What natural process contributes nitrogen to the soil?

The process by which nitrogen is contributed to the soil through leguminous crops is particularly significant due to a unique symbiotic relationship they form with nitrogen-fixing bacteria located in their root nodules. These bacteria convert atmospheric nitrogen, which is not directly usable by plants, into ammonia or other compounds that plants can assimilate. When leguminous crops, such as soybeans or peas, are grown and later decomposed, they enrich the soil with nitrogen naturally. This is a vital aspect of sustainable agriculture because it reduces the need for synthetic nitrogen fertilizers and improves soil health over time.

While decomposition, another natural process, releases nutrients into the soil, it does not specifically focus on nitrogen fixation; its role is more general as it breaks down organic matter. Photosynthesis primarily converts carbon dioxide and sunlight into energy, and watering serves to supply moisture to the plants, but neither of these processes plays a direct role in adding nitrogen to the soil. Therefore, the ability of leguminous crops to harness atmospheric nitrogen is a key mechanism for replenishing soil nitrogen levels.