Key Technical Points for Soil Testing and Formulated Fertilization in Sweet Potato Cultivation

1. Fertilizer requirements: Of the three major elements nitrogen, phosphorus and potassium, sweet potato has the greatest demand for potassium, followed by nitrogen, then phosphorus. Sweet potato likes potassium, and increasing potassium fertilizer has a clear effect on both yield and quality. Sweet potato absorbs few nutrients at the seedling stage; from the branching and tuber-setting stage to the vigorous stem and leaf growth stage, the rate of nutrient uptake accelerates and the amount absorbed increases, then gradually decreases near the later stage; by the stage of rapid tuber enlargement, nitrogen and phosphorus uptake declines, while potassium uptake remains at a relatively high level. With nitrogen deficiency, the older leaves first show chlorosis, the number of leaves and branches decreases, the leaves become smaller, the internodes shorten, and the leaves tend to yellow and senesce early, seriously affecting yield. With phosphorus deficiency, the leaves become smaller, turn dark green and lose their gloss, stem elongation is hindered, and large yellow spots appear on the older leaves, turning purple, and they soon drop off. Potassium deficiency is shown by small leaves, shortened internodes and petioles, and dark green leaf color, with brown spots appearing on the underside of the leaves in the later stage. Sweet potato is sensitive to chloride; when chlorine-containing fertilizers are applied above a certain amount, the starch content of the tubers decreases, and the tubers do not store well.

2. Fertilization technique: At a medium fertility level, the fertilizer application rate for sweet potato over the whole growing period is, per mu, farmyard manure 2500-3000 kg (or commercial organic fertilizer 350-400 kg), nitrogen fertilizer 12-14 kg, phosphorus fertilizer 6-8 kg, and potassium fertilizer 9-11 kg. Nitrogen and potassium fertilizers are split into a base fertilizer and two topdressings, while all of the phosphorus fertilizer is used as base fertilizer; chemical fertilizers and farmyard manure (or commercial organic fertilizer) are applied together.

Base fertilizer: Apply per mu farmyard manure 2500-3000 kg or commercial organic fertilizer 300-400 kg, urea 4-5 kg, diammonium phosphate 13-17 kg, and potassium sulfate 5-7 kg. Slow-release fertilizer is recommended.

Topdressing: At the tuber enlargement stage, apply per mu urea 16-19 kg and potassium sulfate 13-15 kg.

Foliar topdressing: In the middle and later stages of sweet potato growth, spray the leaves with a 0.5% urea dilution, or a 2%-3% superphosphate solution, or 5% wood ash water, or a 0.2% potassium dihydrogen phosphate solution. Spray once every 6-7 days, for 2-3 sprayings.