Testing the Validity of Stanford's 1.2 Rule for N Fertilizer Recommendation

We evaluate an old and widely accepted rule of thumb for fertilizer management in corn production: apply 1.2 pounds of nitrogen fertilizer per bushel of corn expected. This “1.2 Rule” has dominated fertilizer management recommendations for almost fifty years, and similar algorithms have been used all over the world to make fertilizer recommendations for other crops. Here we show that the 1.2 Rule only makes economic sense if the production satisfies two restrictions: (1) to be of the von Liebig functional form, i.e. the function has a “kink” and a “plateau,” and (2) the kinks of the von Liebig response curves for different growing conditions lie on a ray out of the origin with slope 1.2. Non-linear estimation techniques and non-nested hypothesis framework are used to test if the 1.2 Rule satisfies these restrictions. We conclude that there exists little scientific justification of the 1.2 Rule, and that its long-term and widespread use basically resulted from its long-term and widespread use.