Although Calathea like a soil pH of around 6.5 and coffee grounds tend to be around a pH of 6.5-6.8, making it acceptable from a pH standpoint, it is not recommended to add coffee grounds directly to your Calathea soil. In order to make any nutrients in coffee grounds available to plants, the coffee grounds must be composted first. Adding coffee grounds directly to soil before composting will rob the soil of nitrogen and can actually harm your plants. Usingany uncomposted kitchen scraps will actually create conditions that are favorable to fungus, mold, bacteria and also fungus gnats.