Higher nitrogen prices have
rekindled interest in the agronomics and economics of nitrogen management
for crop production.
Depending on which nitrogen carrier you use each pound of nitrogen
credit is worth anywhere from 25 to 36+ cents. A credit of 40 lb N/A
will keep at least $10 to $14+ in your pocket.
There are many sources of plant available nitrogen for which credit
can be taken; residual soil nitrate or ammonium, soil organic matter,
residues from a previous crop or a seed cover crop (cereal grains
or a legume), applied animal manure, and/or applied compost. Many
times these sources of plant available nitrogen are overlooked. Taking
the amount of nitrogen available in these materials as a credit against
the total nitrogen requirement of the crop can reduce the size of
your nitrogen fertilizer bill.
Organic soils (ones with over 30% organic matter) frequently release
60 to 100 lb N per acre during a growing season. Even soils that contain
4-8% organic matter may mineralize 20 plus lb N/A. A clover cover,
seeded in August the previous year, may provide 40 to 80 lb N/A.
A cereal grain cover crop seeded the previous September or early October
may recycle 20 to 40 lb N/A. The exact amount depends on how long
the cover crop is allowed to grow in the spring.
Composted manures or municipal sludge’s may provide four to
12 lb of available N per dry ton equivalent. Animal manures may provide
from four to 20 lb of available N per dry ton equivalent depending
on composition. Liquid manures vary greatly depending on whether it
comes from a lagoon or liquid pit. The question is frequently asked,
how much of the nitrogen in the cover crop or applied manure will
be available to the crop. An analysis of composts or manures will
provide information about the total nitrogen content. Based on that
result and estimate can be made of how much will become available
during the growing season. For composts only about 10–15% will
be available the first year. For animal manures around 50% will be
available the first year.
Another approach is to sample the soil four to six weeks after the
crop has been planted. This is about the time that many vegetable
crops will be topdressed with nitrogen. By this time a significant
amount of the nitrogen contained in the incorporated cover crop or
applied manure or compost will have been converted, by microbial activity,
to the ammonium and nitrate forms. The amount of available nitrogen
in the soil at this time can be used to adjust the amount of topdress
nitrogen to apply.
Summaries of past pre-topdress soil nitrate tests have shown that
in 20-25% of the cases the soil contained enough available nitrogen
so that no additional nitrogen was needed. In about another quarter
of the cases the topdress nitrogen rate was able to be reduced by
50%. An investment of $7 to $10 for a timely available soil nitrogen
test can be beneficial by a reduction in monetary outlay for nitrogen
fertilizer or by an improvement in crop quality and yield when the
test indicates a need for more N that would have been routinely applied.
The nitrogen requirement of vegetable crops during the first four
to five weeks after seeding or transplanting is small, usually less
than 25 lb/A. A good management approach is to supply 25 to 40 lb
N/A in the starter fertilizer. The young developing root system utilizes
nutrients from the fertilizer band more effectively during the early
stages of growth than from fertilizer that is broadcast and incorporated
into all the soil. Then base subsequent nitrogen needs on a soil nitrogen
test. At about four to five weeks vegetable crops enter into a rapid
growth stage and the nitrogen requirement increases greatly. Hence,
nitrogen broadcast at that time will be used effectively. Nitrogen
broadcast prior to seeding or transplanting is at greater risk to
be lost by leaching or denitrification (conversion to nitrous oxide)
under wet soil conditions than topdressed nitrogen. Preplant broadcast
applications are best limited to not more than 50 lb N/A. For some
vegetable crops multiple topdress nitrogen applications at a moderate
rate of 25 to 40 lb N/A are better than one large application. However,
this may not be practical for all vegetable crops, especially vine
crops.
Checking the nitrate content of the petiole sap provides a good indicator
of the nitrogen status of many vegetable crops and the need for supplemental
nitrogen. Many carrot and potato growers are successfully managing
nitrogen inputs by using this diagnostic test. Interpretation guidelines
are available for many vegetable crops. Applying nitrogen only when
needed is not only good economics, but is also good for maintaining
water quality.
For long-term nitrogen management, look to incorporating legume covers
or cereal grain cover crops into your vegetable cropping systems.
Legumes fix nitrogen from the air as well as recycle residual nitrogen
and other nutrients. Cereal grains are important for recycling nutrients
as well as maintaining an active microbial population in the soil.
Field studies at the Kellogg Biological Station have shown that spreading
compost on a green cover crop improves the release of nitrogen. Using
sources of nitrogen that are a natural part of farming system or community
can improve the agronomics of vegetable production and also keep money
in your pocket.
