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nitrogen availability in legumes | (Read 86 times) |
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paul wheaton
Administrator
Posts: 1338
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July 14, 2008, 08:33:34 PM |
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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?
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SueinWA
Posts: 303
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October 15, 2008, 01:54:05 PM |
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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
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