Ames, Iowa
May 4, 2009
Source:
Integrated Crop Management News
by Palle Pedersen, Department of Agronomy, Iowa State University
Extension
Soybean is a legume with a seed content of approximately 38
percent protein. Because of its seed protein concentration,
soybean has a large nitrogen demand and continues to increase as
yield increases. Total nitrogen accumulation for soybean is
supplied by two sources: the nitrogen fixation and from the
residual soil nitrogen pool.
Nitrogen (N) fixation is the symbiotic process of converting
atmospheric nitrogen into a usable form for the plant and is
critical for producing high soybean yields. For
nitrogen-fixation to occur, the nitrogen-fixing bacteria known
as Bradyrhizobia japonicum need to be readily available in the
soil or must be applied to the seed.
When the soybean seed germinates, the rhizobia bacteria invade
the root hairs of the seedling and begin to multiply. Nodules,
which are structures that house the bacteria, form on the roots.
Under field conditions, nodule formation can be seen shortly
after emergence but active nitrogen fixation does not begin
until about the V2 to V3 growth stage. After this, the number of
nodules formed and the amount of nitrogen fixed increase with
time until about R5.5 (midway between R5 and R6), when the
fixation decreases sharply.
Nitrogen fertilization of soybean is not recommended because it
typically does not increase grain yields in Iowa production
systems. The total number of nodules that form decreases
proportionately with increasing amounts of applied N. In
addition, N fertilization will inactivate nodules or cause them
to become inefficient, proportionately to the amount of N
applied. Although the soybean plant can use both symbiotically
fixed N from bacteria and inorganic soil N (both mineralized and
fertilizer N), soil N is used in preference to fixed N if
available in the soil.
Soybean does not respond with increased yield to the addition of
N, even though plants remove a significant amount of it from the
soil. In Iowa soils, which have appreciable organic matter and
mineralized N, up to 50 percent or more of the total N
accumulated by soybean can come from the soil system rather than
N2 fixation. Increasing N supply by adding fertilizer, animal
manure, sludge, or a green manure crop simply substitutes N from
these sources for N that would otherwise be fixed by the
bacteria in nodules on the roots, and hence an economically
wasted input.
Today, most fields in the Midwest have experienced soybean in
the rotation, likely increasing the population density of
Bradyrhizobia bacteria in the soil. Improved inoculant
technology coupled with higher commodity costs, ease of
application, and low cost inoculant products have many growers
reconsidering the use of inoculants.
I have since 2003 conducted numerous inoculation trials every
year to assess new inoculants on the market. Overall, I have not
seen a consistent response to use of soybean inoculant in fields
in Iowa with a history of soybean. Current recommendations for
states of the upper Midwest are to use an inoculant if fields
have no history of soybean production in the past three to five
years, soil pH is below 6.0, sandy soil, low organic matter, or
have been flooded for more than a week.
Consider fields that were flooded last year
This year, we need to consider the fields that were flooded last
spring. Some of these fields were flooded over an extended
period of time, and plants were injured or died. These fields
may need to be inoculated because the anaerobic conditions may
have reduced the level of Bradyrhizobia bacteria in the soil.
Palle Pedersen is an assistant professor of agronomy with
research and extension responsibilities in soybean production.
Pedersen can be contacted at
palle@iastate.edu or by calling (515) 294-9905.
This article was published originally on 4/28/2009 The
information contained within the article may or may not be up to
date depending on when you are accessing the information. |
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