law of diminishing returns

In economics, diminishing returns is also called diminishing marginal returns or the law of diminishing returns. According to this relationship, in a production system with fixed and variable inputs (say factory size and labor), beyond some point, each additional unit of variable input yields less and less additional output. Conversely, producing one more unit of output costs more and more in variable inputs. This concept is also known as the law of diminishing returns, law of increasing relative cost, or law of increasing opportunity cost. Although ostensibly a purely economic concept, diminishing marginal returns also implies a technological relationship. Diminishing marginal returns states that a firm's short run marginal cost curve will eventually increase. It is possibly among the best-known economic "laws."

A simple example
Suppose that one kilogram (kg) of seed applied to a plot of land of a fixed size produces one ton of harvestable crop. You might expect that an additional kilogram of seed would produce an additional ton of output. However, if there are diminishing marginal returns, that additional kilogram will produce less than one additional ton of harvestable crop (on the same land, during the same growing season, and with nothing else but the amount of seeds planted changing). For example, the second kilogram of seed may only produce a half ton of extra output. Diminishing marginal returns also implies that a third kilogram of seed will produce an additional crop that is even less than a half ton of additional output. Assume that it is one quarter of a ton.