Those leaves are called the cotyledons. They are the starter leaves of the plant. They are meant to eventually die off.
When the larger fan leaves at the bottom of the plant start yellowing, that typically indicates a Nitrogen deficiency, and is a signal to start providing nutrients to the plants.
When the larger fan leaves at the bottom of the plant start yellowing, that typically indicates a Nitrogen deficiency, and is a signal to start providing nutrients to the plants.