Fertilizing your plants is essential to ensure they get the nutrients they need to grow. However, over-fertilizing can be just as harmful as not fertilizing at all. The key is to find a happy medium. Depending on the houseplant fertilizer you use, you will likely need to apply it every two weeks. Fertilizers with a high concentration of nitrogen and phosphorus should be applied. Apply more often because these nutrients break down quickly in the soil. A lower concentration fertilizer like potassium phosphate (K2P) should only be used monthly. In general, you should avoid mixing different types of fertilizers.