Bill lives in montana and likes to grow zucchini. he applies fertilizer to his crops twice during the growing season and notices that the second layer of fertilizer increases his crop, but not as much as the first layer. what economic concept best explains this observation?

The economic concept that best explains this situation is the law of diminishing returns. The law of diminishing returns explains that as an increased input is put onto an object with all other factors held constant, the output of that product will continually decrease as more and more of that input is added.