What About Rhizobia Inoculants?

“What about rhizobia inoculants? I don’t recall any mention of them in the ‘Seeds Available from ECHO’ listings. Isn’t it likely that many of the legume seeds will need rather specific rhizobia inoculants at planting time?” Bob Tillotson, Thailand. “Does the seed [velvet bean] need to be inoculated to fix nitrogen or will it naturally do it on it’s own?” Jim Triplett, Guam. Similar questions regarding legume inoculation come up often. The following attempt to answer these questions is based on an article by Dr. Paul Singleton with NifTAL which was sent to us by one of our readers, Brian Hilton. The article, “Enhancing Farmer Income Through Inoculation of Legumes with Rhizobia: A Cost Effective Biotechnology for Small Farmers,” addresses a series of questions. We will summarize these and add a few others.

What are rhizobia and what do they do? Rhizobia are a genus of soil bacteria that infect the roots of legumes and can fix (make available to the plant) atmospheric nitrogen. Unlike disease-causing bacteria, rhizobia enter into a symbiotic relationship with the plant. The legume provides the bacteria with energy and the bacteria provides the legume with nitrogen in a form it can use.

Does one rhizobium work with every legume? No, rhizobia are selective and grouped according to which legume species they will colonize. The rhizobia of some species, e.g. leucaena, are very specific. Others cross-inoculate many species. For example the “cowpea family of inoculant” will inoculate Acacia albida, Cajanus cajan (pigeon pea), Desmodium spp, Lespedeza spp, Mucuna (velvet bean). Some species, such as peanut, called “promiscuous,” can be inoculated with any of a number of rhizobia. Often one rhizobium strain will provide some biological nitrogen fixation (BNF) but will be less effective than another. Unless some strain of inoculant suited to the legume species you are growing is present in the soil, no BNF will take place.

Which of my crops are most likely to respond to inoculation? Responses are likely from species whose rhizobia are quite specialized such as soybeans and leucaena. Areas with a distinct long dry season of 6-8 months are also likely to respond due to existing rhizobia populations dropping off more quickly under these conditions.

How do I know if I need to inoculate my plant? Rhizobia live in nodules on the roots and can be easily seen. Well nodulated legumes will have nodules on the tap root. (Dig the plant and remove the soil carefully or the nodules will fall off). Not all nodules are effective, however. Cut several nodules in half. Nodules that are effectively fixing nitrogen will usually be red or pink inside.

How are rhizobia introduced? Most commonly legume seeds are coated with the appropriate inoculant just prior to planting. A sugar or gum arabic “sticker” is used to attach the powdery inoculant to the seed. If healthy, nodulating plants of the same species are already growing in the area the proper rhizobia should already be available and need not be purchased. Just add about 5 grams of soil from such a plot to each hole as seeds are planted.

Can I maintain my own inoculant? Yes. After a successful crop, soil will always retain some inoculum until the next season. Replanting the same species in the same soil year round will serve to increase inoculum for that crop. But, this practice may also increase the occurrence of some diseases.

Why doesn’t ECHO carry inoculant for the legume seeds it distributes? This would seem to be the wise thing to do. However, it is challenging enough to preserve and monitor the viability of our stored seeds. Viability of inoculum is even more difficult to monitor and maintain which is why we leave this enterprise to those set up to do the job well.

How much rhizobium is needed to inoculate a seed? It takes about 100 grams of inoculant to sufficiently treat one pound of leucaena seeds. A hectare of soybeans requires 286 grams of inoculant. Quality is more important than quantity. The best inoculant contains a billion rhizobia per gram, but it doesn’t take long for quality to drop. This is why inoculation is done just prior to planting. Since you can’t tell if inoculant is good or bad by looking at it care should be taken to use a good source and handle it properly. Inoculant should be protected from heat, light and desiccation and used as soon as possible. If a cool storage area is not available, a pot buried in a shady area is a good option. If transportation is required, a container covered with a damp cloth works well.

Where can rhizobia be obtained? Many countries manufacture inoculants for a number of crops. Contact your local agricultural extension agency or national department of agriculture to see if they have the inoculant you are looking for. If it needs to be imported, probably the best source would be the NifTAL (NifTAL Project and Mircen, 1000 Holomua Ave, Paia, Maui, HI 96779-9744, phone: 808/5799568, fax: 808/579-8516, Cable: NifTAL@UHCCUX). They will provide small quantities for major tropical legumes, including trees, free to developing countries. ECHO has been keeping a running list of sources we have come across to date. If you would like a copy please let us know. More information on this topic can be obtained by contacting NifTAL. Other possibilities include the international agricultural research center nearest you (e.g. CATIE, CIAT, ICRISAT, IITA, IRRI, etc.), UNESCO (Microbial Resource Centre, Karolinska Institute, 10401 Stockholm, SWEDEN), the BNF Resource Centre (Soil Microbiology Research Group, Rhizobium Building, Soil Science Division, Department of Agriculture, Bangkok 10900, THAILAND, fax: 662-5614768) or Liphatech Company, 3101 West Custer Avenue, Milwaukee, WI 53209 USA. 800/558-1003; 414/3511476. FAX: 414/351-1847.

Some concluding remarks: Each situation is different. If farmers can obtain inoculant quickly and reasonably it can be a low-cost input with high returns. If planting something like soybeans for the first time in an area, special efforts should be made to obtain proper inoculant. Legumes will grow without rhizobia, they will just require mineral sources of nitrogen like other plants. Even with proper inoculation, factors like low phosphorous, low pH and insect damage will limit yield. It should also be noted that it can take up to 20 days for biological nitrogen fixation to get going, so an application of nitrogen just after germination can help even if rhizobia are present.