The answer is B. Plant a crop of legumes, such as clover or alfalfa.
It is known that plants cannot directly use atmospheric nitrogen. But, some legumes have a symbiotic relationship with bacteria that live in their root system. These bacteria are called Rhizobia and have the ability of nitrogen-fixation. In the root nodules, they use atmospheric nitrogen to convert it into ammonia, and later to ammonium, which can be used by plants. When legumes die, nitrogen from their remaining is released back to the soil where it is available to the other plants.
