As there are no suggestions, a good crop that enriches the nitrogen content of the soil in about 4 percent is the field peas (Pisum sativum). The field peas have in their roots symbiotic bacteria (Rhizobia) which have the ability to fixate nitrogen. This nitrogen is then converted into ammonium which is used by the pea plant. When these plants are left in the field to die after a harvest, i.e., all of the nitrogen from the plant is absorbed by the soil getting richer for future crops.