The soybean is a legume who’s seeds generally contains 37% to 45% protein by weight. Depending on the protein content, a bushel of soybeans will cont ain between three and four pounds of nitrogen. The production of a 60-bushel per acre crop requires in excess of 300 pounds of nitrogen.
Some of the nitrogen comes from the oxidation of soil organic matter with the balance produced by bacteria residing in nodules on the plant’s roots. Each legume species requires different bacteria to fix nitrogen from the air and produce ammonium for use by the plant.
The Inoculation Process
The bacterial strain of Rhizobium used by soybean is Bradyrhizobium japonicum, and the process by which it gains access to the plant is very complex, but fairly well understood. Soybean plants release chemical signals (flavanoids) into the soil as they germinate and the plants emerge. These signals are picked up by the rhizobia in the soil and in inoculation material, and produce a return signal “Nod factor signal” to the plant. This return signal results in the plants’ roots preparing for infection by the bacteria.
The Nod factor signal causes the root hairs to curl, trapping rhizobia that are present on the surface of the root. An infection thread develops, and the rhizobia multiply until they reach the inside of the root. The cells of the root also begin to divide and form a specialized structure called a nodule. The rhizobia continue multiplying inside the nodules increasing their size. The enzyme (nitrogenase) that fixes nitrogen from the air to form ammonium cannot function in the presence of oxygen. In order for nitrogen fixation to occur, the plant must produce Leghaemoglobin in side the nodule to absorb oxygen in the root to prevent it from interfering with the nitrogen fixation process. Since Leghaemoglobin is red, the inside of an actively fixing nodule is pink. Sugars produced in the leaves travel to the root system and move into the nodules to provide energy that the rhizobium uses to extract nitrogen from the air to make the nitrogen compounds which the plant then uses to produce protein. The better this process works, the greater the grain yield and amount of protein produced.
Both plant and bacteria genes govern the entire nitrogen fixation cycle. Stresses on the bacteria or the plant such as cold, flooding, drought, and low soil pH can all interfere with this process. Cold temperatures delay the recognition of both the plant and bacteria signals. The plants and bacteria have to produce more signals in order to begin nodulation, which delays the onset of nitrogen fixation. Once enough signal is received and nodules develop, ammonium is produced and provided to the crop.
Drought reduces the moisture in the soil that protects the bacteria while it lives on the seed surface after planting. A dry seedbed and dry seed will quickly draw moisture from the inoculation material causing the bacteria on the surface of the seed to dry and die. If enough bacteria die, there will be little nodule formation and insufficient ammonium production for a good yield. Other stresses, including low soil pH, seed treatment chemicals, and starter fertilizers, can also kill the bacteria or inhibit nodulation. In fact, soil nitrogen levels in excess of 30-50 lbs per acre can stop both nodulation and nitrogen fixation altogether.
SOURCE: Dr. Jim Beuerlein, The Ohio State University