Are your soybeans fixing as much nitrogen as possible and how much nitrogen can a soybean plant fix?

Soybeans are a legume and take nitrogen out of the air and fix it into a form that plants can use. You can dig up soybean roots and see whitish nodules on them. If when pressed between your fingers a nodule oozes pink sap it means the nodules are active and fixing nitrogen.

But just because the nodules are present doesn’t mean there are enough of them or that they are efficiently fixing enough nitrogen for the plant. Since we can’t actually see and count all the nodules (some slough off during digging or are too deep to extract from the soil), nor can we know how much nitrogen they are contributing, we just assume if nodules are present the soybeans are fixing nitrogen and the plant’s needs are being met. But can we improve a plant’s ability to fix more nitrogen?

Here are three facts that we know about soybeans and nitrogen that have become “rulings:”

• Soybeans fix their own nitrogen and don’t need supplemental nitrogen fertilizer.
• Soybeans require 4.5 to 6.0 lbs. of nitrogen per bushel of grain produced.
• Soybeans fix enough nitrogen to produce a 50- to 60-bushel crop.
• Nitrogen fixation produces 40 to 60% of the nitrogen required by the soybean plant.

From the above we know that a 75-bushel crop requires from 338 to 450 lbs. of nitrogen per acre, that the soybean plant will fix only enough nitrogen for 50 to 60 bushels, and that the rest has to come from the soil.

But what we don’t know is whether nitrogen fixation is both maximized in lbs. produced per acre and optimized for the plant and its environment. To date there is no easy way to know this and no easy way to measure it in the field or a laboratory. My concern with the debate over whether soybeans need nitrogen fertilizer is the assumption that soybeans fix all the nitrogen they need. Coupled with this is the fact that we can’t yet measure nitrogen fixation in plants, neither how much is actually being contributed nor how much could potentially be contributed to what the plant needs.

For nitrogen fixation to occur, rhizobia populations need to exist in the soil. Native populations thrive in the soil as long as soybeans are in the field every second or third year. Rhizobia bacteria require the availability of molybdenum (Mo), a soil element. In general, Mo is present in soils in sufficient quantity, but its availability is affected greatly by the soil pH. When planting soybeans or other legumes, farmers should lime the soils to raise the pH to between 6.5 and 7.0 and electroconductivity readings less than 1 dS/m (non-saline). Soils need to be well drained and aerated. Also, excess nitrate in the soil will shut down nitrogen fixation since plants won’t spend energy fixing nitrogen when they can get it free from the soil.

Native populations of rhizobia exist in the soil. But the argument is that native populations of rhizobia are not as efficient at fixing nitrogen or as adapted to more marginal conditions. Inoculating seed with improved rhizobia bacteria improves overall nitrogen fixation and profitability.

Nitrogen fixation is a wonderful trait to have since it feeds the crop nitrogen. We can only wish to have this trait in corn and wheat. However, as growers are we doing all we can to optimize nitrogen fixation in the field or are we letting nature decide for us? I often think we need to pay more attention to this, but until we can measure fixation in the field for ourselves and learn new ways of improving fixation, all growers can do is optimize the soil environment and perhaps apply an inoculant every time they plant.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.