A Detailed Guide to the Growing Environment and Cultivation Techniques of Sweet Potato

The Growing Environment of Sweet Potato

(1) Temperature. Sweet potato likes warmth and fears cold. When planting slips, at a soil depth of 5-10 cm and a soil temperature of around 10°C, roots will not form; at 15°C it takes 5 days for roots to form; at 17-18°C rooting is normal; at 20°C roots form in 3 days; at 27-30°C roots can form in just one day.

When the air temperature is 25-28°C, the stems and leaves grow quickly; above 30°C the stems and leaves grow even faster, but the tubers swell slowly. Above 38°C respiration consumes a great deal, and the stems and leaves grow slowly; below 20°C the stems and leaves also grow quickly; at 15°C growth stops; if the temperature stays below 10°C for too long or frost occurs, the stems and leaves wither and die.

When the soil temperature is between 21-29°C, the higher the temperature, the faster the storage roots form and the greater their number, but the tubers are smaller. A soil temperature of 22-24°C is more favorable for the formation of storage roots. A soil temperature of 20-25°C is most suitable for tuber swelling; below 20°C or above 30°C swelling is slower; below 18°C some varieties stop swelling; below 10°C they are prone to chilling injury; at -2°C the storage roots freeze. During the tuber swelling period, a larger day-night temperature difference is favorable for tuber swelling and the accumulation of nutrients.

(2) Light. Sweet potato likes light. Under sufficient light, the leaves are darker in color, the leaf age is longer, the stems and vines are thick and strong, the conducting tissue of the stems is well developed, and the yield is higher. If there is insufficient light, the leaves turn yellow, more leaves drop, the leaf age is short, the stems and vines are thin and long, the conducting tissue is poorly developed, less organic nutrition formed by assimilation is transported to the storage roots, and the yield is low.

A long daily light period is favorable for stem and leaf growth, the vines become longer, and the number of branches increases. A daily light period of 12.5-13 hours is more suitable for tuber swelling. A daily light period of 8-9 hours is favorable for budding and flowering, but not suitable for tuber swelling.

(3) Water. Sweet potato is a drought-tolerant crop, but too much or too little water is unfavorable for increasing yield. Sweet potato fears waterlogging, especially after tuber formation, when waterlogging has a great impact on yield. When the soil is alternately dry and wet, the internal and external growth rates of the storage roots become uneven, often causing skin cracking. In short, sweet potato fears both waterlogging and drought. As people say: “Dry soil grows woody roots, wet soil grows fibrous roots, and neither dry nor wet soil grows storage roots.” To obtain a high yield of sweet potato, irrigation should be timely and appropriate according to specific conditions, drainage should be thorough and timely, and in dry land cultivation and soil moisture conservation should be strengthened.

(4) Nutrients. Sweet potato has a strong ability to absorb fertilizer and tolerates poor soil, but to achieve high yield, sufficient fertilizer must be applied. In addition to nitrogen, phosphorus and potassium, sulfur, iron, magnesium, calcium and others also play important roles. Among the three major elements, sweet potato requires the most potassium, followed by nitrogen, and the least phosphorus. According to analysis, every 1000 jin of sweet potato contains 3.5 jin of nitrogen, 1.75 jin of phosphorus and 5.6 jin of potassium. Therefore, increasing potassium fertilizer and applying nitrogen and phosphorus fertilizer timely and appropriately has a significant yield-increasing effect.

(5) Soil. Soil with a deep plow layer, rich in organic matter, loose, well aerated and well drained, such as sandy loam and sandy soil, is best. When the soil is heavy and clayey, the storage root skin color is poor, rough, the tuber shape is irregular, the yield is low, and storability is poor. However, sandy loam and sandy soil generally have lower fertility and poor water retention, so fertility should be gradually built up through measures such as fertilization before high yields can be obtained. Sweet potato is relatively tolerant of acidity and alkalinity, with an adaptive pH range of about 4.5-8.5, but 5.2-6.7 is preferable. When the soil salt content exceeds 0.2%, sweet potato should not be planted.

Cultivation Techniques for Sweet Potato

Seedling Raising Methods

Sweet potato needs to be raised as seedlings 2 months before transplanting, that is, spring sweet potato begins seedling raising in late January, autumn sweet potato in mid-to-late June, and early winter sweet potato in early-to-mid August. One-time seedling raising may be used, or propagation may be done in stages.

Each year, seed selection and seedling raising are carried out for purification and rejuvenation. Seeds should be selected from fields free of viral disease and scab disease and with a uniform variety. Medium-sized seed tubers with a long, even shape, no ridges or grooves, straight form, smooth skin, and no disease or insect spots or wounds should be chosen as seed. Before planting, soak in 1.0% copper sulfate solution or 2% sodium hydroxide solution for 5-10 min, or soak the seed tubers for 2 h in biological agents permitted for organic production such as Bt and Beauveria bassiana. It is best to use insect-free gauze greenhouse conditions to propagate virus-free seedlings.

The seedling bed base fertilizer should be applied sufficiently. Generally, for every 667 sq m, apply 500-1000 kg of self-composted organic fertilizer and 50 kg of mineral potassium-magnesium fertilizer. Before sowing, disinfect the seedling bed with lime-sulfur mixture or lime, or fumigate the shed for disinfection.

Seedling Stage Management

During the sweet potato seedling raising period, topdressing should be applied in a timely manner according to seedling condition. The fertilizer used should be biologically organic fertilizer certified as organic or permitted for use by an organic certification body (total NPK nutrient content ≥6%, organic matter content ≥40%). The total amount of topdressing during the seedling raising period is generally 250 kg per 667 sq m. At the first “red bud” stage, generally apply dilute fertilizer water; when the seedlings are 10-13 cm high, a second topdressing may be carried out. After each fertilization, sprinkle and wash the seedlings with clean water to prevent fertilizer from sticking to the seedlings and causing seedling burn.

Earthing up can be done in 2-3 rounds. When the seedlings are about 10 cm high, the first earthing up may be carried out, and after 7 d a second earthing up is done, with a total of 3-5 cm of soil added. For earthing up, it is best to use fertile, loose, fine soil mixed with burnt mud ash or decomposed compost, evenly scattered into the seedling bed. Earthing up can be combined with applying liquid fertilizer, doing earthing up first and then fertilizing, so that the soil and the base of the seedlings are closely combined, which helps early and abundant new root development.

After sowing, spray once every 15 d with biological agents permitted for organic production such as Bt and 300-fold tea seed cake water, scatter wood ash once; and place sex pheromone traps around the seedling bed or outside the seedling shed.