Answer: Yellowing leaves is quite common and can be caused by a variety of things, including transplant shock, over- or underwatering, shock from being moved from one location to another (such as from the greenhouse where it was raised to a home environment, or indoors for winter when it spent the summer outdoors), and lack of nitrogen. Since you did not mention the type of plant, I'll give you some basic info.
How often to water depends on the type of plant (i.e., cactus require very little water) and the soil's characteristics.
You probably noticed that fertilizers have 3 numbers on the container. These numbers refer to the percentage of nitrogen (N), phosphorous (P), and potassium (K) in the fertilizer. These 3 elements are referred to as macronutrients because plants need them in fairly large (i.e., macro) amounts to thrive. In general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers (and eventually fruit), and potassium keeps the root system healthy. Container plants can't seek nutrients by sending their roots out to the surrounding soil, so they need regular fertilizer applications. Use a balanced fertilizer and follow package instructions.
Moving plants indoors from outdoors can be a real shock to their system because the conditions are usually so different. Yellowing and dropping leaves is a typical reaction. Don't fertilize for a month or so, but continue watering at a slightly reduced level. Resume fertilizing at about half strength. The plant might go dormant for a while and then new growth appear.
In spring, slowly acclimate your plant to moving back outdoors by putting it outside in a sheltered location for a couple hours at a time, gradually increasing the time period. In fall, do that in reverse, rather than abruptly moving from outside to inside, which will help lessen the shock. I hope this info helps!
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