What level of N fertilizer can be safely added,
without inhibition of nodulation and nitrogen fixation?
At very low levels of soil N, starter doses of N fertilizer enhance
early plant growth and photosynthesis
and actually improve early nodulation
and nitrogen fixation.
At higher levels, nitrogen fertilizer can inhibit nodule formation,
and reduce nitrogen fixation by already-formed nodules. A common
problem is that N levels at the beginning of the growing season
may limit nodulation and nitrogen fixation, but not be sufficient
to supplement the plant's N needs for the entire growth cycle. Yields
are then reduced. A common recommendation is that some decline in
nodulation and nitrogen fixation is likely when residual plus added
fertilizer N in the top twelve inches of soil exceed 30 lbs. of
N/acre.
Cultivar differences in
tolerance of the symbiosis
to soil N have been shown in a number of crops, but have yet to
be used commercially.
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