Window boxes are containers.....containers need potting soil.......and potting soil - to work the way it is intended - shouldn't include any common soil amendments.

Unless it is impregnated with slow release fertilizer, potting soil is void of any nutrients. Nutrients must be provided by the gardener in the form of regular fertilization. Heating and drying isn't zapping the nutrients. They leach out with the need for frequent watering that all containers require. That's why they instruct one to fertilize containers "weekly, weakly" or once a week at half strength.

There is also a limit as to how long potting soil will continue to function as intended. Most bagged potting soils contain ingredients that start to break down and begin to collapse and compress after a single growing season. That affects how well the soil drains and how easily plant roots are able to access oxygen. Most experienced container gardeners will change out inexpensive bagged potting soil every new growing season. A better quality soil could stretch to two growing seasons. If not a complete re-do/replace of the potting soil, then at the very least a 50-50 refresh with new soil.

Thank you, this is very helpful. Any suggestion for ’container fetilizer’?

Any fully water soluble or liquid fertilizer. Miracle Gro works perfectly fine and contains most trace elements as well. Or Dyna Gro's Foliage Pro.