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Inoculate soybeans going into CRP

Legumes fix nitrogen with the help of bacterium 

There is a trend across MFA’s trade territory—a shift of acres away from CRP ground back into crop production. When brought back into production, most CRP acres will go to soybeans the first year. In some cases growers prefer corn the first year. Most of the CRP acres that are coming back into production haven’t seen a crop in 10 to 15 years, some fields have been out of production even longer. I have had the opportunity to visit with several producers about their plans to establish soybeans across these acres. Most of the conversations have revolved around weed control and the planning to fight weeds present in the CRP and ones that might emerge with the crop. But one thing producers sometimes overlook is the importance of inoculating soybean seed. I’ve visited with some producers who plan on planting soybeans into CRP land who were not concerned or didn’t plan on inoculating. That’s a recipe for less than optimal yield. 

Inoculants are very important to soybean production. Soybeans are a member of the Leguminosae family and this family has the ability to live in a symbiotic (beneficial to both) relationship with Bradyrhizobium japonicum. This bacterium is specific to soybean and cannot inoculant any other legume. 

For the relationship to exist and be beneficial to both, B. japonicum must be present in the soil in relatively high numbers at planting time. In fields where soybeans haven’t been planted for three to five years, it is essential to inoculate your soybeans. Studies from some of the Midwest universities show a benefit from inoculating your soybeans every year while others say you only need to inoculate when soybeans have been absent for several years. I suggest that you do a study on your own farm to determine what best fits your operation. A simple side-by-side evaluation of both practices can help you determine what system to use. 

When B. japonicum is present in the soil, it attaches and colonizes on the soybean root hairs immediately behind the growing root tip. Sometimes people confuse the nodules produced by B. japonicum with soybean cyst nematode. SCN are white to yellow in color and the B. japonicum is a wart-like structure. B. Japonicum will grow rapidly and start producing nitrogen around the V2 to V3 stage. If you are curious if your nodules are producing nitrogen you can cut one of the nodules in half. The inside of the nodule will have a pink or red interior. This color doesn’t define the nitrogen efficiency of the nodule or plant. Nodules will fix nitrogen for six to seven weeks.

Soil moisture, soil temperature, soil pH, diseases and micronutrient availability are some of the factors that influence the ability of B. japonicum to fix nitrogen. 

Soybeans get about 35 percent of the nitrogen they consume from the soil. The rest of the nitrogen that the soybean plant needs comes from the symbiotic relationship of soybean and B. japonicum. Most of the 35 percent comes from organic matter or fertilizer that has been used in previous crops. If the soil nitrogen level is very high it will inhibit nodules and the B. japonicum will shut down until the soybean utilizes the soil nitrogen. Soybeans can require more than 300 pounds of nitrogen per acre, and most of it is needed during the initial flowering through seed set stages. During pod fill it is not uncommon for the soybean to produce three pounds of nitrogen per acre per day. 

Several companies are selling inoculants with growth promoters. One of the common growth promoters on the market is LCO. These promoters are nutrients or chemical enhancers that stimulate root growth or enhance the communication between bacteria and roots to make the nodules form quicker than normal. We are evaluating these growth promoters and other products at the MFA Training Camp site, and will provide the results of these studies when they are available. 

I hope that if you are putting a soybean crop on first-year removed CRP ground that you will inoculate your soybeans. Contact your local MFA agronomist to develop a plan for your inoculant needs.

Dr. Weirich is director of agronomy for MFA Incorporated. Read more of Dr. Weirich's columns at

Originally published in the April Today's Farmer magazine ©2013 MFA Incorporated. All rights reserved. Here is a LINK to the original version of this story.

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