A simple in-season soil test
can help sweet corn growers avoid overfertilizing during the growing
season based on soil nitrate nitrogen levels at a specific crop growth
stage. The pre-sidedress soil nitrate test, or PSNT, was first developed
for use on field corn and then adapted to sweet corn production. As
a gauge of soil nitrate nitrogen levels prior to in-season sidedress
applications, it helps growers reduce or eliminate those inputs where
adequate mineralized nitrogen is present.
Joseph Heckman, an Extension soil fertility specialist at Rutgers
University in New Brunswick, N.J., was a pioneer in adapting the PSNT
test to sweet corn. He says it can be an especially useful tool for
avoiding over fertilization of sweet corn in soils that have been
amended with compost or manure or otherwise contain inherently high
organic matter levels. “It’s a test that is pretty much
used all across the country with some variation from state to state,”
Heckman says.
According to Heckman’s recommendations, growers in New Jersey
and surrounding areas should sample soil nitrate nitrogen levels at
early sweet corn growth stages, about six weeks after planting when
plants are established between six and ten inches tall.
“When you plant use just enough fertilizer to get the crop established
(typically about 25 pounds per acre),” he says. “The test
can then help you decide whether you need to add or apply additional
N fertilizer to grow the crop for the remainder of the season.”
If PSNT lab levels come back higher than 25 ppm, Heckman says he would
not recommend adding additional nitrogen sidedress. Rates below the
25 ppm level should be sidedressed with additional N, although the
test doesn’t provide specific recommendations as to application
rate. Generally growers who need additional nitrogen apply up to between
140 and 160 pounds N, depending on soil type, he says. Consider split
applications on sandy soils, applying the first half at the 6- to
10- inch growth stage and the second about two weeks later.
“There are no real hard and fast guidelines as to how much to
put on when you know you need it. But we know if you are at a 10 ppm
you are really low, so you need the full recommended application rate
for nitrogen,” he says. “If you are between 20 and 25
ppm you would put on a low rate of N, say about 25 percent of usual.”
Heckman notes that the test in New Jersey is most useful on soils
with high organic matter, where manure or compost is added or a legume
cover crop has been previously incorporated, and additional N applications
might be applied simply for insurance.” The PSNT helps growers
avoid potential over-fertilization in those scenarios, which might
be a waste of money and leave unused nitrogen in the field after harvest
that might be leached into groundwater.
Heckman notes that his soil nitrate-N level guidelines are regional
and that growers should consult with their local Extension office
or other experts to get PSNT recommendations for their area.
In western Oregon, Extension soil specialist John Hart with the Oregon
State University Extension Service in Corvallis, recommends sweet
corn growers make sidedress nitrogen applications based on PSNT tests,
particularly in situations where they could save money on nitrogen
because of N inputs from a previous crop.
For example, growers rotating sweet corn into a field previously planted
to grass for seed should realize that decomposing roots from the grass
provide a good source of mineralized nitrogen for the next crop.
Growers should use PSNT levels in combination with additional sidedress
N applications, if necessary based on that reading, to reach a recommended
level of 175 pounds total N per acre.
Soil should be sampled 12 inches away from the plant when planted
sweet corn reaches 12 inches high. The sample should be drawn from
about 20 to 30 subsamples in a 40-acre field, and lab test should
convert the PSNT level from ppm to pounds nitrate nitrogen per acre.
A field with a PSNT below 135 pounds per acre should be supplemented
with up to 40 pounds additional sidedress to reach the recommended
175 pound level. Growers with readings above 175 pounds can save money
and can protect against over-application by avoiding additional sidedress
N. The gray area in the middle, between 135 pounds and 175 pounds,
is where growers will have to make their best decision.
“This middle area needs to take into account a grower’s
experience with his variety, management of the field, personal choice
and how much yield risk he is willing to take,” Hart says. “He
will have to make the decision there based on his own judgement and
past experience.”
Heckman says growers can verify they are applying adequate nitrogen
throughout the season to meets the plant’s needs by using a
nitrogen stalk test at harvest. The stalk test operates on the assumption
that excessive nitrogen is stored in the stalk when there is abundant
nitrogen in the soil. And laboratory tests of the stalk at harvest
can provide information about whether a grower is generally applying
optimum, excessive or inadequate nitrogen during the season.
“The purpose of the test is to get a report card on the crop,”
he says. “It’s too late to make adjustments for the crop
you just harvested, but you can use it for purposes of finding out
if your N program is on target.”
Stalk tests in New Jersey have shown in the past that growers are
about split into thirds between applying too much nitrogen, not enough
and adequate amounts. This means many growers could save money on
nitrogen inputs, and other may be able to boost yields by adding more.
An optimum stalk-test will read between 1.6 percent and 2.2 percent
total nitrogen by combustion in the stalk at peak harvest.
The test should be done on samples of sweet corn taken from sections
of the lower stalk on a day when the sweet corn is perfectly ripe.
An 8-inch stalk should be cut off between 6 inches and 14 inches above
the ground and analyzed for total nitrogen by combustion. The $10
lab test can provide at a small expense a snapshot of how well growers
are meeting their crop’s nitrogen needs by harvest.
“That’s a test I would like to see more widely used,”
Heckman says. “It’s fairly new and has widespread utility.”
