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Nitrogen inoculants are better than nitrogen fertilizer in pulses

Nitrogen inoculants deliver better nutrition than nitrogen fertilizers for most pulse crops by synchronizing nitrogen fixation to plant growth, supplying nitrogen to the plant in the quantities that it needs when it needs it.

Nitrogen inoculants deliver incredible value. For example, a 50-bushel pea crop requires 150 pounds per acre of nitrogen. A well-inoculated pea crop can fix up to 80% of its nitrogen needs or in this case about 120 pounds of nitrogen. If nitrogen fertilizer costs $0.40* per pound, fertilizer would cost $48.00 per acre (120 pounds x $0.40 per pound). With inoculants priced from $3.50 to $12.70 per acre depending on formulation, using an inoculant would save $35.30 to $44.50 per acre. That is a great return on investment.
*Representative fertilizer prices as of spring, 2005.


Nitrogen Fixation - Mother Nature at Her Finest

By: Dr. Fran Walley

Department Head,
Department of Soil Science
University of Saskatchewan
Saskatoon, SK






Nitrogen fixation really is a remarkable process and perhaps one of the best examples of Mother Nature at her finest. The process begins with a finely coordinated series of events taking place in the soil. Soil bacteria, called Rhizobium, take the first steps to identify an appropriate legume host plant root, using a series of coordinated chemical signals. Having recognized, and been recognized, by a specific host plant, the bacteria infect the plant roots, ultimately resulting in the formation of nodules capable of supplying the plant with a much needed source of nitrogen. The signals used for recognition between the bacteria and the plant root are very specific and thus the Rhizobium species must be carefully matched to appropriate legume hosts.

Two Unlikely Partners
Surprisingly, the host plant is able to recognize the Rhizobium bacteria as a ‘friendly invader’, allowing the bacteria to invade the roots. Keep in mind that Mother Nature also has provided the plant roots with a number of defense mechanisms to ward of invasions by unwanted disease organisms, so it is surprising that these defense mechanisms are dropped when Rhizobium come knocking-but that is just what happens. The rhizobia invade the plant roots and the plant responds by developing a nodule in which the bacteria are housed. From the standpoint of the Rhizobium, the housing conditions within a plant nodule are very nice indeed and the plant goes out of its way to be a good host. Specifically, the plant provides the bacteria with a supply of food in the form of photosynthates, together with a carefully regulated chemical environment that is perfectly suited to rhizobial requirements for nitrogen fixation. In turn, the Rhizobium bacteria respond by physically changing their form and initiating the production of an enzyme that is necessary for converting gaseous nitrogen into a form that is plant available. The end result is that two unlikely partners are able to convert a virtually unlimited source of gaseous nitrogen into a reliable and significant source of plant available N.

Inoculants Capitalize on Mother Nature
Using an inoculant capitalizes on this remarkable process and helps Mother Nature along. The Rhizobium bacteria are supplied in the inoculant at levels far exceeding populations normally found in the soil. Moreover, the bacteria are placed in direct or in near contact with the legume host plant roots, so the communication and recognition stages are bound to succeed. Finally, the Rhizobium species and the host plant can be perfectly matched, further ensuring a successful relationship.

Phosphorus – a Critically Important Nutrient
Is there more that can be done to ensure a productive relationship between the Rhizobium and the host plant? Absolutely. To ensure the nitrogen fixing relationship is maximized, it is critically important that the host plant is able to divert energy and resources to the bacteria that have taken up residence in the nodules. Clearly, the plant needs to allocate its resources to ensure its own survival, so a lot of energy is spent early on producing leaves and thereby expanding the photosynthetic surface. Consequently, the growing tips of the plant are a powerful ‘sink’ for energy and internal plant resources. As the plant matures, setting seed becomes a priority, and most of the plant’s internal resources will flow to this developing sink. In the meantime, the nodules remain a hub of chemical activity with the process of nitrogen fixation requiring a expenditure of energy. If the plant is nutritionally starved due to low levels of key nutrients – in particular phosphorus – the supply of energy to the nodules can be seriously compromised and the bacteria in the nodules will not function at peak efficiency. A nutritionally starved plant simply can’t afford to be a good host. Although good overall plant nutrition is important for promoting nitrogen fixation, phosphorus, in particular, is a critically important
nutrient. Phosphorus stands out because much of the energy in a plant is stored in plant chemicals that are phosphorus based. For example, adenosine triphosphate, or ATP, is a chemical produced within a plant that serves as a ‘bank account’ of energy. If phosphorus is limiting to a plant, the production of ATP is limited, which in turn limits the amount of energy available to support energy consuming processes within a plant. The consequence? Stunted growth, poor seed set, and, of course, in the case of pulse crops, reduced nitrogen fixation. Thus, to maximize nitrogen fixation, it is critical that the pulse crop be provided with all its needs to be the best host possible for the invading Rhizobium bacteria. In this case, the key to being a good host is being able to provide food energy in abundance. The pay – off for the plant is extremely valuable - a source of nitrogen perfectly timed to meet the needs of the growing plant.


Small plot test site near Rosetown SK, demonstrates the difference between a nitrogen and phosphate inoculant vs. a nitrogen only inoculant.