Plants need minerals to grow. Back in the ‘better living through chemistry’ days, we’d add a bunch of nitrogen rich fertilizer to the soil and wait for the plant to take off. A lot of that fertilizer never got absorbed by plants, and often the excess washed into streams, lakes, and bays, wreaking all sorts of havoc with the water systems as a result.
You can help your plants better absorb nutrients in the soil by adding compost.
Here’s a very simplified version of how it works: Nitrogen, Phosphorous and Potassium invite Carbon to play a game of musical chairs. The ‘chairs’ in this story are the roots of a plant. Carbon’s a volatile kind of kid, and if the environment around the roots is rich in organic matter, Carbon is easily bounced off of her chair and back into the soil. Nitrogen, Phosphorous and Potassium then jump in to that void, and, voila, the plant now gets the minerals it needs to grow.
Compost/Organic matter improves a soils Cation Exchange Capacity- otherwise known as it’s ability to facilitate the exchange of nutrients between a plant and the minerals in the soil.