Final answer:
Legumes put the most nitrogen into the soil through nitrogen fixation.
Step-by-step explanation:
Fertilizers, legumes, lightning, and decomposition all contribute to nitrogen in the soil, but the one that puts the most nitrogen into the soil is legumes. Legumes have a symbiotic relationship with nitrogen-fixing bacteria, which convert atmospheric nitrogen into usable forms that can be absorbed by plants. This process is called nitrogen fixation. Additionally, nitrogen-based fertilizers, such as ammonium nitrate and urea, are widely used in agriculture to boost soil nitrogen levels. Decomposing organic matter, including plant residues and animal waste, contributes nitrogen. Overall, a combination of these factors and practices is often necessary for maintaining optimal soil nitrogen content.