In the 1840s, the French chemist Jean Baptiste Boussingault began investigating the nitrogen requirements of plants. He discovered that some plants, such as legumes (peas, beans, etc.), thrived perfectly well on nitrogen-free soil even when watered with nitrogen- free water. Not only did they grow, but their nitrogen content steadily increased. Boussingault could only conclude that they were absorbing nitrogen from the air.

(We now know that plants do not achieve this themselves; instead, certain strains of nitrogen-fixing bacteria, which grow in root nodules, are responsible.)

However, Boussingault further demonstrated that animals absorb nitrogen not from the air but only through their food. He refined the more qualitative findings of Magendie by precisely analyzing the nitrogen content of various food items and comparing it with growth rates. When using only a single food as a nitrogen source, a functional relationship could indeed be established. However, this relationship resulted in different growth rates for different foods despite the same nitrogen content. From this observation, it could only be concluded that some types of protein were more suitable for bodily nutrition than others. The reason for this remained unclear until the end of the century. Nevertheless, around 1844, Boussingault was able to compile a list based on empirical data, showing the relative utility of various foods as protein sources.

He is credited with laying the foundation of agricultural science.