nitrogen availability in legumes

A couple of years ago, and agronomist told me that the nodules on the roots of legumes form as the pods form. 

I'm looking at "roots demystified" and it seems to suggest that the nodules are formed before the plant even flowers. 

It also suggests that you can get far more N in the soil by tilling legumes at just the right time.  Interesting.  Although I always kinda thought that you would get about 30% as much N in the soil by not tilling at all - which seems like it could be a fair trade.  I was getting the feeling in the book, that it might end up being something more like 5%.  Anybody have any good info on this sort of thing?

this may help in that it may raise more questions than answers...which in itself may help deepen the understanding or rather, reveal the complexity of nitrogen fixation through plants.
http://agron.scijournals.org/cgi/content/full/98/2/302

From the Kansas Rural Center Sustainable Agriculture Management Guides- Cover Crops and Legumes http://www.kansasruralcenter.org/publications/covercrops.pdf

[snips]

"An ideal plowdown time for legumes is between early bud and early bloom. A legume cover crop that begins lowering will increase the amount of carbon which will slow down the availability of nitrogen for the next crop. The nitrogen produced by the legume will remain in the plant tissues whether the plant is living or dead. As long as the legume sod attained its maximum growth prior to fall dormancy, it should not matter whether the plant regenerates in the spring or if it has winter killed (“Green Manures,” 1983)."

"It is best after plowdown of a mature, carbon-rich green manure to wait until the breakdown is well underway before planting the following crop. Soil microbes multiply rapidly to break down the carbonaceous plant matter, and they consume a lot of nitrogen in the process. This process leaves little nitrogen available for plant growth until breakdown has been completed to the extent that the microbes begin to die and release their nitrogen reserve back to the soil. A general rule of thumb applicable to most crops and climates is to allow breakdown of the green manure crop for at least two weeks. When the tilled-in materials break apart or crumble easily, the breakdown may be considered far enough along for the next crop to be planted."

"Deep-rooted legumes, such as alfalfa, used in crop rotations, are believed to cycle nutrients upwards from subsoils. Winter cover crops trap nutrients that otherwise might have been lost from the root zone and recycle them for the next crop. Legumes have an advantage over other soil-conserving crops because of their ability to decompose more rapidly due to their lower carbon-nitrogen ratio."

"The portion of green-manure nitrogen provided to a following crop is usually about 50-60% of the total amount contained in the legume. Approximately 40% of the plant tissue nitrogen becomes available the first year following a chemically burned, no-till legume mulch. Approximately 60% of the tissue nitrogen is released when the cover crop is incorporated as a green manure rather than left on the surface as a mulch.  Lesser amounts are available the next two growing seasons, but increased yields are apparent. Nutrients from decaying plant material are more readily available for use by succeeding crop plants than those nutrients derived from soil minerals or particles. During decomposition of organic matter, carbonic and other organic acids are formed. These organic acids react with insoluble mineral rocks and phosphates precipitates, releasing phosphates and exchangeable nutrients.  If a cover crop is put up as hay, the nitrogen in hay is removed with the crop. If it is grazed, the animals return about 80% of the nitrogen in their excrement"


Also, many people say that if you inter-plant a legume (like beans or field peas) among another crop (like corn), it's a waste, because the soil doesn't get the nitrogen from the root nodes because the beans/peas hasn't been ground up and tilled into the soil, and by the time the plant dies, it's used up all the nitrogen for itself.  This is only partly true.  ALL the nitrogen from the plants is not available but, as with many things in nature, it's simply part of an ongoing  process.  The nitrogen-fixing nodules are on the roots, and the roots grow and die, releasing the nitrogen to the microbes to turn into a form the corn can use.

Using an interplanted legume among a growing crop also provides the benefit of shading the soil, helping to retain moisture and reduce the too moist/too dry stresses to the corn.

Even the tiny feeder roots of trees only live for a few days, and that included legume trees.

Sue

A podcast that talks about this a bit:  http://www.richsoil.com/permaculture/143-podcast-012-helen-atthow-soil-conifers-fukuoka/

I have always been under the impression that the nodules will not release the nitrogen into the soil until the legume dies and begins decomposing.  The leaves gather nitrogen from the atmosphere and transport it to the nodules where it is converted into a form that the plant can utilize, and sent back up the stalks to the leaves/flowers/fruits which can now use them for growth.  I was told that the nodules will only release "an insignificant amount" (<1%) of N into the soil as long as the plant is living/growing.

The timing for incorporating the plant into the soil depends on your ultimate goal.  For maximum N, you incorporate while the plant is still green and growing.  For maximum organic matter, wait until maximum growth has been achieved.  If you do not incorporate the plant into the soil, some N will be lost back into the atmosphere.

Most no-till farmers state that it is usually 2-3 years before they see any measurable improvement to their crops.

What If I waited to harvest some legumes then tilled the parent plant under?

THAT is the ultimate use of the plant (if it is a legume with an edible crop).  Your crop will have consumed a portion of the N, which you (or your livestock) will benefit from.  Therefore, you will be incorporating slightly less N into the soil, but close to maximum organic matter.  A win-win situation.

There was a time in the 20'2-30's when a farmer with depleted soils would plant a season of sweet peas just to enrich his soil.  Though it would help his soil, he had to forfeit a season of crops.  Now, the farmers are planting soy beans for the same reason, but soy yields a crop that can be sold.  With the current trend, sweet peas may become extinct since they do not produce a marketable crop.  As a side note: 85% of all soy beans grown in the US are GMO, and soy bean crops use more chemical herbicides than any other crop in the US.  The only crop that comes close to soy beans in herbicide use is lawns (home+golf course).  Next is corn.  If you eat any product that lists soy oil or protein in the ingredients, you're eating GMO.

I read cotton was the worst offender.
http://en.wikipedia.org/wiki/Organic_cotton#Ecological_footprint

High levels of agrochemicals are used in the production of non-organic, conventional cotton. Cotton production uses more chemicals per unit area than any other crop and accounts in total for 16% of the world's pesticides.[7]

Cotton wins the prize for most total chemicals because of the high use of insecticides.  I was stating the facts for herbicides which soy is the primary abuser.  Since cotton is a non edible crop, farmers feel safe drowning it in anything they can find.  Maybe that is why I cringe whenever I see cotton seed oil listed as an ingredient in foods.

Thanks for the info and link on the availability of nitrogen!  I'd like to read more in order to help me better map out my planting.

I'm looking at "roots demystified" and it seems to suggest that the nodules are formed before the plant even flowers. 

Does clover count?  A couple years ago, with my preschooler, we dug out some plants from the yard to learn about root structure(tap/fibrous).  The clover had nodules then, and it had not yet flowered.  Do you think it varies by plant?

I plant crimson clover, white clover and dutch clover all the time. I've seen nodules very early (yes, before flowering takes place)..I interplant my edibles with a nice cover of crimson and have had great success as you might expect..

Peace -

Paul and Jack Spirko discuss nitrogen release as it relates to "pruning and pulsing" in this podcast: http://www.richsoil.com/permaculture/371-podcast-054-jack-spirko-modern-survivalism/

Paul and Jocelyn review chapter 6 of Gaia's Garden, in which Paul talks about pulsing, and release of nitrogen no matter whether the nitrogen-releasing plant is dead or alive: http://www.richsoil.com/permaculture/373-podcast-055-gaias-garden-chapter-6/

paul wheaton wrote:
A couple of years ago, and agronomist told me that the nodules on the roots of legumes form as the pods form. 

If that's what he said he was talking out of his posterior

I did a PhD on this. LOADS of myths, misconceptions and wishful thinking.

The nodules generally start to form when the plant is just a seedling, and the legume plant can then fix anything between 0 and 100% of its nitrogen needs. If soil nitrogen is in short supply, if there is competition from other plants,it feeds its rhizobia and fixes more. If soil nitrogen is readily available perhaps due to lack of competition it kicks out those unnecessary freeloading rhizobia and gets its N from the soil instead. Often in these situations it will subsequently get smothered by plants that are better at using soil N. (for this reason, a mixture of legumes and non legumes is better than a legume monoculture. If the non legumes take over you probably had plenty enough N to begin with. The relationship between the plant and the bacteria can switch between mutualism and parasitism depending on the environment.

Legumes do not "want" to feed nitrogen to other plants ,and exudation from the roots is not the most important route by which this occurs. It's a slow process which often happens at inconvenient times such as winter. Over many years this release of nitrogen is highly significant but in one growing season less so.

Most of the fixed nitrogen is in the leaves, flowers and seeds  not the roots. Rather than hopefully waiting for the legume plant to kindly share its fixed nitrogen with neighbouring plants it's often better and quicker to intervene. A cow grazing a field of grass and clover transfers lots of fixed nitrogen from clover to grass in dung and urine. Cutting and mulching is also good especially if you put the mulch somewhere else (legumes fix more N and persist longer when N is in short supply, paradoxically it helps to have low fertility areas with legumes to make the whole system more fertile).

Casey Halone wrote:
What If I waited to harvest some legumes then tilled the parent plant under?

Good. When beans and peas are green there is still a lot of fixed N in the leaves and stem which can be left to rot, composted, fed to animals and returned to soil that way.

However when beans and the like are harvested fully ripe and dry they don't leave much fixed N behind in the soil, it's all in the beans (but you can still enrich the soil with them if you have a compost toilet or feed them to animals), sometimes maincrop beans can actually deplete soil N slightly. If the soil is left bare till the  next spring the benefits from the bean crop  residues will be small at best.

Pignut wrote:
If that's what he said he was talking out of his posterior

I did a PhD on this. LOADS of myths, misconceptions and wishful thinking.

The nodules generally start to form when the plant is just a seedling, and the legume plant can then fix anything between 0 and 100% of its nitrogen needs. If soil nitrogen is in short supply, if there is competition from other plants,it feeds its rhizobia and fixes more. If soil nitrogen is readily available perhaps due to lack of competition it kicks out those unnecessary freeloading rhizobia and gets its N from the soil instead. Often in these situations it will subsequently get smothered by plants that are better at using soil N. (for this reason, a mixture of legumes and non legumes is better than a legume monoculture. If the non legumes take over you probably had plenty enough N to begin with. The relationship between the plant and the bacteria can switch between mutualism and parasitism depending on the environment.

Legumes do not "want" to feed nitrogen to other plants ,and exudation from the roots is not the most important route by which this occurs. It's a slow process which often happens at inconvenient times such as winter. Over many years this release of nitrogen is highly significant but in one growing season less so.

Most of the fixed nitrogen is in the leaves, flowers and seeds  not the roots. Rather than hopefully waiting for the legume plant to kindly share its fixed nitrogen with neighbouring plants it's often better and quicker to intervene. A cow grazing a field of grass and clover transfers lots of fixed nitrogen from clover to grass in dung and urine. Cutting and mulching is also good especially if you put the mulch somewhere else (legumes fix more N and persist longer when N is in short supply, paradoxically it helps to have low fertility areas with legumes to make the whole system more fertile).

Nice breakdown bro. Good suggestions.

Paul and Helen duke it out over nitrogen availability in their recent podcast reviewing Geoff Lawton's food forest DVD: http://www.richsoil.com/permaculture/417-417/

Here, Paul interviews Geoff Lawton: http://www.richsoil.com/permaculture/514-podcast-089-geoff-lawton-1/

They discuss Paul and Helen's ongoing nitrogen availability debate.

From recent reading, it appears that legumes should be harvested when they are between 10% & 50% in flower. After this point, any N in the nodules will be sent up to the pods. Creating the seeds will almost completely deplete any N from the nodules.

If they are terminated at the 10-50% flowering stage, the N will remain in the nodules. Then as the soil food web (SFW) processes them, they will become available to subsequent crops.

John Polk wrote:From recent reading, it appears that legumes should be harvested when they are between 10% & 50% in flower. After this point, any N in the nodules will be sent up to the pods. Creating the seeds will almost completely deplete any N from the nodules.

If they are terminated at the 10-50% flowering stage, the N will remain in the nodules. Then as the soil food web (SFW) processes them, they will become available to subsequent crops.

The N mostly isn't in the nodules, it's just fixed there. The nodules are like little starchy potatoes, with very little protein or N in them. As the plant flowers it transfers nutrients from its leaves and roots to flowers and seeds. Actually all plants do this, especially annuals, which is why you often see old leaves senescing, that is turning brown, and dropping off as the plant flowers. The reason for choosing the early flowering stage is that at this stage, the plant is at it's peak of growth but can still easily decompose. Flower stalks often get dry and woody as the seeds develop, and seeds can remain dormant for years without rotting, so the nutrients in them are unavailable to plants (incidentally, I have often wondered how much soil nutrients are bound up in weed seeds)