The tiny white threads that cover many plant roots are actually mycorrhizal fungi. Mycorrhizal fungi are not always apparent above the ground. While some do grow mushrooms to disperse their spores above ground, others do not. For example, truffles grow beneath the ground. When you fertilize your plants with commercial fertilizers you may be doing more harm than good.
Most commercial fertilizers contain salts.
Salts not only burn the roots of your plants, but they kill mycorrhizal fungi and other healthy organisms in the soil. While your plant may look greener, in the long run you are jeopardizing your plant's health by killing the mycorrhizal fungi they rely so heavily upon. If your plants need supplemental nutrition, use organic products, which are far less likely to harm either your plants or the organisms in your soil. These pathogens preclude some species from growing in certain locations. They also may cause some species to die, letting new species start growing in an area.