Plan For Phosphorus And Potassium Fertilization
URBANA, ILL.
With a promising harvest under way, it's a
good time to replenish fields with phosphorus
and potassium, said Fabian Fernandez,
University of Illinois Extension
specialist in soil fertility and plant nutrition.
“High fertilizer prices for a few years were followed
by a wet fall and late harvest last year,”
Fernandez said. “This resulted in many fields
not receiving all the phosphorus and potassium
needed to maintain optimum test levels. Since
many fields have already been harvested and
soil conditions are ideal for tillage or to drive
equipment over the field, now’s a great time to
plan for phosphorus and potassium applications.”
Fernandez said how phosphorus and potassium
are applied does not matter as long as you
apply it so test levels are adequate for crop production.
For farmers interested in strip-till this fall,
Fernandez recommends waiting until at least
the middle of October to avoid heavy rains that
can flatten the berm created during the tillage
operation.
Applying phosphorus and potassium annually
or biennially is debatable. U of I research indicates
that as long as needed fertilizer is applied,
there is no yield benefit hinging on whether the
application is done every year or every other
year.
“However, we have seen that for biennial applications
it is better to apply fertilizer before the
corn crop and have soybean as a residual
feeder,” he said. “Research has shown that having
corn in the second year after fertilization can
cause yield reductions, especially in no-till systems.
Conversely, soybean yields were not affected
in response to the time of fertilization.
Even if a biennial application results in time
saving and one less pass over the field, if your
experience tells you that your soil does not build
up, I would suggest always applying on an annual
basis.”
Both fall and spring applications have proven
to be effective alternatives to provide nutrients
to the crop and there is no agronomic difference
in terms of one timing being better at increasing
nutrient availability relative to the other.
“Fall is normally the preferred time since typically
there is more time and equipment available
in the fall than during the planting season
in the spring,” Fernandez said. “Also, soil compaction
is less of a concern when driving heavy
equipment loaded with fertilizer in the fall because
soil is typically drier than in the spring,
and phosphorus and potassium applications
combined with tillage operations are more feasible
in the fall.”
One potential drawback for fall applications is
the fact that the nitrogen accompanying phosphorus
in di-ammonium phosphate (DAP, 18-
46-0) and mono-ammonium phosphate (MAP,
11-52-0) is more susceptible to loss even if applied
late in the fall. However, the amount of nitrogen
present in these applications is not very
high and the benefits of a fall application typically
outweigh the potential for any small nitrogen
losses.
If phosphorus and potassium are at adequate
levels, there is no need to make any significant
change in the short term when going into a rotation
with more corn.
For farmers planning to make a long-term
commitment to more corn, remember that overall
corn can remove more phosphorus and less
potassium than soybean. Thus, fertilization
plans should be adjusted accordingly.
Finally, Fernandez said that before deciding
placement method and when and how much to
apply, the single most important thing to know
is the test level of the soil. To find out, there is
no substitute to a regular (every four years) soil
sampling program. Δ