24 Jul No Nodulation on Your Soybean Crop? Why Not?
As the middle of the growing season approaches, it comes time to check crops for disease and pests, and to check your legume crops for nodulation. This week on Growing Possibilities we will be discussing nodulation, with a focus on soybeans. What is nodulation? How do I check for it? What can I do if it does not occur?
Nodules on the roots of all legume crops are the result of a symbiotic relationship between leguminous plant roots and nitrogen-fixing bacteria. Symbiotic bacteria fix nitrogen from the air in the surrounding soil and provide a source of N for their host plants, who in return provide bacteria with nutrients and a source of carbon. Each legume crop has a specific species of bacteria with which it will form nodules. In soybeans, this species is Bradyrhizobium japonicum, which should be present in all soybean inoculants.
About 4-6 weeks after planting, nodules should begin to form on soybean roots. To check for nodulation, carefully dig out a plant with the surrounding soil still intact. Gently wash or break away soil from the roots (dipping the plant into a bucket of water is a good way of removing soil) and look for nodules adhered to the plant roots. With on-seed inoculation, nodules are more prevalent at the crown (Fig.1.a), and with in-furrow inoculation, nodules are more prevalent on lateral roots (Fig.1.b). Slicing open the nodules should reveal pink/red tissue, indicating that nitrogen fixation is occurring and your rhizobacteria are active (Fig.1.c). White-grey tissue inside nodules usually means these nodules are young and not yet fixing nitrogen. Grey or green tissue inside a nodule indicates that it is no longer fixing nitrogen and will be shed from the roots.
If you are checking your crop and do not see nodules in the suggested time frame, what is going wrong? And what can be done to fix it? There are a variety of reasons why nodules may fail to form following inoculation. These include:
- Low soil pH (<5.5) (1).
- High soil pH (≥8) (2).
- Cool soil temperatures (3).
- Presence of high background levels of N. High levels of available soil nitrogen (more than 50 lb/acre) will result in your crop preferentially using nitrogen from the soil, inhibiting nodulation (4).
- High soil salinity (5).
- Soil compaction. Compacted soil will prevent aeration of soil and cause a lack of nitrogen for bacteria to fix.
- Nutrient deficiency or toxicity, including phosphorous, potassium, iron, molybdenum, manganese, calcium, and zinc (5).
- Application of the wrong species of bacteria. Each legume crop has a specific species with which it has a symbiotic relationship, and usage of another species will not provide adequate, if any, nodulation. Always make sure you are using an inoculant that is recommended for your crop.
- Failure to follow all label directions, including proper storage conditions, application directions, length of time between inoculation & treating and planting, and awareness of seed treatment/pesticide combination compatibility. Storage condition recommendations still persist after application of the product, and should be followed for the duration of the on-seed window until the seed is planted.
In the event of a crop with no nodulation, there is little that can be done to establish nodules. If it is still early enough in the season, reseeding with a higher rate of inoculant can be done, but application of inoculant in-season will not lead to nodulation. Higher rates of inoculant are recommended in fields with little or no history, or virgin soil, of any particular legume crop. The recommended best practice for inoculating virgin ground is dual inoculation, which refers to both an on-seed and in-furrow inoculant application.
1) Bjederbeck VO, Bjorge HA, Brandt SA, Henry JL, Hultgreen GE, Kielly GA, Slindard AE. 2005. Soil improvements with legumes. Ed: BJ Green, VO Bjederbeck. Government of Saskatchewan. Available on-line at: http://www.agriculture.gov.sk.ca/Default.aspx? DN=4b50acd7-fb26-49a9-a31c-829f38598d7e.
2) Staton M. 2014. Identifying and responding to soybean inoculation failures. Michigan State University. Published 4 Feb 2014.
3) Conley S. 2015. Non-nodulating soybean questions. Agri-View Briefs. Published 23 July 2015. Available online at: http:// www.agriview.com/briefs/crop/non-nodulating-soybean-questions/article_9b2f79e6-dd9d-5277-b414-4e80a63fd84f.html.
5) Epp M. 2015. Why nodulation fails. Grainews. Published 26 March 2015. Available online at: http://www.grainews.ca/2015/03/26/why-nodulation-fails/.