Data from a published paper [Rehm (1984)] were used to illustrate different interactions encountered between N and P fertilizers in a five‐year field test of a mixture of grasses. In a 5 × 5 factorial experiment, sequentially additive, synergistic, slightly synergistic, and Liebig‐synergistic responses were all observed. Therefore, there were instances of Liebig‐type limiting factors, Mitscherlich‐type limiting factors, and in‐between gradations. There was little indication of antagonism between N and P. The relationships observed between yields and N doses indicate an increasing critical dosage for P with increasing N.