Why do lettuce plants need nitrogen and potassium fertilizer during the head-filling stage?

During the stem elongation and head-filling period, lettuce needs a sufficient supply of nitrogen and potassium fertilizer to fill out quickly. The specific application method is as follows:

Apply nitrogen and potassium fertilizer once each at the early, middle and late stages of lettuce stem elongation and head filling. Each time, apply 15-20 kg of urea and 8-10 kg of potassium sulfate per mu, or 20-25 kg of potassium nitrate, diluted 150-200 times with clean water, biogas slurry, or decomposed and rotted human and livestock manure water, then evenly drench the soil around the base of the lettuce plants, ideally so that the fertilizer solution fully soaks into the soil without running off.

??? In addition, during the stem elongation and head-filling period, spray the leaves once every 7-10 days with a mixture of 0.3% urea and 600-times amino acid potassium solution, or with a 0.5% potassium nitrate aqueous solution, for 2-3 consecutive sprays. Spray evenly to wet all the leaves, ideally until water droplets begin to drip down, to supplement nitrogen and potassium through the leaves.