The Late Spring Nitrate Test (LSNT) has long been a useful
in-season check on nitrogen availability, but manured fields have always sat a
little differently in the interpretation guide. The original recommendation
still says it plainly: soils receiving recent manure applications, or corn
following alfalfa, don’t behave like “typical” soils and will get some slower
nitrogen release. They tend to mineralize more plant-available nitrogen after
sampling than a “typical” soil, meaning the standard LSNT critical values may
not tell the full story. In practice, that means we’ve always known manured
systems don’t fit neatly into a single threshold but we also want to try to use
the LSNT as a tool to help us manage better.
The newer LSNT decision support work reinforces something
many of us have seen in the field: even when we try to quantify nitrogen
availability, there is still a lot of variability. That raises an important
question, should we be thinking differently about manured fields altogether,
rather than just adjusting the threshold or are the principles still the same?
And the good news is, the principles are at least similar, but the bad news is
we have to think a little bit deeper about the characteristics of the manure we
are using, when we applied, and how the growing season has been in terms of
mineralization potential thus far to improve interpretation.
One useful way to ground that question is to step back and
look at soil nitrogen dynamics in-season. The FACTS soil
mineralization tool is a helpful reference here, especially the “how are we
comparing to average” thus far through the growing season comparison. If the
season is tracking below average for mineralization, manure-derived organic
nitrogen is likely also lagging behind expectations, meaning more of that
nitrogen may show up later in the growing season and we won’t see as much as usual
on our late spring nitrate test. If mineralization is trending above average,
the opposite is true, we should expect manure nitrogen to be contributing more
aggressively than a “typical year” assumption would suggest.
But not all manure behaves the same, and that is where
interpretation gets more interesting. The ratio of ammonium-N to total-N is a
simple but powerful indicator of how quickly a manure behaves like a fertilizer
versus a slow-release organic source. I gave a summary of this in the Talkin’
Crap episode, Available
or Not: The Nitrogen Guessing Game in Manure Planning, and you can see our
summary of average Midwestern
manure properties on this handout. In summary, high ammonium manures, such
as liquid swine manure, tend to act much more like commercial nitrogen
fertilizer at application, while more organic-heavy manures shift their
contribution toward delayed mineralization.
That distinction shows up in the research. Woli et al.
(2011), working with John Sawyer, evaluated liquid swine manure as a nitrogen
source for corn and found LSNT critical values near 25 ppm, but also documented
cases where yield response did not occur even at lower LSNT readings. One
explanation is that manure history and mineralization dynamics were still
supplying nitrogen beyond what the soil nitrate snapshot captured at sampling
time. Is this a manure challenge? Maybe, but when you look at the latest data from
the Iowa Nitrogen Initiative, we see fields that exhibit the same sort of response,
that despite being below the critical threshold they don’t need as much N as an
average field. Perhaps it is more common in manured fields because we are
building soil health and quicker nitrogen cycling, but there are other ways to
get there.
A similar pattern emerged in work by Ruiz Diaz, Sawyer, and
Mallarino with poultry litter. They showed LSNT results were strongly tied to
the ammonium fraction applied with the litter, while the organic nitrogen
contribution was less immediately reflected in the test (probably because
mineralization is relatively low until that May time period and starts to pick
up rapidly around the time, we’d be taking the late spring nitrate sample).
However, they also noted that this organic fraction does not disappear, it
continues to mineralize and contribute nitrogen later in the season, which
aligns with what we’ve seen in subsequent field work here in central Iowa.
Taken together, these results suggest that LSNT on manured
fields is not just a question of “what is the number,” but “what nitrogen cycle
are we sampling?” A manured field is often a moving target: part fertilizer
response, part soil mineralization, and part delayed organic nitrogen release.
That is exactly why variability in LSNT results is higher in these systems and
why a single cutoff value will always feel a bit blunt, and a reason the newest
tool focuses on the probability to a yield response.
So where does that leave us? LSNT is still valuable, but in
manured systems it should be interpreted as one snapshot within a broader
nitrogen story, not the full story itself. Understanding manure type
(especially NH₄-N to total N ratio), current soil mineralization conditions,
and field history becomes just as important as the number in how you tailor
your nitrogen management decision. The LSNT tells us what nitrogen looks like
today. Manure systems require us to also think about what nitrogen is still on
its way.
