Bark and moss can also harbor bacteria, fungus, or gnats, and make it very difficult to accurately test the soil moisture needs. These are important for health and vigor, flowers, and healthy foliage. Fertilizer provides nitrogen. You can check this with a pH meter, but these are not extremely precise. Moisture Plant species can vary dramatically in water usage, but there are factors which can change how much a given plant will use.
Plants prefer to be evenly moist, with a reservoir of water at their roots to use when they dry out.