Q.My gardenia is indoors next to a sliding glass door.
Is this a good place to put my plant? Right now it is loosing its leaves and there are no more blooms. Some of the plant died and I cut the dead branches off. How do I find out the acidity of the soil and what kind of fertilizer should I use and how often?
You need to be sure your plant isn't receiving direct sunlight through the glass. Gardenias would be happier outside in semi shade with you bringing them into the house in the winter if you have issues with frost. Fertilizing with an acidifying fertilizer like Miracle Gro Miracid is a good option. This fertilizer will also work for azaleas, camellias, ferns and other acid lovers. To encourage blooms once the plant is healthier use something like water soluble Super Bloom alternating with the Miracid. Be careful not to over feed.
This is most likely a watering issue, one of your containers may be draining water more quickly. Soil can dry out at different rates.
Container plants may need additional watering when days are hot, even twice a day in some cases.
https://www.gardeningknowhow.com/ornamental/flowers/african-daisy/african-daisies-osteospermum.htm