Nitrogen fixing plants are a cornerstone of permaculture plantings. Through a symbiotic relationship with organisms in their roots, these plants can take nitrogen from the atmosphere and turn it into bioavailable fertilizer useful to themselves and also becoming available to their neighbors over time through root die back, leaf fall, and chop and drop practices. While it does not replace the need to bring in phosphorus, calcium, potassium and other nutrients depleted by harvests, this strategy provides a free source of a very essential fertilizer.