In the eyes of many, bacteria are typically seen as villains. However, not all bacteria are bad; some can be considered “good guys.” If you pull up a soybean plant and look at its roots, you’ll see numerous round nodules. These nodules are treasures for the soybean as they are factories producing nitrogen fertilizer, a beneficial activity performed by bacteria.
Around 1866, Russian biologist Winogradsky, German biologist Hellriegel, and Weirfass, almost simultaneously discovered a type of bacteria living in the root nodules of soybeans—rhizobia. Later, in 1888, it was discovered that these rhizobia could absorb nitrogen from the air and convert it into nitrogen fertilizer for the crops, turning the nodules into nitrogen fertilizer factories.
Under a microscope, you can see these nitrogen-producing bacteria: some look like short, thick sticks; others like balls, and some appear in ‘Y’ or ‘T’ shapes.
The life of rhizobia is quite interesting: originally, the roots of soybeans do not have any nodules. However, when they encounter rhizobia in the soil, these bacteria invade the soybean roots and stimulate the cells within the roots to divide more rapidly, resulting in the formation of nodules. These nodules become the “dormitories” for the rhizobia and the “workshops” for producing nitrogen.
Normally, rhizobia absorb some nutrients—sugars—from the soybean roots for survival. In return, the soybean absorbs a lot of nitrogen fertilizer from the rhizobia, enabling its leaves to grow lush and green. They truly help each other, hence they are called “symbiotic organisms.”
Don’t underestimate the capabilities of rhizobia—they can extract nitrogen from the air to make nitrogen fertilizer, a skill that humans lacked more than a hundred years ago. Despite the critical need for nitrogen fertilizer for crops, and although both humans and crops live in an atmosphere that is nearly 78% nitrogen, neither could directly absorb and utilize atmospheric nitrogen. It was only after the development of the chemical industry that humans began to use atmospheric nitrogen to produce nitrogen fertilizers like synthetic ammonia directly.
Not only soybean roots but many leguminous plants like mung beans, peas, beans, and peanuts also develop root nodules. Rhizobia have been immensely helpful to humans: each acre of leguminous crops can absorb 7-27 kilograms of nitrogen from the air annually! Consequently, fields of leguminous crops often do not require nitrogen fertilizers, or only need a small amount.
Even after a year of growing soybeans, although the soybeans and most of the rhizobia die, a significant portion of the nitrogen produced by the rhizobia remains in the field. Thus, crops like rice or wheat planted subsequently often grow better. Agriculturally, this leads to crop rotation, alternating legumes with other crops to enhance yield, known as the “crop rotation method.”
Nowadays, people also cultivate rhizobia in factories on a large scale, creating “bacterial fertilizers.” Applied to fields, these hardworking rhizobia can similarly increase the soil’s nitrogen content, boosting crop yields.