Potato Cultivation Techniques
The potato (Solanum tuberosum), also known as the Irish potato, white potato or earth apple, is an annual herbaceous plant of the Solanaceae family. Its tubers are rich in starch, protein and mineral salts, and are highly nutritious. They can be used as a vegetable, as a staple food or as feed, and also as a food or industrial raw material, giving them an extremely wide range of uses. The potato has a short growing period and high yield, and tolerates transport and storage; it can be intercropped or relay-cropped with crops such as corn and cotton, and is praised as a crop that does not occupy land. In vegetable production it fills gaps in the off-season supply, and its market is extremely broad. The potato's cultivated area is expanding very rapidly both at home and abroad.
(I) Biological Characteristics and Traits
Potatoes propagated from tubers have no main root, only fibrous roots, which are distributed within the tilled soil layer and penetrate rather shallowly. Seedlings grown directly from true seed have roots that penetrate more deeply. The potato stem can be divided into three types: aboveground stem, stolon and tuber. The aboveground stem is 20 cm to 70 cm tall, triangular or polygonal in cross-section, with straight or wavy stem wings, which serve as the basis for identifying different cultivars. The stolon, also called the underground stem, extends horizontally and is distributed within the tilled layer, ranging in length from 2 cm to 30 cm; for cultivation it is best to choose cultivars with short stolons, to facilitate harvesting and intercropping with other crops. The tuber is the productive organ of the potato, formed by the accumulation of nutrients and swelling at the tip of the stolon. The tuber bears eyes, and the closer to the top end, the denser the eyes. Tuber skin colors include white, yellow, pink and red. Flesh colors include yellow and white. On the international market, yellow skin with yellow flesh is preferred. The leaves are odd-pinnate simple leaves. Potatoes propagated asexually from tubers: in the Yangtze-Huai River basin, many cultivars do not flower. To conserve nutrients and promote tuber enlargement, flowers should be removed promptly even in cultivars that do flower. The potato is a naturally self-pollinating plant, with a cymose inflorescence and a funnel-shaped corolla; the petals are white, light red, purple-red or blue-purple, and there are no nectaries. The fruit is a berry, and the seeds are sesame-like, with a 1000-grain weight of 0.4 g to 0.6 g and 80 to 300 seeds per fruit. In the Yangtze-Huai River basin, cultivars that can flower and set seed normally in spring include Fengshoubai, Youyi No. 3, Hongwenbai and Fengshou. Potatoes grown from true seed have a longer growing period, and management is labor-intensive; most existing potato cultivars are hybrids, and their progeny segregate in traits, so the size, shape, skin color and flesh color of the tubers produced are very inconsistent. There is as yet no successful example of large-scale use of sexual reproduction in production.
The potato prefers warmth, fears cold and does not tolerate heat. Tubers that have passed dormancy develop roots at 4°C to 5°C and sprout at 5°C to 7°C; the optimum temperature for young sprout growth is 18°C, and 21°C is suitable for stem and leaf growth. Tuber formation is best at 15°C to 18°C; above 25°C tubers grow slowly, and above 30°C the accumulation of nutrients is unfavorable. If stems, leaves and tubers are exposed to -1°C to 2°C for a long time, the plants will freeze to death and the tuber eyes will die. Because the tuber stores water during sprouting, the potato is more drought-tolerant than other vegetables. During plant growth, water demand is greater from the early bud stage to the flowering stage; in the Yangtze-Huai River basin, apart from irrigation when necessary, spring rainfall is generally abundant, so particular attention should be paid to ditching and drainage in the field. The potato is a short-day crop, but through artificial selection and long cultivation its response to day length is no longer obvious, and it can form tubers in either spring or autumn; however, if planting is too dense during the growing period, insufficient light will cause excessive stem and leaf growth and weaken the plant's disease resistance. The potato is a light-loving plant, but tubers exposed above the soil surface, if exposed to sunlight for a long time, will turn green and their solanine content will increase. Solanine has an astringent taste, and eating too much can cause poisoning in humans and livestock, and even death. Therefore, when eating, the green skin should be peeled off and the potatoes thoroughly cooked, so that solanine decomposes under high temperature. In cultivation, attention should be paid to intertillage and hilling so that tubers are not exposed above the soil surface. The potato requires deep, fertile sandy loam; when grown in heavy clay soil, tuber shape is irregular, eyes protrude, and yield and starch content are low. The potato grows well in slightly acidic soil at pH 5 to 6. The potato requires relatively large amounts of nutrients, especially potash; the preferred ratio of nitrogen, phosphorus and potassium is 4:8:12. When applying potash, potassium sulfate should be used rather than potassium chloride. In addition, the potato has special requirements for trace elements such as boron and copper.
(II) Cultivar Selection
Cultivation systems, cultivar types, cultivation characteristics and production problems vary from region to region. According to a survey by the Keshan Institute of Agricultural Sciences, Chinese potatoes are divided into 4 major regions: the northern single-cropping region, the central plains double-cropping region, the southern double-cropping region, and the southwestern mixed single- and double-cropping region. The Yangtze-Huai River basin belongs to the central plains double-cropping region; because the growing period is too short, early or early-to-mid-maturing cultivars should generally be chosen, as late-maturing cultivars often only grow stems and leaves and meet with damage from high or low temperatures before they can form tubers, resulting in failure. In addition, cultivars should be chosen that are high-yielding, have a short dormancy period, weak dormancy intensity, and resistance to virus diseases, scab, early blight and ring rot. Cultivars available for selection in the potato double-cropping region are shown in Table 2-22.
(III) Spring Potato Cultivation Techniques
1. Raising Seedlings
(1) Seed selection: For seed potatoes, choose tubers that are free of pests and diseases, free of freezing injury, smooth-skinned, fresh and medium in size. Diseased tubers, rotten tubers, deformed tubers, and tubers with protruding eyes or rough, cracked skin are not suitable as seed. Within a certain range, seed potato size is directly proportional to the final yield. Small seed potatoes contain fewer nutrients, the plants grow weakly, and yields are low. Generally, tubers of 50 g to 100 g are best; if seed potatoes are insufficient, they should not be smaller than 15 g. Per mu (1 mu = 667 sq m), 120 kg to 150 kg of seed potatoes are generally used, and in high-yield fields the amount used can reach 400 kg.
(2) Cutting: The main purpose of cutting is to save seed potatoes and break dormancy; cutting and trimming the heel end and breaking the skin can accelerate internal metabolism. However, when cutting, the knife must be disinfected to prevent the spread of virus diseases. When a diseased tuber is found, the knife and cutting board can be disinfected with 75% alcohol, 0.2% mercuric chloride solution, 5% carbolic acid, 0.1% potassium permanganate, 0.1% domiphen bromide, etc.; several knives can also be sterilized by boiling in salt water, and a different knife can be used when a diseased tuber is encountered. Potato eyes show obvious apical dominance; in production, to make tubers sprout earlier and evenly, the cutting method of dividing the apical bud equally should be advocated.
(3) Sprout promotion: To achieve early maturity and high yield in potatoes, spring potatoes in the Yangtze-Huai River basin can be sown directly in the field as early as around the Spring Festival. However, in years when winter is particularly cold, the potatoes may suffer freezing injury. To be safe, hotbeds, cold frames, greenhouses, plastic film mulching and other methods can be used to promote sprouting. When promoting sprouts in a bed, soil must be used to separate the potatoes from the fermenting material. Arrange one layer of potatoes and one layer of soil; if the bed surface is insufficient, 3 to 4 layers can be stacked, but stacking too thickly easily damages the sprouts. During sprout promotion, maintain 15°C to 18°C; if the temperature is too high, sprouting is faster but the sprouts are thin and long. Water sparingly; excessive humidity will rot the tubers. When the sprouts are 2 cm to 3 cm long, take them out of the bed and sow.
To speed up potato sprout promotion, gibberellin treatment is sometimes used before sprouting. For whole uncut tubers, the concentration used is 5 ppm to 10 ppm, soaking for 3 to 10 minutes; for cut pieces, the concentration used is 0.5 ppm to 1.0 ppm, and they should be taken out immediately after soaking. After soaking, the seed potatoes must be spread out and air-dried for 4 to 5 hours. The advantage of gibberellin treatment is fast sprouting; the disadvantage is that the sprouts are slender and the plants tend to age prematurely. In spring, if conditions allow sprout promotion in a greenhouse, hotbed or plastic tunnel, gibberellin treatment can be omitted.
2. Planting In the lower Yangtze region, the average temperature in February has already reached about 5°C; sowing tubers deep in mid-to-late February generally will not cause freezing damage. Although early sowing does not mean early emergence, low-temperature conditions favor root development. With early and deep sowing of potatoes, even if emerged sprouts suffer late frost damage, secondary buds can still sprout again from the seed tuber, so there is no danger of all being frozen to death. If conditions permit, promoting sprouts and planting late is more reliable.
Potato planting methods can be divided into three types:
(1) The "starry sky" planting method: both row spacing and plant spacing are about 20 cm, and this method can plant more than 11,000 plants per mu (1 mu = 667 sq m). Because of the high density, the rows close early, ventilation is poor, and there are many diseases; the potatoes produced have many "green-shouldered" tubers and many small tubers, with poor quality and low yield.
(2) Wide-row planting method: row spacing is increased to 40 cm to 50 cm, plant spacing remains 20 cm, and about 6,000 plants can be planted per mu (1 mu = 667 sq m). With wider rows, ventilation is good, there are fewer diseases, intertillage and hilling are convenient, and there are few "green-shouldered" tubers; this is currently the most common sowing method.
(3) Double-row high-ridge cultivation method: the large row is 70 cm to 90 cm, the small row is 10 cm, and plant spacing is 25 cm to 30 cm, planted in a triangular arrangement, with more than 6,000 plants per mu (1 mu = 667 sq m). This planting method provides even better ventilation and allows repeated hilling; the tubers produced are large and the yield is high.
3. Field Management Field management of spring potatoes should achieve early maturity through measures such as checking the field and filling gaps, topdressing, intertillage, hilling, and irrigation and drainage. Potato growth should be early in the early stage, steady in the middle stage, and late in senescence in the late stage. Before flowering, strive for early topdressing and early hilling; after flowering, irrigate according to weather, soil and plant conditions, so as to promote plant growth and tuber formation, use tubers to control top growth, maintain balanced growth of the potato plant, and prevent excessive growth and premature senescence.
Potatoes have many pests and diseases. At present, the more common diseases include virus diseases, late blight, early blight, ring rot, black leg, scab and black scurf. The main insect pests include the 28-spotted ladybird beetle, white grubs, wireworms and cutworms. Among these pests and diseases, potato virus diseases are the most serious; they are the main factor affecting yields in the double-cropping region. But to this day there is still no specific chemical control, and prevention can only be carried out through agricultural techniques. The control methods are described below:
(1) Establish a sound seed reservation system, select disease-resistant cultivars, and reserve seed by methods such as single-plant mixed selection, line selection, and elimination of diseased tubers. Generally, the ratio of seed reservation field area to production field area is arranged at 1:10, to ensure self-reserved excellent disease-free seed potatoes.
(2) Choose cool locations and times for seed reservation. According to experiments, the virus content of tubers formed at a high temperature of 25°C is 4 times higher than that of tubers formed at a low temperature of 15°C. Therefore, potato seed reservation in the Yangtze-Huai River basin should choose high-cold regions for seed production, or develop autumn potatoes sown in spring, using seed potatoes sown in winter and harvested in spring in protected facilities for autumn planting.
(3) In autumn, sow small whole tubers from superior disease-free single plants of spring potatoes, to prevent the spread of virus when potatoes are cut.
(4) Strengthen water and fertilizer management, enhance plant disease resistance, and inhibit the rapid development of viruses. For other fungal diseases such as late blight and early blight, spray Bordeaux mixture at 1:1:150, 80% zineb at 600 to 800 times, or 75% chlorothalonil at 600 to 800 times; according to the incidence of disease, spray once every 5 to 7 days, for a total of 3 to 4 sprays.
4. Harvesting and Seed Reservation Whether spring potato tubers are mature can be judged from the following aspects: stems and leaves turn from green to yellow; when the stem is pulled and shaken, the tubers fall off easily; when the tuber is rubbed with a finger, the skin comes off; when the tuber is cut with a knife, the wound dries easily. All these indicate that the tubers have matured and can be harvested. To secure high yield, potatoes for market supply can be harvested before June 20; if delayed to the plum rain season, the tubers rot easily.
During the spring potato harvest period, temperatures rise from low to high, and the later the harvest, the more virus accumulates in the tubers. If seed is to be reserved, harvesting should be completed around June 10. Spring potatoes should be transported as they are harvested; those that cannot be transported in time can be placed in tree shade or covered with potato stalks for shade. If tubers are exposed to the sun, they rot easily and do not store well.
(IV) Key Points for Autumn Potato Cultivation
The autumn potato growing period has high temperatures early and low temperatures late, and the growing period is relatively short, so yields are lower than those of spring potatoes. However, in the tuber-forming season of autumn potatoes, temperatures turn from hot to cool, less virus accumulates in the tubers, and disease incidence is mild; using autumn potatoes as seed potatoes for the following spring gives spring potatoes vigorous growth, less degeneration and high yield. Autumn potato production is not only for obtaining fresh autumn potatoes for the market, but more importantly for obtaining spring seed potatoes. The key points for autumn potato cultivation are described below:
(1) Select seed potatoes: Harvest early from the seed reservation field for spring potatoes. Remove injured, diseased and rotten tubers before using as seed.
(2) Switch to sowing small whole tubers: From superior disease-free single plants, harvest small tubers of 2.0 g to 2.5 g and keep them as autumn seed potatoes. Because the seed potatoes are small, cutting is unnecessary, which reduces the chance of disease infection. Each small whole tuber has an apical bud and shows obvious apical dominance. To overcome the disadvantages of small seed potatoes and low yield, sowing density can be increased, that is, 10,000 to 12,000 plants can be planted per mu (1 mu = 667 sq m); at harvest the tubers are not very large, but they are just right for use as seed potatoes the following spring.
(3) Delay the sowing date: To avoid dry, hot weather, the sowing date for autumn potatoes in the Yangtze-Huai River basin can be postponed to late August to early September. Sow when morning and evening temperatures have begun to cool.
(4) Attend closely to field management: The autumn potato growing period is short, and after sowing there are many pests and diseases and serious gaps in the stand; to obtain yield, every effort should be made to secure a full stand. During the autumn drought, watering, loosening the soil and weeding should be carried out promptly to promote stem and leaf growth and ensure autumn tuber yield; loosening the soil combined with hilling can both prevent tubers from being exposed and turning green and prevent tubers from freezing.
(5) Harvest appropriately late: The autumn potato growing period is short; on the principle of avoiding freezing injury, one day later in harvesting means one portion more yield. In the Yangtze-Huai region, harvesting is generally in mid-to-late November.
(V) Storage and Processing
1. Storage During potato storage, mechanical damage should be avoided as much as possible, and tubers that have suffered heat, cold, or infection by pests or diseases should be strictly removed. Commercial potatoes intended only for food should be stored at 2°C to 5°C; tubers intended only for seed should be stored at 8°C to 20°C. To prevent tubers from losing water and shriveling, the storage room should maintain a relative humidity of 80% to 90%. Winter temperatures in the Yangtze-Huai River basin are not very severe, so pits or trenches are often dug outdoors in sheltered, sunny, high and dry places for storage; warehouses are also used for indoor rack storage or heap storage, but the heap should not be too thick, to allow ventilation and facilitate inspection.
2. Processing The potato is both a vegetable and a food and feed. Industrially it can be used to make starch, vermicelli, syrup, maltose, malt sugar, glucose, monosodium glutamate, rayon, synthetic rubber, glutinous rice paper, perfume, lactic acid, citric acid and motion picture film, among others. These industrial products are mostly manufactured by specialized factories, and their process flows are omitted here.