Usable forms of nitrogen can intentionally be increased in the soil by
lightning.
planting more trees.
planting fewer beans.
allowing organic matter to decompose.
Usable forms of nitrogen can intentionally be increased in the soil by allowing organic matter to decompose. When organic matter such as leaves, grass clippings, and other plant residues decompose, the process is facilitated by soil microbes that break down the material into simpler compounds, including forms of nitrogen that plants can use, such as ammonium and nitrates.
Here's how the other options impact soil nitrogen:
- Lightning: Although not intentional, lightning does indeed increase the amount of usable nitrogen in the soil. Lightning can cause atmospheric nitrogen (N2) to combine with oxygen to form nitrates (NO3), which are then deposited in the soil via rain. However, this is a natural phenomenon and not something that can be practically controlled or used as a farming strategy.
- Planting more trees: Trees can indirectly increase soil nitrogen through nitrogen-fixing species or by the accumulation and subsequent decomposition of organic material, such as leaf litter. Certain tree species form symbiotic relationships with nitrogen-fixing bacteria, which convert atmospheric nitrogen into forms usable by plants. However, simply planting more trees doesn't guarantee increased soil nitrogen unless those trees are nitrogen-fixing species.
- Planting fewer beans: This option would actually lead to a reduction in soil nitrogen over time. Beans and other legumes are known for their ability to fix atmospheric nitrogen into the soil through symbiotic relationships with Rhizobia bacteria in root nodules. By hosting these bacteria, legumes convert nitrogen from the air into ammonia, which can then be used by the plant and, once the plant dies and decomposes, by other plants in the ecosystem. Hence, planting fewer beans would decrease the natural input of usable nitrogen into the soil.