small areas. Mechanical—manual control consists of a combination of between row and between plant cultivations, as follows:

1. Mechanical cultivation using an inter row harrow and a manual cultivation between plants within 10 to 15 days of planting.

2. Hilling up around plants, 25 to 30 days after planting with the aim of reconstructing the hill to eliminate weeds and the incorporate fertilizers. If necessary, hand hoeing between plants should be done at this stage.

In Cuba a second mechanical cultivation might be necessary for dry season planting.

Use of chemical products

There are many products on the market which can be used and their use will depend upon the chemical to be used.

FERTILIZATION

Due to the sweet potato's great yield potential (foliage and tubers) it requires a considerable amount of fertilization; this varies according to variety and soil type. The percentages of N.P. and K in the organic matter of sweet potatoes are as follows:

When 15 ton/ha of sweet potato are harvested approximately: 70 Kg of N; 20 Kg of P₂O₅ and 110 Kg of K₂O are extracted from the soil (Jacob and Uexkull, 1968).

Method and timing of fertilizer application

There are several different ways of applying fertilizer, including:

1. In the furrow.

   1. Placing the fertilizer in the furrow and covering with soil from the hill.

   2. Planting the stem in a new hill after opening the furrow at a higher level than the fertilizer.

2. In the hill. The fertilizer is placed at points 30 cm apart within the furrow.

3. At 30 days after planting. The fertilizer is continuously spread along the the length of one side of the hill, following which it is covered using a double mould—board plough passed down the centre of the furrow.

4. At 20 or 30 days before planting (in sandy soils). After spreading the fertilizer it is incorporated using a harrow.

The ratio and amount of fertilizer to be used in sweet potato production has been established in different countries. Examples are shown in Table 4.

IRRIGATION

Overall the sweet potato plant requires soils with a high moisture content. For vegetative growth its needs are moderate, though during the first month of growth when the tubers are developing the moisture requirement increases. During the final days of the cycle the moisture requirement reduces.

Furrow and sprinkler irrigation techniques can be used. It is best to carry out one irrigation before planting if the soil is not moist. The net application of irrigation should be between 200 to 250 m³/ha every 7 to 10 days. This should be stopped about 15 days before harvest (INRA, 1972).

DISEASE AND PESTS

The sweet potato plant is not seriously affected by fungus diseases or virus attacks. Though it can suffer from insect attack.

Tetuan (Cylas formicarius elegantulus S.) is the most important pest that affects the sweet potato. The female lays its eggs in stems and roots and the larvae make galleries in the tubers which affects flavour (Dias Sanchez, 1980). In order to control this pest it is best to use “seed” from areas free of the pest, though various insecticides can be used to combat the pest; these are applied to the soil and stems.

Biological control

In Cuba the insect “Hormiga Leona” (Pheidole megacephala frabricio) has been used successfully to control the pest. This is achieved by distributing its colonies or nests around the fields at a rate of 13 per ha., 30 to 45 days after planting. Distribution of the fungus (Blauberia bassiana Bal.) over the plants 15 days after planting has also been successful.

Nematodes in general do not constitute a serious problem. The principal species are Rotylenchulus remiformis Lenford and Oliveira.

Overall, biological control, complimented by agronomic practices such as crop rotation is the best way to achieve success in sweet potato cultivation.

HARVESTING

The time of harvest largely depends upon the variety and soil moisture content during the first month of plant development. However the actual commercial varieties used take different times to harvest, for example:

• early varieties take 3 to 4 months

• medium varieties take 4 to 6 months

• late varieties take more than 6 months

The growth cycle of any variety can be altered by an excess of moisture in the first 3 months after planting. This produces an intense foliar development and a late formation of tubers. Often in these cases the number of tubers are reduced.

Methods of harvesting

Harvesting can be carried out in 3 ways: manual, semi-manual and mechanical.

The manual method is the simplest. It is usually used by the small scale producers and involves the use of a digging stick to lever the tubers out of the ground.

Semi-manual: This is the most frequently used method in Cuba and involves the removal of the foliage with the help of a harrow which clears the foliage from the area to facilitate the final harvesting. The elimination of foliage must be carried out 24 hours before harvesting. After the foliage is removed a double mould board plough is passed down the centre of the hill leaving a ridge in between the original two and ensuring that the soil does not cover part of the adjacent ridges. The tubers exposed after the first pass are picked up by hand and removed prior to making a second pass. Tubers are then again collected by hand.

Mechanical: This system is not ideally suited to the conditions of Cuba. Where this system can be applied satisfactory results can be achieved with a potato harvester. With this equipment the tubers can be collected in bulk in the field or on a trailer running along side the harvester. The presence of foliage or inadequate soil preparation can make this type of harvesting more difficult.

Conservation and storage

The tubers of sweet potato can be conserved in good condition for some time if they are stored in good conditions; for example:

1. All tubers damaged by insects or fungus disease should be removed.

2. All tubers with mechanical damage should be removed.

3. Storage should be carried out in 45 Kg bags in store rooms with good ventilation and low humidity. The bags should be raised off the ground on wooden pallets. Stacks should be no more than 10 bags high with space around each for the circulation of air.

4. Once the tubers are bagged they should be transported within 24 hours.

In countries such as USA, USSR and Japan some producers of sweet potato store their products in refrigerators at a temperature of 13–15 degrees C and with 80% relative humidity. In this way the tubers can be stored for 4 to 6 months.

Bibliography

Cairo, P. 1980. Soil. Editorial Pueblo y Educacion Habana Cuba.

Carballo, N. 1979. Effect of soil humidity on the sub - period of growing in the cultivation of sweet potato (Ipomoea batata L) I Forum cienific technic. Santa Clara, Cuba.

Del Carpio. 1969. The genetic imp rovement of sweet potato in Peru. Vida Agricola Peru.

Dias Sanchez, J. 1980. Determination of the fly activity of Cylas formicarius elegantulus (F) using a light trap. Centro Agricola, Central University of Las Villas, Cuba.

FAO. 1987. Production Yearbook. FAO, Rome.

INRA. 1972. Irrigiation standards. Technical collective of Irigation. La Havana, Cuba.

Jacob, A. and Uexkull. 1968. Ferti lization. Edicion revolucionaria. Instituto del Libro. La Havana, Cuba.

Lopez Zada, M. and Carmen Alvar ez. 1971. Comparative study of sweet potato yield with different times of foliar nitrogen application. Centro Agricola Fac. de Ciencias Agropecuarias. Universidad Central de las Villas.

Ministry of Agriculture. 1990. Rec ommendations for agamic multiplication of tropical crops. Cuba.

Rodriguez, Nodal, A.A. and Morales Tenon, T. 1990. Recommendations for agamic multiplication of tropical plants. Santo Domingo, Cuba.

Youg, C.K. 1962. Effects of thermo Ipomoea batata under controlled conditions. Plant physiologyc 36 No. 5 pp. 680–684, 1961.