Longevity of nitrogen in soil following a nitrogen fixing plant

Asked March 22, 2014, 5:34 PM EDT

I am a CMG in El Paso County. I teach a class entitled "Soil: The Foundation of a Successful Garden" for our spring and fall class series. I had a question from a class attendee that I have been unable to find a satisfactory answer from online research. When growing a legume crop, such as beans, we know that that crop can fix nitrogen for it's own use. Some general gardening resources suggest that it would be beneficial to rotate a different crop into that site the following year to take advantage of the nitrogen in the soil. However, we also know that nitrogen does not generally remain in the soil from year to year. Also, some .edu sites suggest that the entire plant, such as corn, would need to be plowed into the soil in the fall for the plant to decompose any nitrogen left in the plant and for the soil to benefit. So, the main question is how long nitrogen remains available in the soil following the growth of a legume. Thank you. Joan Nusbaum CMG '04

El Paso County Colorado

2 Responses

Joan,
thank you for sending in this interesting question..
I have been looking at a few websites from different extensions in different states. They all mostly state, that rhizobacteria occurs naturally in many soils. This bacterium helps to change the nitrogen in the atmosphere, to nitrogen that can be absorbed by either the legume plant or stored in the root nodules for future needs The roots should remain in the soil during the fall clean up. The amount of nitrogen produced will depend on the amount of bacteria in your soil as well as on the amount of crop you plant.
Each rhizobium bacteria is specific to a crop, or sometimes 2. You can bye inoculated seeds for a cover crop9 legumes , hairy vretch) etc) then mow and till the crop under as green manure,in addition to the root nodules. So when you look at all this..... there is no guarantee to have a certain amount of nitrogen fixed in the garden bed without taking a soil sample.
I would imagine after a few years of trialing this, a home gardener would be able to estimate what the nitrogen amount in a certain bed might be, given that he had a soil test done in the previous years, on a regular basis and used inoculated seeds and/or cover crop.
Another point to consider is the loss of nitrogen due to erosion, leaching out during rainfall etc.
To plow some plants under that will decompose easy makes sense, ie. lettuce, spinach.
I know, I have not answered your question with specific numbers, but I think the only way to find out those numbers would be to do a soil test before your seed out your crop, in the fall after your harvest and again in the spring.
Thanks again. Let me know if you learn more about this.