Overwatering and under watering are usually the reason that a Katy plant's leaves are turning yellow. While Katy plants can withstand being a little too moist, it's bad for the plant to have a lot of water in the soil. This is because the leaves will absorb the extra liquid and eventually turn yellow. There are other causes, however, like plant disease or low sunlight. The majority of the time, yellow leaf foliage is due to a potassium deficiency. You can add some kind of potassium source, such as bone meal, perlite, potash, or blood meal when we water our plants.