Why can plants not use atmospheric dinitrogen directly?

1. Atmospheric nitrogen, the diatomic molecule known as dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems. 2. Plants cannot take advantage of it because they lack the enzymes needed to convert it into biologically useful forms. 3. Nitrogen can instead be fixed, meaning converted to ammonia, by biological, physical or chemical processes. 4. The ammonia that results can be transported into plant tissue and built into amino acids and then proteins. 5. Nitrogen matters because it is part of nucleic acids and proteins, so a plant cut off from it cannot grow. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 31 "Soil and Plant Nutrition", section 31.3 Nutritional Adaptations of Plants_