X Cultivation methods
As a highly popular crop, sweetpotatoes are cultivated not only by professional farmers but also by amateurs, including elementary-school pupils learning about agriculture. Sweetpotatoes also enjoy a special popularity in family gardens. A number of special guides to cultivation techniques for sweetpotatoes are available. (For more information, see the "Reference" section toward the end of this book.) Accordingly, we have summarized here only key points concerning cultivation practices.
(1) Conditions for seed sweetpotatoes
First, thoroughly investigate the characteristics of several varieties, and select a variety suitable for your intended purpose. Prepare seed potatoes that seem to be good examples of your chosen variety, based on factors such as shape, skin color and regularity. All should be free of disease and healthy, and about 200 to 300 g in size.
(2) Quantity of seed sweetpotatoes
Though the required quantity depends to some degree on the variety chosen, it is generally necessary to prepare about 40-80 kg of seed sweetpotatoes per 10 are. Sprouting patterns, numbers of sprouts, and growth speeds of sprouts also differ according to variety. For "Norin No. 1" and "Kokei No. 14," for example, to start plant production and complete planting at the same time, Kokei requires 3 times more seed sweetpotatoes than Norin. Large quantities of seed sweetpotatoes must be prepared, if for example, the roots are large and nursery beds are narrow, if the bedding soil is poor, the sprout cutting time is early and the frequency of sprout cutting is low, or if the number of roots to be planted is large.
(3) Disinfection of seed sweetpotatoes
Even if disease-free seed sweetpotatoes are secured, some may already have been affected by black rot, low temperature, or poor air circulation. As a result, all roots must be disinfected before use. Two disinfection methods are available: hot water and chemical-based methods. In the former, potatoes are immersed in 47-48 degrees C hot water for 40 minutes. In the latter, benomyl is used.
(4) Pre-sprouting treatment
Until the seed potatoes begin to sprout, provide a higher than ambient temperature and appropriate humidity. After sprouting, raise sprouts at closer to ambient temperature until they become high-quality, healthy sprouts. If proper temperature and humidity are provided, sprouting will be complete in several days. In small-scale private cultivation, germination can be easily forced by using a rice sprouting cabinet. For larger-scale operations, a curing shed should be used.
Although the temperature for forcing germination varies by variety, it is generally about 30 degrees C. Structure forced germination such that the sprouts reach about 10 mm in length over 3-5 days.
(5) Nursery bed preparation
Choose the time to prepare the nursery bed after considering your planting schedule. The standard period from planting of seed sweetpotatoes to the first sprout collection is about 40-50 days, though the type of nursery bed used can affect this timing by up to 1 week.
(6) Planting
Align the tops of seed sweetpotatoes by height, laying larger ones in the center of the bed (which is warmer) and smaller ones along the bed periphery, so that their ends lay in the bed soil at an angle of 15 degrees. After planting, cover roots with soil so as to hide their tops, then irrigate fully with lukewarm water.
Scatter 5-7 liters/m2 of smoked charcoal over the covering soil. Cover the bed with straw to prevent heat radiation and drying.
(7) Nursery bed fertilization
Fertilizer amounts differ greatly depending on the fertility of the bed soil. When ordinary field soil is used, as in open-field and cold nursery beds, the standard amount of fertilizer per m2 is 6 kg of compost, 20 g of nitrogen, 10-15 g of phosphoric acid, and 15-20 g of potassium. When fertile bed soil prepared in the fall of the previous year is used, there is no need for fertilizer. Instead, simply add fertilizer while observing sprout growth. When forming a new nursery bed, use compost, apply 100-150 g of nitrogen, 100-150 g of phosphoric acid, and 150-200 g of potassium per m2.
(8) Nursery bed management
Keep nursery beds at a relatively high temperature until the seed sweetpotatoes have sprouted. After sprouting, keep the temperature at 22- 25 degrees C in the daytime and 18 degrees C at night as the sprouts lengthen. When sprouting begins , remove the cover gradually. When the sprouts lengthen to about 5 cm, expose sprouts to the sun for 2-3 hours during the daytime; at about 10 cm, remove the cover and expose sprouts to the sun fully. The moisture of the bed soil should be about 70%, or enough so that a handful of soil does not lose shape when squeezed together.
While observing the sprout growth, add a maximum of 5-10 g per m2 of nitrogen fertilizer at a time. By spraying 1.0-1.5 liters per m2 of 1% urea liquid 3 days before sprout cutting, rooting and initial growth will be accelerated.
During this time, sprouts are often damaged by soft rot disease, black-rot, virus disease, Fusarium wilt (stem rot), Imokoga, or Nakajiroshitaba. To prevent such damage, it is important to take the measures as shown in the following table.
(9) Cutting
While the period of time from first sprouting to sprout maturation varies by variety and nursery bed, it averages 40 days. Generally, sprouts 25-30 cm long with 7-8 nodes and fully developed leaves are regarded as mature. When collecting sprouts, cut them one by one carefully with a knife, leaving 1-2 nodes near the ground.
The frequency of sprout cutting is governed by the sprout raising method used, by the nursery bed area and by cultivation area. In general, sprouts can be collected 3-4 times in early harvest and 5-6 times in general harvest. The quality of the 2nd-4th cutting will be higher than that of the first cutting. The quality of the fifth and later cuttings tend to decline due to increased fiber levels. Disinfect sprouts with benomyl and warm water. Sprouts can be preserved for 4-5 days in a windless, shaded area, or for 1 week if there is proper humidity.
○ Type of nursery beds and number of days to sprout collection (first)
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Open-field nursery bed, cold nursery bed |
Suitable for southwest in japan, warm districts. Sprouts make use of natural heat in warm places. This approach takes many days, and the sprouts are not uniform - though such nurseries are low-cost and labor-saving to run. |
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Cold nursery bed using vinyl |
The temperature of the nursery and soil is raised by covering the cold nursery bed with vinyl film. This approach can be used in the Kanto area in combination with an easy fermentation hotbed. |
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Forced-germination cold nursing bed using vinyl; forced-germination nursery bed using greenhouse |
After forcing germination, sprouts are transplanted to the vinyl cold bed and greenhouse nursery bed. It is important to prepare the nursery bed early and to keep its temperature high beforehand. |
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Fermentation hotbed nursery bed |
Trample on rice straw, compost, manure, and rice bran, to make them generate heat, and cause the sprouts to grow under high-temperature conditions. The nursery bed can be used for early planting. |
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Electric heat hotbed |
Sprouts are raised by maintaining temperature using electric heat from a hotbed wire. Appropriate temperature for sprouting can be obtained easily. This approach enables planned cultivation practices to be pursued, though there are added equipment costs. |
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House nursery bed |
Cold nursery bed in heated greenhouse, and hot nursery bed using electric heat and hot water in non-heated type greenhouse. Since the beds are large, thermal efficiency is high. They are suitable for a large group of farms. |
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Note: |
The above numbers were obtained by calculations using a yield of 80 kg/10 are for seed potatoes, 5,000 planted sprouts, and four sprout collections. |
○ Diseases in nursery beds and countermeasures
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Disease and insect pest |
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Soft rot |
The surface of the bed soil has a white, weblike appearance; the seed sweetpotatoes become soft and rotten. Upon discovery, immediately remove the seed sweetpotatoes, and irrigate the bed with benomyl. |
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Black rot |
Black spots appear at the base of sprouts. Leaves turn yellow and stop growing. Immediately after discovery, remove the seed sweetpotato, collect the sprout, leaving 5-6 cm from the base, and immerse about 5 cm of the sprout's base in 47-48 degrees C warm water for exactly 15 minutes. Otherwise, immerse the sprout in a 500-fold benomyl water-dispersible powder solution for 20 minutes. |
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Virus disease |
Leaves become odd-shaped, shrink, and appear dappled. Remove them. |
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Fusarium wilt or Stem rot |
Leaves turn yellow and thicken. When bent, they break and become glossy. Cracks often appear in them. Upon discovery, remove and burn the affected leaves. |
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Sweetpotato leaf folder and Sweetpotato leaf worm |
Sweetpotato leaf folder cause leaves to roll up while Sweetpotato leaf worm eats them. Discover these pests at an early stage and spray affected areas with insecticide. |
(1) Soil disinfection
Sweetpotatoes are susceptible to soil insect pests such as the root-knot nematode and root-lesion nematode. Such damage not only delays initial growth and causes nutritional disorders but also makes the sweetpotatoes malformed and produces dark-brown or brown spots on their surfaces. Sweetpotatoes thus affected have no fresh market value.
To avoid such unfavorable conditions, select highly resistant varieties, practice rotation, and cultivate pest resistant crops such as peanuts. During cultivation for fresh consumption, disinfect the soil with chlorpicrin and DD agents.
(2) Plowing, land preparation, and fertilization
Plowing is effective for softening cultivated soil, improving air permeability and drainage, and preventing weeds from growing. However, in volcanic-ash soil zones, plowing may leave the soil subject to drought damage and can also reduce yields, depending on climate conditions. Excessive rotary plowing should thus be avoided in such zones. In zones with more clay-like soil, however, plowing is indispensable. In these zones, operating efficiency can be improved by applying compost, lime, and phosphoric acid at the time of plowing.
(3) Ridging
In fertile soil and humid soil, form higher ridges; in infertile and dry soil, form lower ones. High ridges improve soil permeability, allow leaves to receive light, create large soil temperature differentials to curb the growth of aboveground parts of sweetpotatoes, and increase yields. Moreover, high ridges facilitate harvest operations including vine cutting and lifting. In infertile and dry fields, though, high ridges may reduce crop yields. This risk must be taken into account. For crops intended for fresh consumption, mulch cultivation using plastic film is frequently employed.
(1) Planting time
If the soil temperature is no lower than 18 degrees C, early planting of sweetpotatoes produces higher yields. But when planting is too early and temperatures are too low, roots do not differentiate smoothly, the sweetpotato stem is long, the number of roots decreases, and their shape becomes irregular. And since roots take root over a wider range, harvesting efficiency (particularly in the lifting phase) declines. Finally, excessively early planting leads to higher levels of damaged sweetpotatoes and a less valuable crop.
Determining just how early sweetpotatoes can be planted depends considerably on soil conditions in the immediate area; even within the same district, the appropriate timing for planting can vary considerably. Planting can begin earlier in sandy soil, sandy loam, and south facing slopes, but must be delayed in volcanic ash soil, alluvial soil and north facing land. Generally, planting begins in late April to early May in western Japan including Kyushu, early to late May in eastern Japan including the Kanto area, and early June in the Tohoku area. It should be noted that early planting entails competition with preceding crops and requires added labor for weed control .
Meanwhile, if planting time is delayed into June in the Kanto area or mid to late June in western Japan, yields decrease. At such times, the number of sprouts to be planted must be increased.
(2) Planting method
Transplanting methods include horizontal, improved horizontal, concave, fish hook, oblique, and upright planting. Adopt a method that minimizes planting damage while also considering storage root growth, soil and sprout properties.
In horizontal and improved horizontal planting, large sprouts are planted sparsely and roots grow at each node. Accordingly, both methods require good-quality sprouts (e.g., those raised in open soil) and are suitable for warm districts such as those in western Japan. By contrast, the hook, oblique, and upright planting methods are utilized for early-cultivation fields intended to yield low volumes of larger-sized potatoes. These latter methods are effective for areas where short, thick sprouts are planted densely, such as the Kanto area.
Since labor saving is accelerated by use of mechanized planting, transplanting machines are being developed; at present, they offer about twice the operating efficiency of human labor. Future improvements would be desirable.
O Planting methods and their characteristics
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Horizontal planting |
Sprouts are planted shallowly and horizontally, so that many sweetpotatoes may grow from the nodes of sprouts |
Sweetpotatoes grow easily, but are apt to dry; also, their rooting is poor. |
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Improved horizontal planting |
In order to improve rooting of sprouts, cut ends are planted deeply in the soil. |
Sprouts produce many sweetpotatoes and are resistant to drying, but planting efficiency is lower. |
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Concave planting |
Sprouts are planted in a concave form, with the central part slightly deeper. |
High planting efficiency, less planting damage, good yields, but irregular potato shapes. |
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Fish hook , oblique, and upright planting |
Sprouts are planted in hook, oblique, and upright form, in mulch cultivation for early harvesting. |
Sweetpotatoes grow in shallow 1-2 nodes of the sprout; per-plant yields are low. |
(3) Planting density
Planting density for sweetpotatoes varies depending on the degree of growth expected of a given variety, field fertility, the properties of the cultivated soil and its intended use. As a result, actual planting densities vary greatly by region. Generally speaking, the standard number of sprouts is 4,000-5,000 per 10 are for sweetpotatoes intended for secondary uses, and 3,000-4,000 per 10 ares for those intended for fresh consumption. The standard ridge width is 70-100 cm, ridge height 20-35 cm, and hill distance about 25-40 cm. Mechanization will play an important role in future agricultural practices. In order to aid efforts to mechanize the sprout planting and harvesting operations, certain cultivation parameters are now being standardized. Standard practices call for a ridge width of 90 cm, ridge height of 20-30 cm, and hill distance of 25-40 cm (15 cm for cell-forming sprouts)
(1) Amount of fertilizer applied
Among fertilizer components, sweetpotatoes have the greatest sensitivity to nitrogen, followed by potassium and phosphoric acid. According to trial data, production of 1 ton of sweetpotatoes requires 3.4 kg of nitrogen, 2.0 kg of phosphoric acid, and 9.0 kg of potassium. Note the potassium absorption value, which is especially large. Compared to other crops, sweetpotatoes experience greater declines in yield when nitrogen and potassium fertilizers are not applied. The effect of compost application on yield is extremely high in sweetpotatoes. Conversely, excessive nitrogen causes "Tsuruboke" (excessive growth of vines), leading to poor root formation, inhibited root growth, and sharply lower yield.
In general cultivation, a standard fertilizer application for sweetpotatoes is 3-6 kg of nitrogen, 4-8 kg of phosphoric acid, and 8-12 kg of potassium per 10 are.
The tuber forming from the root is said to absorb potassium and enlarge. To thicken the tuber, it is important to apply potassium fertilizer to the soil's deep layers in order to increase potassium concentrations, so that potassium can be absorbed until the later periods of root growth.
○ Fertilization effects
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Nitrogen Nitrogen promotes the growth of the sweetpotato's aboveground parts as well as dry matter production; it also helps root growth and increases yield. With excessive nitrogen, however, the aboveground parts grow too thick, causing "Tsuruboke" as well as sharply lower yields. Accordingly, an appropriate amount of fertilizer should be determined by studying the reaction of each variety, the relative fertility of the field, and planting times. Phosphoric acid Sweetpotatoes absorb relatively little phosphoric acid. Though this acid's effect on yields is small, deficiencies cause the plant's leaves to turn deep green and its aboveground parts to weaken. Use of high levels of phosphoric acid enables the storage roots to lengthen, starch content to increase, and storability to improve. Also, steamed sweetpotatoes become starchy and sweeter. Potassium Potassium affects storage root growth substantially. Use of high levels of potassium accelerates the migration of assimilation products, improves the photosynthetic ability of leaves, and enlarges tubers. The ratio of potassium to nitrogen should be about 3: 1.
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○ Tsuruboke (excessive vine growth)
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(2) Fertilization methods
With fertilization, the key point is to match it to the growth of the sweetpotatoes. For nitrogen, ensure that the potatoes fully absorb nitrogen in their initial and middle periods of growth when the above-ground parts grow actively, and inhibit absorption in the later period - so that roots can grow better. For phosphoric acid, full absorption is also important, as it helps roots spread during the plant's initial period of growth. For potassium, to ensure absorption over the plant's entire period of growth, perform deep-layer fertilization and use slow-acting fertilizer. The initial fertilization is critical for areas with clay or loam soil. In areas with sandy soil and sandy loam, it is advisable to consider periodic fertilization as a countermeasure against runoff.
(3) Effects of compost and barnyard manure
Application of compost and barnyard manure to sweetpotatoes is far more effective than with other crops, thanks to the synergistic effect of fertilizer components and physical improvement of the soil. Since fertilizer components remain largely in place and are slow acting, a compost/manure application works to promote both enlargement of the sweetpotatoes and growth of plant foliage. Physically, it eliminates excessive drying and humidification by improving air permeability and water retention, thereby promoting root development. In addition, manure is effective for activating microbial activity and supplementing trace elements in the soil.
For amounts of 1 ton or less, compost and barnyard manure can be applied to ditches at the time of ridging, but for larger amounts, it is better to spread the fertilizer over the entire surface before plowing.
(1) Weeding
Sweetpotatoes generally have strong resistance to weeds because the vines cover the ground surface. However, since vines do not yet cover the ground for 40-50 days after planting, weeds can grow readily; if left alone, these weeds will inhibit storage root growth. Most weeds come out 10-20 days after plowing and ridging, so it is effective to use herbicides before planting.
In open field cultivation, weed-control measures such as intertillage and ridging can be performed in conjunction with specific weed-killing measures.
(2) Intertillage and ridging
Intertillage and ridging promote the growth of the plant's vines, enlarge storage root, increase yields, and assist in weed control. These steps should be performed 1-2 times within 30 days after planting. Specifically, till the ground lightly with a cultivator or ridge control loader, and keep down weeds. During the first pass, ridge the ground lightly with a small ridge plate; during the second, fully ridge the ground up to the roots of sprouts with a large ridge plate or a ridger. When using mulch cultivation, note that intertillage and ridging are unnecessary.
○ Effects of intertillage and ridging
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(3) Additional fertilizer
Generally, if initial fertilizer is applied sufficiently, additional fertilizer is not necessary. However, in open-space cultivation - where fertilizers can run off easily if sandy soil, high temperatures, or strong rains are a factor - it is recommended to apply additional fertilizer, in conjunction with intertillage and ridging, about 30 days after planting, when sprouts take root and roots (future tubers) differentiate.
Fertilizer should contain 1.0-1.5 kg of nitrogen and three times as much potassium as nitrogen, so as not to induce "Tsuruboke."
(4) Disease and pest control
Diseases and insect pests affect sweetpotatoes relatively infrequently. Diseases that do occur include Fusarium wilt (stem rot), soil rot (Pox), black rot, and kokuseibyo. Insect pests include imokoga, nakajiroshitaba, gold bugs, root-knot nematode and root-lesion nematode. Take care to control such diseases and pests before they can cause damage.
(1) Harvest
Harvest time for sweetpotatoes is governed by their maturity and purpose of use. For storage roots for table use, there are multiple harvests: an extremely early one for high-grade vegetables, followed by an early harvest, middle harvest, general harvest, and harvest for storage. The timing of harvests is also affected by market prices. For use in starch-related products, early and late maturing varieties are grown in combination to reflect the operating period of factories. These varieties are generally harvested from mid-October to early November in the Kanto area, and from early October to late November in Kyushu.
○ Patterns in sweetpotato growth
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(Kyushu, Shikoku) |
In southern regions of Japan, the plant's vines grow steadily after planting, and begin thickening in early to middle September. At that time, storage roots begin bulking and rapidly enlarge. They continue to bulk until the plant's foliage turns yellow and falls off. Starch yields reach their highest levels in late October to early November. |
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(Kanto) |
In northern regions, the plant's vines begin growing rapidly immediately after planting. Even during the harvest period, foliage barely turns yellow or falls off. Storage roots begin to enlarge quickly in mid-July, and achieve about 70% of their growth during August and September. |
(2) Harvest methods
Sweetpotatoes were formerly harvested using a series of steps including vine cutting, digging, preparation, sorting, shipment for sales and storage. In recent years, however, due to the greater use of mulch cultivation, it has become necessary to add another step: mulch removal. To assist this process, machines that perform simultaneous vine cutting and mulch removal have been developed.
As with planting, harvesting accounts for a large share of the time required for sweetpotato cultivation. Recently, compact riding harvesters - used to harvest storage roots intended for fresh consumption - have become popular among sweetpotato farmers. Larger, self-propelling harvesters suitable for harvesting storage roots intended for processed foods or starch-related products have also been developed, and are expected to become widely used.
The appropriate temperature for sweetpotato storage is 13-15 degrees C, with 80-90% humidity. Sweetpotatoes can be stored in a variety of locales including buried in the ground, underground holes, heated curing rooms, or warehouse-type facilities. While sweetpotatoes were once stored in underground holes by individual farmers, large-scale interior (container) storage has become more prevalent in recent years due to the growing scale of collective farms and to the need for year-round shipping to fresh markets.
(1) Underground hole storage
Storage types include both vertical and lateral deep holes suitable for long-term, large-scale storage; and underground type, round hole type and ditch types suitable for simple, smaller storage. After storage, cover the hole with straw and vinyl to protect the stored storage roots from rainwater and cold air, and leave them for about 2 weeks until the respiration storage roots' heat has settled. Then, provide a ventilation opening, cover it with soil, and increase the soil amount as the weather becomes colder. Close the ventilation opening in mid-December to seal the storage areas. The standard storage temperature is 13 degrees C and 80-90% humidity. Generally, decay and loss run to about 10-20% of the stored product. Storage must be ended by the end of April because the potatoes will be begin to sprout as soil temperatures rise.
(2) Curing storage
In curing storage, intended for longer term storage, cork layers are formed in damaged parts of potatoes and under the epidermis, and the damage is cured to prevent infection by pathogenic microorganisms. By forming cork layers, resistance to disease-related damage is increased. Compared with underground hole storage, curing storage can reduce decay and improves the commercial value of sweetpotatoes. Freshly harvested sweetpotatoes release more respiration heat and moisture, easily retain temperature and humidity, and cause the cork layer to form more quickly.
To prepare a curing storage facility, thoroughly sprinkle water throughout the entire storage area, close it, and heat the sweetpotatoes. Usually electric heat sources are used. The standard temperature is 32-33 degrees C at 100% humidity. Under these conditions, about 4 cork layers are formed in 100 hours. Next, open the storage area to reduce the temperature over 12-24 hours to a level suitable for storage. The key treatment points are quick heating and heat radiation, and maintenance of uniform temperature and humidity.
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1) When black rot was a problem in the past, sweetpotatoes were cured at 35-36 degrees C to help control that disease. Since it is now of little concern, curing is done at a lower temperature which helps the cured sweetpotatoes store better. |
(3) Thermostat humidistat storage facility
In both underground hole and curing storage facilities, the storage period is limited from November to late April. However, in thermostat-humidistat storage, sweetpotatoes are maintained at an appropriate temperature and humidity in a large-scale low-temperature storage facility; storage can thus be continued until June and July. Temperature is automatically controlled by heating in winter and cooling in summer, while moisture is retained using charcoal and rice husks, both to maintain humidity and prevent disease damage. The sweetpotatoes are stored in containers, where they can be left until shipping of early- harvest sweetpotatoes begins in June or July.
With sweetpotatoes, continuous cropping causes no soil sickness and relatively few problems with insect pests, so cultivation is generally easy. Indeed, continuous cropping is thought to produce better results, and many potatoes for fresh consumption are grown in this manner. When continued for too many years, however, continuous cropping reduces the soil's vitality and yields decline. This trend is particularly true of fields having infertile or sandy soil which cannot hold sufficient nutrients and of fields where high-yield varieties are continuously cropped. In continuous cropping, attention must be paid to Shimonhabyo and root-knot nematode. The former is especially difficult to eradicate once it develops.
To avoid the damage associated with continuous cropping and to maintain soil vitality, it is necessary to establish an appropriate crop rotation system. Perform crop rotation using soybeans, peanuts, and vegetables for 2-3 years alternately, or cultivate sweetpotatoes in summer, and cultivate onions, cabbages, carrots and other vegetables before and after summer. Remember that there will be residual fertilizer activity in fields where vegetables have been planted, so fertilizer-resistant varieties of sweetpotatoes must be selected.
○ Direct planting cultivation
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If the sprout from the storage root is embedded in the soil shallowly and then exposed to the surface, a root will come out from the node and become a storage root. The shape of the vine that grows from this storage root resembles a dragonfly sitting on the ground, so this method is termed "dragonfly" cultivation. |