Upon moving the lemon outdoors for the summer, it appears to have suffered a minor amount of environmental damage - seemingly leaf scorch, either from too much direct sun or low overnight temperatures. The damaged leaves can be removed if bothersome, but are otherwise healthy and still able to feed the plant and removal is not necessary. Plants moved outdoors should be acclimated to the brighter light levels gradually, as existing leaf tissues cannot adjust easily to such a change in conditions. New leaves that grow in outdoor conditions will be better-suited to the change in light levels, ambient humidity, and temperatures. While citrus do appreciate cooler temperatures than tropical houseplants, if they were accustomed to heated indoor spaces during the winter, they need time to adjust to cooler spring evenings and should only be set outdoors after danger of frost has passed.

We do not see signs of nutrient deficiency, though if the plant hasn't been repotted in the past year or two, it would be beneficial to do that this spring. Fresh soil can correct salt buildup (from fertilizer and tap water minerals) which will damage roots in time; damaged roots can manifest as nutrient deficiencies since they are not healthy enough to absorb the nutrients the plant needs. In such circumstances, adding more nutrients only exacerbates the problem. For repotting, remove all of the old soil that you can and replace with fresh potting soil. Cactus and succulent soil blends tend to work well for citrus, since they appreciate excellent drainage. Potting soil pH tends not to be an issue, but you can opt to use fertilizers labeled for citrus or acid-loving plants in general. More guidance on fertilization and other care tips for citrus is available here: https://extension.umd.edu/hgic/topics/growing-dwarf-citrus.

Miri