Abstract An Fe‐efficient and an Fe‐inefficient cultivar of soybean were grown in calcareous Hacienda loam soil with FeEDDHA, P and N fertilizers in factorial combination. With the Fe‐efficient cultivar (Hawkeye), there was response to Fe alone (40 percent) and very small responses to P alone (5 percent) and N alone (13 percent). When N and P were combined there was antagonism. The Fe with P and Fe with N both gave sequentially additive interactions or very near. When all three inputs were combined the responses to Fe, P and N, respectively, were 80, 35, and 44 percent, and the interaction was synergistic. Each nutrient was a Liebig‐type limiting factor. With T‐203 soybean, FeEDDHA gave no response when used alone, and N and P singly and together were very antagonistic. When all three nutrients were supplied simultaneously, the interaction was synergistic with responses to Fe, P and N, respectively, 569, 471, and 116 percent. Even after Fe had been supplied, the N and P were Liebig‐type limiting factors. Y...