Banana peels are chemically composed of potassium, phosphorus, calcium, manganese, sodium, magnesium and sulfur. Potassium is by far the most common element, composing as much as 42 percent of dried banana peels.
Potassium is useful in the growth and development of plants. Plants need potassium to promote nutrient flow among cells, as well as to strengthen stems and protect the plant from disease.
The phosphorus content in banana peels is used by plants to promote flowering and fruiting. Phosphorus also helps improve the durability of plants during winter.
Magnesium and sulfur are needed by plants to form chlorophyll, which is needed for photosynthesis.
For these reasons, dried banana peels are often placed around plants in order to absorb these elements.