Answer: You can get your soil tested to see what the pH range might be, but you'll have to pay for the service and if you're only concerned about a single plant, it doesn't seem to me to be worth the expense. In general, gardenias like an acidic soil. If you didn't mix some peat moss into the soil prior to planting your gardenia, you can begin to lower the pH by feeding with an acidified fertilizer (such as Miracid) and mulching over the soil beneath the plant with some peatmoss. Over time, both will lower the soil's pH and help your plant perk up. If you use a water soluable fertilizer such as Miracid, you can also spray the foliage as you water the plant with the fertilizer mixture. Gardenias like moist soils so water as often as necessary to keep the soil from drying out.
Best wishes with your gardenia!
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