Nitrogen nutrient requirements for soybeans are much less than they are for corn (total volume/acre). Why is that? Soybeans require every essential nutrient that other row crops do, but they can “produce” their own nitrogen (N). Like a few other plants, soybeans are a legume crop, which means they form a symbiotic relationship with Bradyrhizobium bacteria that use the plant sugars in return for nitrogen.
Nodules forming on soybean roots.
Credit: United Soybean Board

First, let’s start with what this will look like on the soybean roots. The process is initially started by the root when it releases compounds that attract the bacteria. Small round nodules will form from the root hairs essentially curling around themselves again and again. The round nodules are produced in response to the bacteria attacking the root, which then colonize the nodule. A light pink color can be seen on the inside and can be used to verify if the nodules are active or not. The nodules will produce and break down continuously from a few weeks after emergence until pod fill.

You may ask why the plant doesn’t just directly use some of the plentiful nitrogen in the air. The answer is because this nitrogen is in a form that is unavailable to the plant. That is where the symbiotic relationship between the bacteria and the plant comes in. Nitrogen in the air is a gas (N2); it is two nitrogen atoms held together very tightly that require energy (sugars) from the plant to be split. The sugars are produced during photosynthesis, and thus translocated from the leaf to the nodule. The bacteria use the sugars as energy for nitrogen fixation. N2 as a result is converted into ammonia (NH3) which then can be used by the plant. The bacteria use the plant’s energy they cannot produce themselves to fix the nitrogen, and in turn provide an available form of nitrogen that the soybean plant can use. Keep in mind that nodule formation will be determined by nitrogen availability in the soil, so if other forms of nitrogen are applied the fixation process will be less necessary.
This is important because nitrogen is essential for many processes that take place within the plant. Nitrogen, broken down into amino acids, is used as important building blocks of DNA and RNA. It is used in the process of photosynthesis and found in ATP, an important molecule for energy.
Soybeans are an efficient plant to say the least. The plant can adapt to a wide range of planting populations, the grain can be used for feed and oils, and their roots can fix their own nitrogen. With nitrogen being at the top of the essential nutrients, its nice to be able to negotiate your own nutrient needs.
  • Are you properly supporting nodulation development?
  • Are your plant nodules lasting long enough through the season?
  • Consider using seed treatments to improve colonialization.
  • Consider applying nitrogen preplant to help supply nitrogen to the plant prior to nodulation development.
  • When striving for higher yielding soybeans, 75-125 bu/ac, additional nitrogen may need to be applied since the nodulation may not have the ability to fully supply this demand. Note: a soybean plant requires roughly 4-5 units of nitrogen per bushel of soybeans.

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About the Author: Cody Pettit

Currently, Pettit is a field agronomist for the Pioneer brand with Corteva Agriscience, covering the east central part of Illinois. Prior to his current role, he was a district sales manager in the seed industry after graduating from the University of Illinois, where he earned a bachelor’s degree in crop sciences. Pettit has a passion for understanding new practices and solutions employed on a variety of farm operations, and is excited for the ever changing future of the agricultural industry.