Examples of this group of plants are alder, ceanothus and Russian olive. The planting of nitrogen fixers such as alfalfa and soybeans as green manure crops is a long-established practice. Trees and shrubs can be used in a similar fashion. For example, alder has been reported as being included in the rotation of Asian rice fields, as a companion plant for apples in the Netherlands. Masanobu Fukuoka interplants acacia trees in his Mandarin orange groves in Japan, and autumn olive is being used successfully as a nurse crop for walnuts in the American Midwest.
Several factors enhance nitrogen fixation.
For maximum production, the soil must be low in nitrogen but otherwise in good condition, with good structure and aeration and adequate levels of calcium, phosphorus and potassium. The nitrogen production of legumes often is enhanced by inoculation with a healthy strain of Rhizobium especially suited to crop and location. Azolla Azolla is a genus of small fern that lives naturally in lakes, swamps and streams. Its agricultural importance lies in its symbiotic relationship with the blue-green algae Anabaena , a relationship very similar to those described above for legumes and actinorhizal plants. Azolla provides nutrients and protection for Anabaena , while the algae produces more than enough nitrogen for both itself and its host.