Potassium should be increased during the winter lettuce expansion period

During the elongation and expansion period of winter lettuce, potassium fertilizer must be applied every 10 to 15 days, for 4 to 6 consecutive times, 40 to 50 kg of potassium nitrate, 60 to 70 kg of potassium sulfate, or 25 to 30 kg of potassium dihydrogen phosphate per mu each time, combined with nitrogen, phosphorus and potassium compound fertilizer or fermented and decomposed flower cake fertilizer, diluted with clean water or biogas slurry or fermented and decomposed human and animal manure 50 to 60 times, and evenly drenched in the soil around the roots of the lettuce plants. At the same time, spray the leaves with 0.1% magnesium sulfate, 0.2% urea, 0.3% potassium dihydrogen phosphate, 1000 times amino acid active liquid fertilizer mixture, or 0.5% potassium arsenate, 1000 times humic acid active liquid fertilizer mixture every 7 to 10 days, spray continuously 3 to 5 times, evenly wet all stems and leaves, preferably with water droplets dripping down, and supplement potassium fertilizer from the leaves.