Legumes and nitrogen fixing

Legumes and their ability to 'fix' (add) nitrogen into the soil in which they were grown: Is it a feasible concept for SFG? Hint: Did Mel write about it in the ANSFG book?

It's that time of year again, and gardeners will be enthusiastically scouring the internet to learn new and exciting things about vegetable gardening. One of the concepts that they will be bringing back to our forum, is the idea that planting peas and beans will make their Mel's Mix so much better because they are legumes, and legumes take nitrogen out of the air and put it into the soil, rather than using the nitrogen in the soil. It must be good advice because nearly every gardening web site and forum says so. Except, have you noticed...nobody talks about the details? Nobody talks about how the nitrogen is fixed, when it is fixed, how much is fixed, and how to take advantage of the concept.

Before we get to the boring stuff, let's get the question out of the way. Planting legumes in a SFG for the nitrogen fix is totally unnecessary, and the benefit is next to none. That famous trowel full of compost that we add at planting time, contains more than enough nutrients, including nitrogen, for whatever it is we're planting. If the amount of acreage that you are growing on is large, legume type cover crops like alfalfa are an economical way to add nitrogen to your field. The use of cover crops was discussed at length in this thread. Cover crops are another row gardening technique that is not necessary in SFG.

Grow your peas and beans because you want to eat them, not for the tiny bit of nitrogen they might provide.

This is a non-technical source that gives a good overview of what the Rhizobium bacteria is about and how it affects nitrogen production in legumes. It also helps us understand the concept of inoculation. By making the bacteria available (inoculating) for the plant to use, it allows the legume plant to produce its own nitrogen the first year, rather than be dependent on withdrawing it from the soil. But, because the nitrogen in our Mel's Mix is so readily available, there may be little difference seen between using and not using an inoculant. In the ANSFG book, Mel doesn't say anything about using inoculant for beans, and for peas, he says: "inoculate to give them a boost". He does not say exactly what that means.

From the:
Cooperative Extension Service
College of Agriculture
New Mexico State University
Guide A-129, A-130
www.cahe.nmsu.edu

"Legume nitrogen fixation starts with the formation of a nodule. A common soil bacterium, Rhizobium, invades the root and multiplies within the cortex cells. The plant supplies all the necessary nutrients and energy for the bacteria. Within a week after infection, small nodules are visible with the naked eye. In the field, small nodules can be seen 2-3 weeks after planting, depending on legume species and germination conditions. When nodules are young and not yet fixing nitrogen, they are usually white or gray inside. As nodules grow in size, they gradually turn pink or reddish in color, indicating nitrogen oxidation has started. The pink or red color is caused by leghemoglobin (similar to hemoglobin in blood) that controls oxygen flow to the bacteria.

Nodules on annual legumes, such as beans, peanuts and soybeans, are round and can reach the size of a large pea. Nodules on annuals are short-lived and will be replaced constantly during the growing season. At the time of pod fill, nodules on annual legumes generally lose their ability to fix nitrogen, because the plant feeds the
developing seed rather than the nodule. Beans will generally have less than 100 nodules per plant, soybeans will have several hundred per plant and peanuts may have 1,000 or more nodules on a well-developed plant.

Some legumes are better at fixing nitrogen than others. Common beans are poor fixers (less than 50 lbs per acre) and fix less than their nitrogen needs. Maximum economic yield for beans in New Mexico requires an additional 30-50 lbs of fertilizer nitrogen per acre. Perennial legumes such as true clovers and alfalfa will return 250-500 pounds of nitrogen per acre.

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. Very 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 residue. 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 a 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."

Conclusions:

• Legumes have the ability to take nitrogen from the air to use for growth, rather than be dependent on soil nitrogen.
  
• Legumes don't put any significant nitrogen into the soil while they are alive.
  
• Most of the nitrogen that is fixed, is in the edible part of the plant.
  
• Growing the right type of legumes can reduce nitrogen fertilizer costs in large scale farming operations.
  
• In SFG, we learn to clean our plants from our boxes for composting. We do not turn plants under because we get much better benefits from adding our own compost.
  
• In the Three Sisters concept, the beans climb the corn, but they don't feed the corn.
  

Makes sense to me.


I asked my local nursery supplier about inoculants. He informed me that sales of such had declined drastically over the years. He too suggested that they were really not needed.

Maybe I don't need the extra nitrogen either.

Boffer that is a very nice, concise, readable, educational post! Thanks for that!!!

Very educational! Thanks for putting so much work into your post. I've always wondered about that, to be honest, and no one ever really went over details about how it was supposed to work. So now I know!

Quote ...
In the ANSFG book, Mel doesn't say anything about using inoculant for beans, and for peas, he says: "inoculate to give them a boost". He does not say exactly what that means.

Sounds like it's just adding natural stuff rather than half a chemical factory , so using perhaps a sprinkle of high nitrogen material like some neat rabbit droppings , fish meal , hoof & horn meal, dried blood or guano

I have learned _so much_ on this forum. Allow me to demonstrate a prime example of how this community functions.

Yesterday at 10 am I posted about last year's peas, and said "I also sprang for the innoculant."

A couple hours later, there is a complete explanation of how Legumes put nitrogen INTO the soil, and really isn't needed in a SFG. Is this a coincidence? Could be, but I think no, this is Boffer, reacting to my ignorance.

Either way, thanks Boffer.

I don't know scientifically if it all works, but to be on the safe side, I put my tomatoes in the next year following the peas since they both grow between my fence posts with trellis or string. I don't pull out my pea roots and all. I cut them back to the ground. I leave the roots there to compost thru the winter thereby, hopefully, gaining nitrogen for my next year's tomatoes. I then plant my peas where the tomatoes once stood.

Thanks, SJ. I've been sitting on the draft for a while. I finished it up now as NR gave me specific instructions to be on my best behavior for a few days because he's been busy in RL, and doesn't have the time to clean up after me! (that's almost verbatim!)

I included the brief paragraph about inoculants because it was pertinent to the topic of N fixing. I realize now that inoculants need their own thread.

Give me a day or so to put some info together.

bump