Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)



First Advisor

Robert Pengra


A search for alternate sources of fixed nitrogen for crop production has been caused by the rising cost of petroleum used in manufacturing nitrogen fertilizers. Nitrogen in its elemental form, N2, is quite stable, and all organisms except a few of the prokaryotic microorganisms can utilize nitrogen only in some chemically combined form. Those microorganisms which can utilize elemental nitrogen are a potential source of fixed nitrogen to crops. Rhizobium is a genus of nitrogen-fixing bacteria which form nodules on the roots of leguminous plants. This is a symbiotic relationship; the plants supply photosynthate to the bacteria which use that energy to grow and fix nitrogen for the plant as well as themselves. These bacteria can fix enough nitrogen to eliminate the need for· nitrogen fertilizer for these crops. Rhizobia will fix nitrogen only 1n association with nodulated plants. Other bacteria do not need to form this type of relationship to fix nitrogen. However, they will use N2 only under certain conditions. Nitrogen-fixing bacteria will use combined form of nitrogen rather than N2 because of the energy required to reduce N2 to ammonia, the form which 1s incorporated into the cells. This energy may be supplied by material leaking from the roots of the plants and would stimulate nitrogen-fixing bacteria in the rhizosphere. The free-living nitrogen-fixers may be able to supply nitrogen for non-leguminous crops if the proper conditions exist in the soil. Finding conditions of the soil which favor these bacteria is a part of the search for alternatives to commercial nitrogen fertilizer. In the summer of 1978 the MPN techniques described were used to estimate numbers of nitrogen-fixing bacteria in the rhizospheres of grasses and soil adjacent to those grasses. This was done from June through August of that year. Samples were collected first from Clay County on the banks of the Missouri River, then from experimental grass plots in Faulk County, and at the end of the season from Brookings County. Altogether 71 sites were sampled. At each sample site duplicate samples of both roots and soil were taken for the counts of bacteria and one larger soil sample was collected for soil tests. If more than one grass species was present at a site, the dominant or healthiest species was selected for the root samples. The plants were dug with a spade. The soil was shaken from the roots, and the sterns were trimmed just above the crown. If the plant was corn, a section of ·the root system was cut away with the spade so that some of the larger roots with laterals could be collected without destroying the plant.

Library of Congress Subject Headings

Bacteria, Nitrifying
Soils -- Analysis



Number of Pages



South Dakota State University