Do Soybeans Need Supplemental N Fertilization?
The nitrogen (N) requirement of soybean is one of the highest of
the agronomic crops.
This is because of a high concentration of protein in the seed.
Soybeans obtain this N by converting non-available N2 gas from
the air into plant useable N with the help of Bradyrhizobium japonicum
bacteria, which infect the roots of the soybean plant and cause formation
of round or oval-shaped nodules on roots. Symbiotic fixation supplies
about half the N needs of the plant, and the remainder comes from soil and
/or fertilizer. Soybeans require about 3.5 lb of N per bushel of seed
Although soybean has a high demand for N, it has historically not
received fertilizer. This is because the amount of N fixed by the
Bradyrhizobia bacteria and the amount of N mineralized from soil has been
assumed to be sufficient to meet the N requirements of soybeans.
However, reports of positive yield responses to late-season
supplemental N application for irrigated soybeans have resulted in
increased interest in soybean N fertilization practices.
For more information on soybean research, click here.
© 1999 Pioneer Hi-Bred International, Inc.
At 12:00 PM 23/01/00 -0600, you wrote:
>Wow, where did you hear that farmers pour pounds of N fertilizer on soybean
>fields? I know of no one who's doing that, or recommending it. Why would
>you pay for an input that's provided essentially for free?
>You're probably right that soybean is not an efficient N user--it's not
>nearly as efficient an N fixer as some other legumes, and it may contribute
>to a net drain on organic N stores in the soil. But wouldn't you agree
>that its overall effect is beneficial compared to the economically viable
>alternatives in a corn based cropping system?
>At 06:18 PM 1/22/2000 -0600, you wrote:
>>If soybeans provide a net nitrogen fix for the soil, then why do soybean
>>farmers pour lbs of N fertilizer on their soybean fields?
>>An article in Science a few years ago (which I will dig out on Monday when
>>I'm back in the office if anyone wants the citation) compared a number of
>>the principle legume crops as to efficiency of N use (ie. kg of grain
>>produced for kg of N fertilizer applied). Soybeans were by far the least
>>efficient N user, by an order of magnitude.
>>Also, soy protein is one of the most unbalanced (in terms of the
>>proportions of essential amino acids considered ideal for human
>>consumption) of any edible protein. In fact, it is barely edible (I
>>wouldn't eat it unless it was heavily processed as in tofu).
>>I have no evidence actually showing that soybean production depletes soil
>>N, but it seens likely that that occurs under the right circumstances. I
>>suspect soybeans have other deleterious effects on the soil, particularly
>>in the conditions they are usually grown in conventional agrculture.
>>Soybeans are still sometimes called the crop of the future and it is
>>claimed will feed the world. I always remember the faces of Mexican farmers
>>when some city do-gooder tells them that their problems will be solved if
>>they just plant soybeans. I seriously doubt it--a much overblown crop.
>>>Date: Sat, 22 Jan 2000 12:49:22 -0600
>>>From: "Laura K. Paine" <email@example.com>
>>>Subject: Re: soybeans deplete soil?
>>>Hello Betsy and everyone,
>>>I don't know of any data that will support the contention that conventional
>>>soybeans deplete soil nitrogen. Both organic and conventional soybeans,
>>>properly innoculated, will fix nitrogen for a net gain in the soil. Most
>>>recommendations suggest that you can credit soybeans for about 40 pounds of
>>>N/acre/season of growth.
>>Mexico, D.F. & San Cristóbal de Las Casas, Chiapas
>>Tel. y FAX 525-666-73-66 (DF)
>> 529-678-72-15 (Chiapas)
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>Crops and Soils Agent
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