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Beginner Vegetables: Nitrogen fixing legumes?

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tucsonjill
Lincoln, NE
(Zone 5a)

October 31, 2008
3:42 AM

Post #5735645

Do all legumes fix nitrogen? I would guess some legumes are better than others in this respect, but don't really know for sure. I'm specifically wondering about peas and beans, if I plant them, will they do "double duty", adding nitrogen to my soil as they add veggies to my diet?

Thanks for any help!
eweed
Everson, WA
(Zone 8a)

October 31, 2008
8:32 AM

Post #5735946

You are here:Home Blogs c. hansen's blog Introduction to Nitrogen Fixation by Legumes
--------------------------------------------------------------------- I would like to focus on nitrogen fixation, a very special relationship between bacteria, plants, the soil, and the atmosphere.
In brief, nitrogen fixation is a process where inert N2 (nitrogen gas) is converted into usable ammonia (NH3). This form of nitrogen is important to plants and animals as it helps to manufacture amino acids, proteins, nucleic acids and other nitrogen-containing components necessary for life.
Nitrogen fixation by legumes is a partnership between a bacterium and a plant. The plant supplies all the necessary nutrients and energy for the bacteria. Examples of legume plants include Alfalfa, Fava Beans, Vetch, Peanuts, Soy Beans, and Clover. Other plants benefit from nitrogen-fixing bacteria when the bacteria die and release nitrogen to the environment or when the bacteria live in close association with the plant.
A common soil bacterium, Rhizobium, invades the root of a legume and multiplies within the root cells forming bump-like masses called nodules. Within these nodules, nitrogen fixation is done by the bacteria, and the NH3 (ammonia) produced is absorbed by the plant. A way to determine whether or not nitrogen fixation is occurring in a plant is to investigate the roots. When fixation occurs the nodules turn from white or gray to pink.
It is a common misconception that nitrogen fixing plants deliver nitrogen directly to the soil via their root systems. The following is from the W.C. Lindemann, a Soil Microbiologist from New Mexico State University:
The amount of nitrogen returned to the soil during or after a legume crop can be misleading. Almost all of the nitrogen fixed goes directly into the plant. Little leaks into the soil for a neighboring non-legume plant. However, nitrogen eventually returns to the soil for a neighboring plant when vegetation (roots, leaves, fruits) of the legume dies and decomposes. When the grain from a grain legume crop is harvested, little nitrogen is returned for the following crop. Most of the nitrogen fixed during the season is removed from the field. The stalks, leaves and roots of grain legumes, such as soybeans and beans contain about the same concentration of nitrogen as found in non-legume crop residue. In fact, the residue from a corn crop contains more nitrogen than the residue from a bean crop, simply because the corn crop has more residues. A perennial or forage legume crop only adds significant nitrogen for the following crop if the entire biomass (stems, leaves, roots) is incorporated into the soil. If forage is cut and removed from the field, most of the nitrogen fixed by the forage is removed. Roots and crowns add little soil nitrogen compared with the aboveground biomass.
Taking the implications of the above paragraph seriously, it is important to till in a legume cover crops in order to utilize the nitrogen fixed from the atmosphere. This process is similar to carbon sequestration process mentioned in the previous blog. When we incorporate plant matter back into the soil we feed the microbial life of the soil foodweb. These microbes mineralize nutrients in the soil, aid aggregation of soil particles, and help to form humus that improve overall health and vitality of the soil.

Above not my material but copied from U new Mexico
tarheel2az
Tonto Basin, AZ

October 31, 2008
1:44 PM

Post #5736417

Very informative! Thanks.


Frank
tucsonjill
Lincoln, NE
(Zone 5a)

October 31, 2008
6:44 PM

Post #5737395

Yes indeed, very informative! I was completely in the dark about how it really works. Thanks for the info, eweed!
dividedsky
Indianapolis, IN
(Zone 5b)

November 9, 2008
4:33 PM

Post #5770197

"When the grain from a grain legume crop is harvested, little nitrogen is returned for the following crop. Most of the nitrogen fixed during the season is removed from the field."

So how do you keep the legume from reseeding itself if you want to grow something else there?
eweed
Everson, WA
(Zone 8a)

November 9, 2008
6:29 PM

Post #5770592

Indy your right you need to turn it in before it matures.

I only use Rye or Winter Wheat. I prefer Wheat because it tills and dies better than Rye.I do like Rye grass because you can plant it in the corn row a month before harvest and will be well started before the corn comes off.

Also works well to stunt the weed growth . To get the best job in the spring before it starts maturing. Mow the field then till then plow

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