ECOLOGICAL AND CULTURAL REQUIREMENTS OF SWEETPOTATO
Introduction
Among the root crops in the Philippines, the sweetpotato ranks first from the standpoint of area planted and production. It is an important source of carbohydrates and its storage roots have many uses. For marginal farmers, sweetpotato is the staple crop eaten boiled with viand or vegetables. It is also used as the raw material in the manufacture of starch, alcohol, carotene-juice, glue or syrup. The starch is one of the materials used in textile or paper industry. Recently, dried sweetpotato chips are being used to take the place of corn in animal feed formulation.
Inspite of the many uses of sweetpotato, the average national production is very low compared to its potential yield. This could be attributed to improper cultural management and the use of traditional agriculture. Less attention given to the crop and the lack of knowledge of the conditions pertaining to its proper cultivation have contributed to its diminished production.
In order to boost production and encourage growers to produce more, it is of prime importance that the ecological and cultural requirements of sweetpotato be fully known and understood.
Ecological Requirements
Soil
Sweetpotato can be grown in different kinds of soil, but sandy loam reasonably high in organic matter with permeable sub-soil is ideal. Good drainage is also essential since the crop cannot withstand water logging. Soil with high bulk density or poor aeration tends to retard storage root formation and results in reduced yields (Watanabe et. al. 1968). A soil pH of 5.6-6.6 is preferred for sweetpotato. It is also sensitive to alkaline or saline soil, hence such soil should be avoided.
Daylength
Sweetpotato does best where the light intensity is relatively high hence shading should be avoided. Daylength of 11 hours or less promotes flowering while daylength longer than 13.5 hours prevents flowering. In the tropics, especially the Philippines, sweetpotato flowers frequently. Root formation in sweetpotato is also promoted by short-day conditions. Short days with low intensity promote storage root formation, while longer days favor vine development at the expense of the tuber production.
Rainfall
Regions with a rainfall ranging from 750 to 1000 mm per annum with about 500 mm falling during the growing season, is best for sweetpotato. The rest of the rain falling during non-growing season makes it relatively easy to propagate and maintain vine growth of cuttings that will be used as planting materials during the next season. Although the crop can withstand drought conditions, yields are considerably reduced if the drought occurs within the first six weeks after planting (Edmund and Ammerman, 1971) or at the initiation of root bulking (Kay, 1973).
Temperature
Growth is best at temperatures above 24C. In general, sweetpotato needs relatively high temperature during the growing period.
Cultural Requirements
Land preparation
The land should be plowed and harrowed twice or until the soil is thoroughly pulverized and crop residues and other organic matter are already incorporated in the soil. The tractor-drawn plow is generally used in land preparation of large-scale commercial farms. Small farmers use the carabao-drawn moldboard plow. Plowing is normally done when the soil is neither too wet nor too dry so that easy cultivation can be attained. In areas where plowing is impossible, hand implements like trowel, bolo, hoe and pick mattock, are commonly used in land preparation.
When the field is thoroughly prepared, making ridges, furrows or mounds should follow depending upon the method and distance of planting.
Preparation of Planting Materials
Terminal cuttings of about 25 to 35 cm are prepared a day or two before planting. The terminal portion is the best part of sweetpotato vine for planting purposes because it can recover immediately and grow faster than the middle and basal portions of the stem. Planting materials should be healthy and free from insect pests and diseases. If the source of planting material is a mixture of several varieties, storage roots of the best variety should be secured and planted in the field until considerable shoots have developed for planting purposes. Under this condition, planting materials derived this way are true to type.
Storage of Planting Materials
When materials cannot be planted immediately due to intense heat or due to unavailability of land area, the cuttings can be stored up to 7 days. To preserve the food reserve of the vines, the leaves should be pruned leaving only a few leaves at the tip of the cuttings. The cuttings should then be tied and bundled, wrapped around the base with wet jute sack and stored in a cool and shady area.
Planting Methods
The recommended method of growing sweetpotato is planting on ridges. It has been shown that the higher the ridge - up to the height of 36 cm (Edmond et. al. 1950), the greater the yield. The optimal height of the ridge in each case will depend on the soil type and the variety to be grown.
Growing sweetpotato on flat beds should be discouraged because the resulting yield is usually low (Kimber, 1970). Planting on mounds gives good yield and is extensively practiced throughout the tropics especially in marginal areas, but the expense on manual labor involved in mound-making tends to make this method impractical. However, if the area is swampy, planting on high mounds is advantageous since this system reduces the risk of water logging in the main rooting zone of the crop.
Planting Time and Distance
It is best to plant sweetpotato early in the rainy season so that it can establish quickly. When the rainy season is very long, planting may be delayed and timed so that the crop matures just as rainfall begins to decline.
In planting, the vine cutting is inserted into the prepared ridge at an angle so that one-half to two-thirds of its length is beneath the soil. The cuttings are normally planted 25-30 cm between hills on ridges that are 75 to 100 cm apart. Each hill contains only one stem cutting. A hectare of sweetpotato field needs 33,333 to 40,000 cuttings of planting materials.
Replanting
One or two weeks after planting the dead and missing plants should be replanted. It is recommended that if the number of missing hills is more than 30% of the population, replanting should be done, especially if the dead plants are adjacent to each other. Missing hills less than 30% need not be replaced for this can no longer reduce yield significantly.
Fertilizer Application
Sweetpotato is responsive to fertilizer particularly if the area planted has been cropped for some time. The exact type and dosage of fertilizer depend on the soil type, the environment and the variety grown. If the soil for sweetpotato crop is still fertile, there is no need for fertilizer application. However, it is recommended that soil analysis should be done.
The general recommendation for soil without known analyses based on the national cooperative testing project is 4 sacks of complete fertilizer (14-14-14) per hectare for relatively fertile soil and 6 to 8 sacks per hectare for poor soil. Care should be taken in applying nitrogenous fertilizer because applying excessive nitrogen delays the initiation of root bulking and promotes vine growth at the expense of root storage growth.
Fertilizers for sweetpotato may be applied in full or split. Upon planting, the whole dosage may be applied at once. In split application, half of the dosage is applied on or immediately after planting, while the second half is applied one month later. The method of first application can be given as a spot application or applied as continuous or discontinuous band beside the ridges after planting. The second application is either a spot or a band application but never the broadcast method.
Manure may also be used to improve the fertility of sweetpotato plots. This is a common practice for smallholdings and in traditional agriculture.
Cultivation and Weeding
The critical period for weed control in sweetpotato is during the two months of growth after planting. After this period, vigorous growth of vines causes rapid and effective coverage of the ground surface, thus suppressing the growth of weeds. For these reasons, it is recommended that the sweetpotato field should be hilled-up 2 to 4 weeks after planting using a carabao-drawn plow. The remaining weeds found in ridges should be removed using a bolo or hoe. Weeding can be done two to three times two months after planting depending upon the weed density.
The use of herbicides to control weeds in sweetpotato is widely practiced in other countries but not in the Philippines due to their high cost. However, some large-scale production enterprises utilize herbicides for weed control.
Harvesting
In the tropics, especially the Philippines, sweetpotato is ready for harvesting 3-4 months after planting. The maturation of the crop varies with varieties and the environmental conditions under which it is grown. Sometimes readiness of the crop for harvesting is indicated by yellowing of the leaves. In other instances, there is no external visible sign of maturity for harvest. If harvesting is done too early, yields are low, but if the crop is also left in the ground too long, the tubers become fibrous, unpalatable, and are prone to various rots and attack by weevils.
In traditional agriculture, sweetpotato is harvested as the need arises, hence there is no fixed time for harvesting. This system of harvesting is good only for small patches of land but is generally disadvantageous since it will expose the plants to weevil attack. Furthermore, it will prevent the utilization of a piece of land for another crop in a given period of time.
For commercial-scale production, harvesting is normally done at one time using mechanical or carabao-drawn implements. The steps for harvesting are as follows:
Do not include weevil-infested and heavily damaged roots in the marketable grade.
After harvesting, there should be no storage roots left in the field because these leftover tubers will be attacked by weevils and become the source of weevil infection for the succeeding crops. If the field is to be planted again with sweetpotato, the newly harvested field should be fallowed and left this way for one month so that debris and other organic matter can be incorporated in the soil.
Source:
Root Crop Digest
Vol. 2 No.2 1987
PRIS ISSN 0016-4325
ECOLOGICAL AND CULTURAL REQUIREMENTS OF CASSAVA
Introduction
Cassava, like any other crop, will only grow successfully when given the appropriate cultural requirements and grown under favorable climatic and edaphic conditions. To insure high productivity of cassava, the following information on the ecological and cultural requirements are provided to guide farmers and prospective growers in its culture.
Ecological Requirements
Light
Cassava, a sun-loving crop, should be grown under open conditions. Yield is drastically reduced under shaded conditions e.g. when grown under coconut. Twelve hours daylength is reported to be optimum for cassava. When plants are exposed to long days, root number and total root weight also decrease. Fortunately, in the Philippines, day-length does not vary much from 12 hours.
Temperature
Cassava tolerates very hot climate but it does best where the mean temperature ranges from 25-30C. A critical temperature seems to exist between a daily temperature of 18-20C below which growth is reduced and yields decline rapidly. At 10C, growth ceases. In the highlands near the equator, cassava will grow in areas where the mean annual temperature is as low as 17C but fluctuations about this mean should be slight. Furthermore, the varieties used in these places have been specifically selected for adaptation under this condition.
Rainfall
Cassava grows very well where the average rainfall is more than 1000 mm a year. It is well adapted in areas with uniformly distributed rainfall ranging from 1000 mm to 3000 mm a year as long as there is good internal drainage. Cassava cannot withstand flooding or even moist soil conditions over prolonged periods but it is tolerant to drought. Cassava farming will rarely succeed in areas where the mean annual rainfall is less than 750 mm a year.
Soil
The best soil for cassava is a light sandy loam of medium fertility. However, it also grows well even under low fertility soil. Cassava grows even under low pH (4.4) as long as aluminum levels do not exceed 80% of base saturation. However, soils with high concentration of salts and of high pH (7.8 and above) can severely affect cassava compared with most other crops.
Cultural Requirements
Land Preparation
In general, one plowing and harrowing is adequate for areas just previously planted or regularly planted every year. For newly opened or fallowed areas, two to three plowings and harrowing may be necessary depending on soil tilth and weed incidence. In areas prone to water logging like in clayey soils, ridges or mounds should be constructed to prevent rotting of cuttings and storage roots.
Depth of plowing from 10 to 20 cm does not seem to affect yield in lighter soils. However, for heavier soils, plowing depth may affect root yield and deeper plowing most probably favor root production.
Variety Selection
Plant varieties that are most suitable to the area. Results of regional variety trials provide the necessary information on the suitability of a given variety to a particular set of environmental conditions.
A good cassava cultivar under Philippine conditions is characterized by high yield, resistance to pests particularly to the spider mites, high starch content, early maturity, low hydrocyanic acid content and good eating quality.
In general, the new cultivars yield higher than the old or traditional varieties even under the farmers’ system of management. The eating quality, however, of the new varieties are not necessarily better than that of the old ones.
Cassava may be classified into sweet (edible) and bitter (non-edible) types. The latter which is mainly grown in commercial plantation has higher hydrocyanic (prussic) acid content than the former. There are no distinguishing morphological differences between the two types but in general the non-edible type is bitter especially when tasted raw.
Selection and Preparation of Planting Materials
To insure good yields, only cuttings that are free from insect pests and diseases, mature, fresh and selected from vigorously growing plants should be used.
The length of the cutting varies from 20 to 30 cm depending on node number. There should be at least five nodes per cutting to have a better chance of sprouting and survival.
Cuttings should be taken from at least 6-month old plants. The best part of the stem to use is the middle part, the terminal part being too young (dries easily) and the basal part too old (lignified with little food reserve). When the age of the plant is uncertain, the size of the pith (central soft portion) may be used as a guide to maturity. The pith’s diameter should be 50% or less than the total diameter.
Stems that are too thin should not be planted. As a guide, the thickness of the stems should not be less half the regular size of the variety. When the stems are thin, not because of poor growing conditions but due to cultural practices like density planting, the thin stems might be planted without affecting field performance.
Stems should be properly stored to maintain their viability and yielding ability. The stems to be stored should be mature, free from insects and diseases and at least 1 meter long. The stems are bundled, positioned vertically (buds facing up) in a well-ventilated area and protected from direct sunlight by covering them with protective materials like coconut fronds.
When using stored stems, only those with enough moisture should be planted. A stem has sufficient moisture if upon cutting, the white latex or sap appears within three seconds. The germinated and rooted parts should be discarded.
Planting
If water is available, cassava can be planted anytime. In the presence of a dry period lasting for several months, it is best to plant at the onset of the rainy season.
Plant horizontally in the furrow when rainfall is uncertain and vertically on the ridge when there is plenty of rain and water logging is a potential problem. Slant or diagonal planting is done when the conditions are neither too dry nor too wet.
There are other things to consider in deciding the position of planting to adopt. In vertical planting, the buds grow earlier giving a headstart over the weeds but harvesting is more difficult and planting takes more time since one has to be careful not to plant the cuttings in an inverted position. Cassavas planted vertically are also more resistant to lodging. On the other hand, those planted horizontally or diagonally are easier to harvest but lodge easily. Planting depth is shallower (5-8 cm) for horizontal planting and deeper for vertical or diagonal planting (10-15 cm). For dry and sandy soils, planting is deeper and for moist and heavier soils, planting is shallower.
Plant only one cutting per hill. When the soil is not fertile and the variety is not branchy, plant about 20,000 cuttings/ha (1 m x 0.5 m spacing). In fertile soil and with vigorously branching varieties, plant about 10,000 cuttings/ha (1.0 m x 1.0 m spacing).
Replant missing hills only if they exceed 30% of the total population and not later than two weeks after planting using longer cuttings when planting vertically.
Weed Control and Cultivation
Control the weeds during the first two months after planting. This can be done by hand weeding in combination with off-barring and hilling-up. Cultivation, which loosens the soil aside from controlling the weeds, is beneficial to the expanding storage roots. Cultivation should not be done beyond two months after planting to prevent damage to the developing storage roots unless the canopy is still limited due to the poor growing conditions. To save on hand weeding, cassava can be planted in straight rows in two directions so that cultivation can be made perpendicularly.
Other methods of controlling weeds in plantations are: high population density planting, inter-cropping with short maturing crops like legumes and using herbicides and mechanical cultivators.
Irrigation
Although cassava is a drought-tolerant crop, yield is also reduced with limited rainfall. Under Philippine conditions, it is not economical to irrigate cassava because of its low economic value. The best way to evade moisture stress is to plant at the onset of the rainy season.
Fertilization
Most cassava plants are planted in very poor soils. If yield has to be increased, it is a must to apply fertilizer. Following the recommended amount based on soil analysis by the Bureau of Soils, yields are at least doubled and sometimes even tripled with the application of fertilizers. In the absence of a fertilizer trial or soil analysis, it is recommended to apply 50-50-75 kg/ha of N-P2O5-K2O in all kinds of soil regardless of season to sustain yield and maintain soil productivity. The recommendation is equivalent to about 7 bags of complete fertilizer (14-14-14) and one bag of muriate of potash (KCl).
Apply all the complete fertilizers, phosphorus, and potassium about 5 cm below and 10 cm away from the plants either in band or on a per hill basis at planting. For high levels of nitrogen fertilization, apply one-half upon planting and the other half two months later or just before the final hilling-up.
For farmers who cannot afford to by fertilizers, crop rotation with legumes or fallowing is recommended. If leguminous plants such as ipil-ipil or madre de cacao abound in the area, incorporate the fresh leaves (5-10 t/ha) during land preparation.
Pest Control
There are many pests attacking cassava. Fortunately, only a few are found serious in the Philippines.
Two species of spider mites of the genus Tetranychus are serious pests especially during the dry season. They suck the plant sap from the under-surface of the leaves causing the leaves to turn yellow then brown before falling off. The damage symptoms appear first among the lower leaves. Recommended control measures are stripping and burning of affected leaves, use of clean cuttings and resistant varieties and planting at the onset of the rainy season. There are a number of available acaricides but these chemicals are expensive and not recommended for practical use.
The scale insects (Saisettia spp.) are another group, which are becoming serious especially under continuous cultivation. They suck the sap from the stems that make them unfit for planting. The use of resistant varieties and clean planting materials are recommended control measures.
Among the cassava diseases, the cassava bacterial blight (CBB) is the most serious and destructive especially during the rainy season. Symptoms of the disease are angular leaf spotting and blight, wilting, die-back, gum exudation and stem and root vascular necrosis. The use of resistant varieties coupled with cultural practices like wider spacing, elimination of infected plants and the use of bacteria-free planting materials are the most promising control measures.
The brown leaf spot is the most common disease infecting cassava but the yield loss is not so serious as that resulting from CBB. The disease occurs throughout the year but is most prevalent during the wet season. Spots are present on both leaf surfaces. The spots appear more or less circular starting from faded green to brown with darker borders. Eventually, the lesions turn somewhat irregular to angular due to the limitation by the leaf margins and the veins. In the advanced stage, infected leaves turn brownish and drop prematurely. The disease is usually not controlled in commercial planting.
Harvesting
The right time to harvest depends on the variety. Some varieties can be harvested optimally from about 10 months for the early maturing to 18 months after planting for the late maturing. However, the eating quality of the roots also affects the decision when to harvest for the fresh market. To determine the best time to harvest, pull out a few (about 10 samples plants) randomly every 10 days starting 9 months after planting to evaluate the eating quality. If there is no further increase in yield and the eating quality is all right, the crop may be harvested.
For the fresh market, only the needed quantity should be harvested at any one time since the roots deteriorate very fast within 2-3 days after harvesting. In this case, staggered harvesting starting from one end of the field to the other end should be practiced so that the harvested area can be immediately used for other crops if desired.
To harvest, cut the tops leaving the stump, about 30 cm. for grasping when uprooting the plant. If the soil is hard, mechanical harvesting aids that grasp the stem as it is raised make harvesting operations easy.
If cassava is to be processed into chips for animal feed or for flour production, harvesting has to be done during the dry season since artificial drying is expensive and uneconomical.
Source:
Root Crop Digest
Vol. 2 No. 1 1987
PRIS ISSN 0116-4325