I have plants outside in a plastic box. There is air and plenty of water. They are turning yellow or light green. Why?
Plants in Hot Box Turning Yellow
I would bet it is from too much water. Only water when necessary. Put your finger into the soil and see if it feels damp to the touch. If it does wait a few hours or a day depending on temperature and check again. Too much watering will also leach the nutrients out of the soil. A liquid fertilizer added to the water once a week should solve your concern.