Remember that soybeans are legumes and fix nitrogen, but they might need a little assistance to make sure the environment is right for bacteria to flourish and maximize nitrogen fixation.
Soybeans are invaded by three distinct types of rhizobia bacteria: rhizobium fredii, bradyrhizobium japonicum and bradyrhizobium elkanii. When soybean seeds germinate, the bacteria invade the root hairs of the seedlings and begin to multiply. These soil-dwelling bacteria have the ability to penetrate soybean roots, which respond by building nodules to house them. The bacteria fix nitrogen in symbiosis with the soybean roots, which provide other nutrients and sugar.
Under field conditions, the first nodules form within a week or two after seedling emergence and become visible as they increase in size. Active fixation begins in the V2 to V3 stage, after which the number of nodules and the amount of nitrogen fixed continue to increase. At the V2 growth stage the soybean plants should have roots six inches down in the soil and at the V3 to V4 stage of growth there should be 8 to 10 large, healthy nodules per plant. The soybean demand for nitrogen is highest beginning at R5 and pod fill.
The amount of nitrogen fixed by the plant will increase with the number of nodules, as well as by the activity of individual nodules. The number of nodules on the soybean root increase until pod fill, when more energy and resources are diverted to the pod. Nodules appear, produce nitrogen and slough off. There is continual development and recycling of nodules for about two months after the first ones appear.
You can always tell if a nodule is healthy and active by its pink or red color inside. A green, brown, gray or white color indicates that fixation is not taking place. You can check the nodule activity by gently digging out plants with a small trowel or hand shovel, rinsing the plant in water, cutting the nodules open and inspecting the inside color.
Soybeans will take up nitrate from the soil before they will start actively fixing their own nitrogen and high concentrations of nitrate will impede nodulation. Nodule numbers also may be low in fields where soybeans are grown for the first time and where no rhizobia bacteria exist, or in fields with high nitrate levels.
Other factors resulting in low nodule numbers include fields that were saturated early in the season or where root rots exist, or those with compacted soil with poor aeration, very dry soils, soil pH below 6.2 or EC (electroconductivity) greater than 1.0.
Nitrogen fixation in soybeans is a gift from nature, considering that soybeans required 4.5 to 5 lbs. of nitrogen per bushel with about 50 percent of that coming directly from fixation. You can help optimize fixation by making sure the soil is in the best condition possible and you have plenty of active and viable rhizobia present in the soil.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at firstname.lastname@example.org.