Learn about organic nitrogen fixation and organic farming methods.

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The Science of Inoculation

What is Inoculation?

Inoculation is the application of commercially available Rhizobia bacteria (inoculants) to legume seed or in the seed furrow while planting the legume seed. An inoculant should be used when there is reason to believe that soil populations of Rhizobia are low. This situation will exist when cleared land is brought into production, legumes have not been grown on a soil for several years (more than 4years) or when the pH levels are low. In a situation of low pH (less than 5.8), the pH should be adjusted by liming prior to inoculation. The Rhizobia bacteria combine with the legume roots to form nodules that in turn carry on nitrogen fixation. Nitrogen fixation is the process by which legume plants obtain the nitrogen they need from the gaseous nitrogen of the air.

How does nitrogen fixation work?

Nitrogen fixation is the utilization of the free gaseous nitrogen in the air by soil bacteria – either in cooperation with the plants (legumes) or without plants – to produce useable nitrogen products which plants and other organisms can use (just as plants would use commercial fertilizers) to grow and produce protein, fiber, and seed. For clarity; the term "useable" nitrogen can be substituted for "fixed" nitrogen.

Nitrogen fixation is the conversion of free gaseous nitrogen in the air to fixed nitrogen which can be utilized by whatever legume is fixing the nitrogen.

Legume growers see fixation in soybeans, peanuts, clovers, alfalfas, dry beans, and other legume field crops; but nitrogen fixation is a universal phenomenon. Wherever nitrogen fixation occurs, it is mediated by the enzyme nitrogenase. In the plant- Rhizobia symbiosis that we are most familiar with, nitrgenase converts gaseous nitrogen to ammonia which is then assimilated into amino acids which subsequently are transported throughout the plant. Most of this nitrogen (fixed in amino acids) goes to the above ground parts of the plant where the plant uses it to make protein, nucleic acids, and other necessary nitrogen containing compounds needed for plant growth and seed production. These processes are no different than when the plant assimilates ammonia fertilizer. Obviously though, the air is a less expense source of nitrogen than fertilizer ammonia.

An added advantage particular to legumes is the amount of fixed nitrogen which is returned to the soil at the end of the crop year. For example, soybeans leave an estimated 55 to 120 pounds per acre of useable nitrogen for next year's crop, while alfalfa is estimated to leave between 120 to 300 pounds per acre of usable nitrogen after plow-down.

Why is inoculation necessary?

Inoculation is necessary when the legume to be grown has never been grown in the area before, when it has been 3 to 5 years since it was grown, or where there are particular stress conditions which do not allow the native soil Rhizobia bacteria to survive and proliferate. Stress conditions such as low pH, low organic matter content, drought, improper nutrient levels, and high soil temperatures can attenuate the level of Rhizobia surviving in the soil.

Inoculants with proven superior strains of Rhizobia [ "strains" are to Rhizobia what "varieties" are to field crops] can replace the Rhizobia which may be resident in the soil but not as effective as the new inoculant strains. It is believed that Rhizobia may become "lazy" or less effective as they reside in the soil for extended periods. Thus, there is the potential benefit of adding high levels of newly selected strains of Rhizobia each time a legume is planted by using a commercial inoculant.

The benefits of inoculation:

79% of the air we breathe is made up of nitrogen and inoculated legumes are able to convert and use this free nitrogen. Properly nodulated legumes can leave 55 to 300 pounds of fixed nitrogen per acre to the soil. The exact amount depends on effectiveness of the nitrogen fixation process, type of legume, length of time the legume is grown, soil nutrient levels, and nitrogen already available. The nitrogen provided by inoculated legumes grown in crop rotation helps boost yield and lower fertilization costs for corn and small grains – in an environmentally safe manner.

Fertilizer Savings: Due to the nitrogen-fixing ability of legumes inoculated with Rhizobia, the need for commercial nitrogen fertilizer is virtually eliminated.

Higher Yields: All legumes average more pounds per acre when properly nodulated, which increases total crop production. For example, studies of soybean on ground thought not to need inoculant have shown and average increase of 2+ bushels per acre.

Effectiveness of Rhizobia Strains: Rhizobia can become less effective at nitrogen fixation over time and fix less nitrogen even though they remain good nodulators. Continuous use of fresh Rhizobia will maximize yield benefits as these Rhizobia will out-compete the indigenous Rhizobia for root nodulation.

Added Nitrogen to the Soil: Properly nodulated legumes add 55 to 300 pounds of nitrogen per acre to the soil.

Benefits Rotated Crops: Nitrogen provided by inoculated legumes grown in crop rotation helps boost yield and lower fertilization costs for corn and small grains.

Environmentally Friendly: Inoculation is a low cost way to ensure nodulation with the proper strain of nitrogen fixing Rhizobia bacteria for increased yield in an environmentally safe manner.

Improved Soil Conditions: Legume plants decompose rapidly, leaving organic matter in the soil which improves its physical, chemical, and biological condition.

Up-to-date Strains: Seed varieties have changed and respond best to up-to-date strains of Rhizobia for maximum fixation.

It is important to note that the presence of a nodule does not assure nitrogen fixation. Native Rhizobia becomes ineffective over time. You can evaluate nodules by examining the color of the nodule at different times durging the growing season.

Inoculation should always be used under the following conditions:

• CRP ground
• Sandy soils
• Soils under stress
• Legumes have been out of rotation for 3-5 years