Corn And Soybeans Are Different
PORTAGEVILLE, MO.
Corn and soybeans are commonly grown in a
1:1 rotation in Missouri. This practice has
several advantages over continuous corn or
soybeans. These include pest control, nitrogen
fertilizer management, and overall soil health.
While both crops of this rotation have similar
nutrient requirements there are several differences
that are important to consider when making
management decisions. These differences
are philosophy, changing yield goals, P & K crop
removal and response to lime.
In the past producers have “grown” corn and
“planted” soybeans. This philosophy implies that
most of the producers management effort is devoted
to corn and the soybeans are largely left
to fend for themselves. This may have made
sense with the value of the corn crop being
much greater than the soybeans. But as soybeans
have increased in value producers are beginning
to “grow” soybeans. This intensified
management effort has lead to increased yield
potential for Missouri soybean crops. In the past
15 years the average yield goal requested by Missouri
soybean producers on soil tests has increased
from 35 to 50 bushels per acre. For
corn, in the same time period, yield goals have
increased from 190 to 210 bu/a. That’s a 40
percent increase for soybeans compared to a 10
percent increase for corn.
The increasing yield of a soybean crop has implications
for soil fertility. The old practice of
feeding the corn crop and letting the soybeans
get by on the left over’s needs to be reevaluated.
While soybeans do not require nitrogen fertilizers
they are heavy feeders of phosphate &
potash. Soybeans will remove a pound of phosphate
and a pound and a half of potash per
bushel of grain. Corn by contrast removes about
1/4 to 1/3 pound of each nutrient per bushel. In
days past, the much higher yields for corn lead
to greater removal of P & K per bushel. But as
average soybean yields have increased; now the
soybean crop removes about the same amount
of P and a much greater amount of K (Table 1).
Now consider an intensively managed soybean
crop with a 75 bu yield, that’s 75 lbs of P and
112 lbs of K going out with the crop. The cost to
replace these nutrients is over $100!
Another important difference between corn
and soybeans are their response to P & K fertilizers.
I complied all of the yield data for corn and
soybeans for P & K trials from my time here at
the Delta Center. The yields for each trial was
expressed as a relative yield with 100 percent
being the yield obtained when the soil test recommended
amount of P & K was applied. This
allowed me to compare years where weather
conditions affected 5yield potentials. With no
applied P & K fertilizer I obtained 85 percent of
the corn and 70 percent of the soybean maximum
yields. When I applied 2/3 of the recommended
rate of P & K the yields increased to 95
percent for corn and 90 percent for soybeans.
From this data it’s clear that the penalty for
being low in P & K is greater for soybeans than
corn.
Corn and soybeans also differ in response yield
to lime. Both crops have the same preferred soil
pH, 6.0-6.5. When I did the same type of comparison
for lime and soil pH as I did for P & K I
found that for the same pH below 6.0 more yield
was lost for soybean than for corn (Table 2). Also
there was soybean yield penalty for at high pH.
This is due to decreases micro-nutrient advisability,
particularly Manganese at pH above 7.0.
Soybeans are different than corn. By recognizing
the key differences in fertilizer needs and
crop removal, along with soil pH preferences
producers can maximize their profits in a
corn/soybean rotation. Δ
DAVID DUNN: Supervisor, Soils Testing Lab,
University of Missouri