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Do I need Legume Soil Inoculant--a way to test

To buy soil inoculant or not to buy soil inoculant for your legume crop, that is the question? You can buy it. There are strains for different legume crops, or you can buy one that is combined, for whatever you plan to grow. Some say it is already in you soil, so to buy it is to waste money. Here is a way to find out if you have enough of the right kind already:

The following information is from Iowa State University. Here is the link:


The question is whether there is a field test one can set up to find out whether suitable rhizobia are in the soil and fixing nitrogen in the roots of your legume crop. The answer is that there is, indeed a simple test one can set up to find out. Here is what they say:

If you wish to conduct a small field trial to determine if a) you have appropriate rhizobia in your soil and b) they are effective in fixing N, you may plant a SMALL area with the test legume. Mark plots and fertilize  portions of the area with N fertilizer (perhaps 100-200 lb N/acre). Extensive nodulation of the plants in the non-fertilized area would mean the proper strains are present that can nodulate the legume. Compare the fertilized and non-fertilized areas. If growth is generally comparable in both areas, the rhizobia are effective in fixing N. If all plants grow poorly, factors other than N may be limiting growth, such as variety adaptation, acidity, fertility, moisture, compaction, or other.


They say that the rhizobial bacteria are doing the best job if there are large nodules on the main root,and some on other roots too. From that, I'd say my photo of fava bean roots shows pretty good work being done fixing nitrogen. This is a very young plant.Already it is covered with nodules, with the biggest on themain root.

 Root nodulesIMG_1426 copy


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Pam Peirce

While the trial you suggest is fine, gardeners who have tiny plots would rather not have to purchase several different innoculants in order to trial them. The beauty of the Iowa State method is that it doesn't require one to purchase any innoculant before doing the trial. Sometimes a gardener is planting in a community garden, for example, and may not know what the previous gardener in the plot has grown there or added to the soil. This is a quick way to check it out.

garden nurseries

One way to evaluate the response of inoculants is to test several inoculants and an untreated control in fields using replicated strip tests. When in doubt about the rhizobial population in a field, it is a good practice to apply inoculum, especially if the legume has never or not recently been grown in that field.

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