The Future is Abundant
A Guide to Sustainable Agriculture
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Seventy-eight percent of the air we breathe is nitrogen. Unlike oxygen, the other important constituent of air, nitrogen is not highly reactive. Nitrogen serves as the atmospheric medium in which the more active gases are carried, kept separated, kept quiet. We cannot do without nitrogen, but we do not do much with it.
Plants, however, do a lot with nitrogen. It is an essential component of all plant life, being one of the basic elements in the chemical structure of proteins. However, nitrogen in its gaseous state cannot be used by plants. Rather, nitrogen must first be converted into various nitrate compounds for it to sustain plants. This conversion, called fixation, occurs in many ways, both naturally and artificially: in fertilizer factories, by lightning storms, by certain plants, and by some special soil bacteria, yeasts, and fungi. In addition, nitrogen compounds are concentrated when consumed by fish, birds and animals, all of whom secrete this concentrated nitrogen in their manure.
In spite of all the nitrogen in the atmosphere, and all the ways in which nitrogen can be fixed into compounds usable to plants, it is still difficult to provide plants with the nitrogen they need. The use of industrially-produced nitrogen for agricultural use has increased from 500,000 tons annually to nine million tons during the last thirty years, at an annual cost of billions of cubic feet of natural gas. That is not a sustainable agriculture. Agriculture that is to survive beyond the last puff of natural gas must get its nitrogen from processes not dependent on fossil fuels. Biological nitrogen-fixation processes are many, and they are effective.
Two groups of herbaceous and woody plants have the ability to fix atmospheric nitrogen in the soil, legumes and actinomycete-nodulated angiosperms. Together in these two groups are at least 1,350 species of plants capable of nitrogen fixation, although only about 25 are extensively used today in agriculture and forestry. Included among the legumes are beans, peas, clover and alfalfa. Certain legumes fix nitrogen because of their symbiotic relationship with Rhizobium bacteria, which form nodules in the roots of their host plants. The plants provide the bacteria with carbohydrates for energy and a stable environment for growth, while the bacteria give the plants usable nitrogen and other essential nutrients. The rest of the soil community and neighboring and succeeding plants benefit from the nitrogen and other rich compounds exuded from the nodules, and from the recycling of the nitrogen as the plant drops its leaves or decays. Rhizobium bacteria usually are annual in nature, developing inthe spring and decaying in the autumn.
A second, less familiar form of nitrogen fixation occurs in plants that are nodulated by actinomycete fungi of the genus Frankia. Although the basic nitrogen fixation process is the same with these plants as with legumes, most of their nitrogen contribution is in the form of falling leaves and decaying litter. Their nodules are perennial rather than annual. 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. In the coniferous forests of the Pacific Northwest, some foresters are studying the use of red alder as a nitrogen-fixing companion for Douglas-fir.
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 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. Under the right conditions, the azolla can double its weight every three to five days, and can fix nitrogen at a higher rate than most legumes. Azolla is used most extensively in China and Vietnam. For example, it is cultivated as a green manure crop on about 60%fo the rice area in Vietnam's Red River Delta. Azolla plants are described by the Vietnamese as "indestructible fertilizer factories," for they were the only nitrogen "factories" which continued to produce even during the worst of the Vietnam war.
In addition to producing nitrogen for crops, azolla absorbs from the water nutrients that might otherwise be lost, storing them with the nitrogen until the azolla mat is incorporated into the soil. Often the stored nutrients include much of the phosphorus applied by the farmer to stimulate azolla growth and nitrogen fixation, thus eliminating the need to apply phosphorus directly to the crop. Another benefit of azolla is that its thick mat on the water's surface curtails the growth of many aquatic weeds, both by blocking their sunlight and by physically inhibiting their emergence.
Two new areas of agricultural research involve nitrogen fixation by algae and bacteria directly in the soil. Battelle's Pacific Northwest Laboratory in Richland, Washington, for example, is studying the use of two species of blue-green algae as a fertilizer for corn and tomatoes. In this study algae, grown in a liquid medium, is sprayed directly onto the soil. The algae multiply on the soil surface, fixing nitrogen from the air and then excreting the nitrogen compounds into the soil. In the first year of the study, tomatoes grown in the algae-treated soil had 45% more plant growth than did plants treated with a commercial nitrogen fertilizer. In addition to nitrogen, the algae provides other organic compounds that help build soils and bind micronutrients, making them more available to plants.
Robert H. Faust, an agronomist and ecological agriculture consultant in Twin Falls, Idaho, has been studying the effectiveness of certain bacteria as inoculants for non-leguminous plants. The nitrogen-fixing bacteria Azotobacter has been used for some years in the Soviet Union, but only recently in the U.S. In 1980, Faust conducted test plantings on a farm outside Twin Falls. Wheat seed treated with Azotobacter yielded 11.6% more bushels per acre than did either untreated or chemically-treated seed. In order for Azotobacter to be effective, soil pH must be between 6.8 and 8, and optimally between 7.2 and 7.8. Faust markets Azotobacter as 'Azobac,' available from Faust Bio-Agricultural Services, PO Box 1150, Twin Falls, Idaho 83301. The Faust Services catalog is available for $1.
Humans and plants have much in common. Both enjoy sunlight, music and tender loving care. And, despite outward appearances, similarities extend to the structure of our cells.
With the exception of one atom, chlorophyll (the center for photosynthesis in plants) and hemoglobin (the vehicle for respiration in animals) are structurally identical. At the center of each chlorophyll molecule is an atom of magnesium which gives plants their characteristic green color. At the center of each hemoglobin molecule, on the other hand, is an atom of iron which causes blood to be red.
Legumes take this similarity one step further. If you carefully remove a pea or clover plant from the soil and slice open one of the nodules formed by the rhizobia bacteria on its roots you will see a pink or red color. This color indicates the presence of hemoglobin - the same compound which is found in the blood of humans and other mammals. This compound exists in neither the free-living bacteria nor the host plant, but is formed in the union of plant roots and bacteria.
So, the next time you extend your fork for a mouthful of peas remember that - for the sake of one atom...
Laura Stuchinsky
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