Textbook of Field Practices in Bioresource Production, University of Tsukuba
Upland farming (1, 2) - A
Sweet potatoes
1. Uses and cultivars
The uses of sweet potatoes are classified into the following four categories: vegetables, starch material, processed food,
and alcohol material. As cultivars to be vegetables, cultivars with bright red skin, spindle or elongated spindle shape, pale
yellow or yellow flesh color, powdery and sweet flesh, and good taste are popular. ‘Beniazuma’ dominates the market, with
‘Benikomachi’ and ‘Beniaka’ being cultivated in smaller quantities. As starch materials, ‘Shiroyutaka’ and ‘Shirosatsuma’,
cultivars with high starch yield, are cultivated. For food processing, ‘Tamayutaka’, a viscous and high-yielding cultivar, is
cultivated for ‘mushi-kiriboshi’ (steamed, cut, and dried sweet potato) and ‘Kokei No. 14’ and ‘Beniaka’ are cultivated for
‘kinton’ (mashed sweet potato) and jam production. For alcohol production, ‘Koganesengan’, a cultivar with a high starch
content, is used, and recently, ‘Joy White’ has been developed as a material for fruity-flavor ‘shochu’ liquor. Other cultivars
suitable for making functional or processed foods have also been developed in recent years, including ‘Ayamurasaki’ and
‘Murasakimasari’ for anthocyan pigment production, ‘J-Red’ for juice production, ‘Sunny Red’ for powder production,
‘Elegant Summer’ whose leaf stalks are edible, ‘Satsuma Starch’ with high starch content, and ‘Konahomare’ with high starch
yield.
2. Raising seedlings
(1) Seed potatoes: A cultivar suitable for cultivation purposes is chosen. Seed potatoes
should have shapes and skin colors characteristic of the cultivar, have no diseases,
weigh 200 – 300 g and be disinfected with warm water or a chemical disinfectant.
(2) Nursery bed: Nursery bed options include natural nursery beds, fermentation
hotbeds, and house raising nursery beds; and the number of days from sprouting to
Raising seedlings using heating wires
pulling of seedling varies by nursery bed type. Fertilizer is applied according to the fertility of the nursery bed soil, and seed
potatoes are planted so that their tops are at the same heights and their tails are tilted at an angle of about 15°. Large seed
potatoes are planted in the center and smaller seed potatoes are planted around the large potatoes. Once planted, potatoes are
covered with a shallow layer of soil (so their topes are not visible). Nursery beds are maintained at 30 °C in a sealed
environment until sprouting, after which point they are cooled to 25 °C. Nursery beds are watered when the surface of the soil
has become dry. When seedlings have reached 5 cm length, the seedlings are exposed to sunlight for a few hours to let them
gradually become accustomed to sunlight, watering is reduced, and starting at 10 days prior to pulling of seedling, seedlings are
acclimated to the outside air and environment.
(3) Pulling of seedling: Seedlings 25 – 30 cm length, which have fully open leaves on seven or eight nodes, are used. Seedlings
are cut off near the ground with one or two nodes left and bundled with cut ends lined up neatly. The collected seedlings are
disinfected using benomyl or other agents and can be held aside for a few days.
3. Field preparation
(1) Soil disinfection: To prevent damage caused by soil pests, such as root-rot nematodes and root lesion nematodes, D-D,
chloropicrin, or other agents are used for soil disinfection.
(2) Fertilizer application: Three major nutrients that need to be satisfied for producing potatoes at 1 t/10a are nitrogen (3.4 kg),
phosphoric acid (2.0 kg), and potassium (9.0 kg), meaning that nitrogen and potassium have especially greater effects. General
Hisayoshi Hayashi and Morio Kato,
AFRC, University of Tsukuba
1
Textbook of Field Practices in Bioresource Production, University of Tsukuba
amount of fertilizer for 10a are 3 – 6 kg for nitrogen, 4 – 8 kg for phosphoric acid, and 8 – 12 kg for potassium. If an excessive
amount of nitrogen is applied to sweet potatoes, the aboveground parts grow excessively and the tuberous roots do not become
thick (excessive vine growth); therefore, it is necessary to use an appropriate amount of nitrogen. Compost and stable manure
are highly effective as they improve the physical properties of the soil, and its fertilizer components work gradually. Compost is
used at 1 – 2 t/10a.
(3) Tillage and ridging: Following the application of compost and fertilizer, fields are plowed and ridges 70 – 100 cm wide and
20 – 25 cm high are made. High ridges improve the air permeability of the soil and effectively thickening of tuberous roots.
Sweet potatoes for vegetables are usually cultivated in a mulch culture that uses polyethylene film. Polyethylene mulch culture
promotes initial growth by increasing soil temperature and controls weeds; however, water supply from rainfall to the ridges is
restricted, so the ridges are made when soil moisture is relatively high to promote the rooting of seedlings.
4. Sprouted vine planting
If the soil temperature is 18 °C or higher, sweet potatoes are known to have higher yields when they are planted earlier.
Planting seasons are late April to early May in Western Japan, mid- to late May in Eastern Japan, and early June in the Tohoku
region and yields decrease if planting is done later than mid-June in Western Japan and later than June in the Kanto region.
Planting methods include horizontal planting, ‘funazoko-ue’ (concave) planting ("ship bottom" planting), ‘tsuribari-ue’
(hooked) planting ("fishhook" planting), oblique planting, and upright planting. With horizontal planting, which is suitable for
warm regions, large seedlings are sparsely planted and potatoes are grown on each node. ‘Tsuribari-ue’, oblique planting and
upright planting are suited to early harvesting cultivation and dense cultivation in the Kanto region. Planting density varies by
cultivar, cultivation method, soil condition, etc., but a planting interval is generally 20 – 30 cm with a density 3,000 – 4,000
plants/10a for vegetables. To avoid damaging seedlings, seedlings are held with the thumb, index, and middle fingers at the
seedling base and inserted obliquely into soil so that three nodes at the base are in soil. To prevent plant death caused by warm
air coming out of the mulch holes, the bases of planted seedlings are covered with an appropriate amount of soil as weight.
5. Cultivation management
Sweet potatoes grow extremely slowly in the initial growth stage and the soil surface is therefore exposed for over one
month after planting, making the field prone to weed damage. For this reason, after planting, herbicides are used to control
weeds. Also, depending on the weed occurrence, a cultivator is used for intertillage. In sandy soil or open culture, which is
prone to the runoff of fertilizer, topdressing is done at
around 30 days after planting, along with intertillage
and molding. Pest control is done at an optimum timing
based on forecasting of occurrence.
6. Mechanism of tuberous root thickening
Sweet potatoes are storage organs formed through
the thickening of roots. Since seedlings are planted
"Funazoko"
normally to grow sweet potatoes, parts of adventitious
roots grown from nodes grow into tuberous roots. The
"Oblique"
"Upright"
Difference in tuberous root formation resulting from different planting
adventitious roots differentiate from young roots to
rootlets, pencil-like roots, or tuberous roots depending on the environmental conditions. A root consists of epidermis, cortex,
Hisayoshi Hayashi and Morio Kato,
AFRC, University of Tsukuba
2
Textbook of Field Practices in Bioresource Production, University of Tsukuba
and central cylinder. The parenchyma cells of the xylem of the central cylinder are not especially lignified, and young roots
with well-developed cambia differentiate into tuberous roots and thicken. Even if the cambium is active, advancement of
lignification results in the formation of pencil-like roots, though if lignification is limited, inactive cambium results in the
formation of rootlets.
7. Harvest
Sweet potatoes can be harvested between approximately three months after planting and immediately before the first
frosting, when tuberous roots are at their largest. However, the harvest time is also determined by purpose of use, market price,
and thickening of tuberous root. Stems and leaves are not resistant to frost and potatoes easily become rotten when the
temperature dips below 0 °C. Potatoes left under 9 °C for a long time also become prone to rotting. Thus, in the even of cold
temperatures, the harvest should not be delayed. Harvesting immediately after rain should be avoided and the best time is when
the soil is reasonably dry. Harvest work consists of vine cutting, mulch removal, digging up, preparation, and storage. Vines are
cut using machines such as a frail mower. Vines near the ground are cut with a sickle and then mulch sheets are removed. For
harvesting, a potato digger or a sweet potato harvester is used. Harvested potatoes are removed from vines one by one and
sorted into damaged or other unsellable potatoes and sellable potatoes. Preparations are made before shipping in accordance
with shipment rules and regulations.
8. Storage
Potatoes should be stored at 13 – 15 °C and a relative humidity of 80 – 90 % in
soil, an underground hole, or a room. In curing storage, a cork layer is formed under
the wounds and skin of the potatoes, which prevents the entry of pathogens and
reduces rotting, thereby improving the potatoes' storage characteristics. Curing is
done by sealing the storage and maintaining the temperature at 32 – 33 °C and the
relative humidity at 100 % for 100 hours and then quickly releasing the heat.
Storage facility of seed potatoes
- To learn about sweet potatoes in general, visit the Website of the Japanese Society of Root and Tuber Crops
(http://www.jrt.gr.jp/index.html).
- Today's practice (Spring Semester): Planting of sweet potatoes
① Cultivar: You will plant ‘Beniazuma’ (five rows).
② To examine the effects of the different planting methods on tuberous root formation, ‘Beniazuma’ is planted using three
different planting methods: the oblique, upright, and "funazoko" (concave) methods. One row (75 m long) is divided into three
sections; oblique planting will be used for the first section, upright planting will be used for the second section, and "funazoko"
planting will be used for the third section. With all these methods, plants are planted so that three nodes are under the soil. The
bases of the plants will be covered with additional soil and compacted.
③After the planting is done for one research row, you will be divided into four groups and each group will responsible for
planting seedlings in one row. Make sure group members are spread evenly over each line and not concentrated in one place.
There should be no spots missing seedlings. The method used for this will be oblique planting. It is important to scrape soil
toward and compact the seedling bases.
Hisayoshi Hayashi and Morio Kato,
AFRC, University of Tsukuba
3
Textbook of Field Practices in Bioresource Production, University of Tsukuba
Special cultivation
The sweet potatoes you will grow will obtain Specially Cultivated Agricultural Product certification and be sold. In sweet
potato cultivation, Ibaraki Prefecture sets conventional application levels of pesticide and fertilizer as follows: nine applications
of chemical pesticide and 3.0 kgN/10a of chemical fertilizer; standards for Specially Cultivated Agricultural Products are four
applications of pesticide and 1.5 kgN/10a of fertilizer. As for the sweet potatoes you will grow, chemical pesticide will be used
twice and cattle mature will be applied at 2 t/10a; thereby restricting the use of chemical fertilizer to 1.5 kgN/10a.
- Today's practice (Autumn Semester): Harvest of sweet potatoes
① You will harvest the sweet potatoes you planted in the spring. First, let's observe the differences in tuberous root formation
resulting from the different planting methods.
② Aboveground parts will be cut using a frail mower.
③ Where mulch sheets cannot be removed because of remaining vines, a sickle will be used for cutting them.
④ Mulch sheets will be removed. They will be rolled while being peeled off and placed in fertilizer bags.
⑤ Potatoes and vines will be dug up together by hand. Do this carefully so as not to cut off rootlets and pencil-like- roots.
⑥ Be sure to make a sketch of the harvested potatoes. Observe how tuberous roots, pencil-like roots, and rootlets are formed.
⑦ After making a sketch for the oblique planting section, do the same for the upright planting section and the "funazoko"
planting section.
⑧ Potatoes will be removed from the stems and gently placed in buckets. Do not throw them into baskets. This is to avoid
damage to the skins.
⑨ Pencil-like roots, rootlets, and stems will be placed in separate baskets.
⑩ Baskets filled with potatoes will be transported from the field.
Hisayoshi Hayashi and Morio Kato,
AFRC, University of Tsukuba
4