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Fertilizer recommendation theory
doesn 9 t work
DETROIT, Mich. - Much
of the cost of growing food
crops comes from altering
the soil m order to improve
its productivity. Amounts of
lime, fertilizer or waste
products needed to do the job
are usually determined by a
soil test.
The recommendations
made by many soil testing
laboratories today are based
on the idea that a specific
ratio of calcium, magnesium
and potassium is required
for maximum soil produc
tivity.
Delaware soil specialist
William C. Liebhardt
challenged this concept
during a recent national
meeting of agricultural
Delaware researcher says Uncaster Farming, Saturday, December 6, 1980-D27
scientists in Detroit,
Michigan.
Reporting to members of
the American Society of
Agronomy, Crop Science
Society of American and Soil
Science Society of America,
he said that, based on a 10-
year study at the University
of Delaware Agricultural
Experiment Station, the
theory doesn’t hold up in
actual practice - at least not
on poorly buffered, sandy
coastal plain soils.
Farmers following
recommendations based on
this theory may add more
calcium, magnesium and
potassium than is needed for
maximum production, he
said These extra nutrients
on light soils
sometimes have no
significant effect on crop
yield, but in other cases they
actually reduce yields.
In either instance, the
farmer is spending more
money than he needs to on
lime or fertilizer. With a
smaller harvest, his profit is
down even further and
there’s less food for the
consumer.
Specifically, Liebhardt
challenged the idea of trying
to achieve a basic cation
saturation ration (BCSR)
between nutrients in soil for
crop production as presently
used. (Potassium, calcium
and magnesium are all
cations.)
The concept, widely used
today, is based largely on
greenhouse soil studies on
alfalfa by soil scientists in
New Jersey during the
1940’5. They proposed that
the ideal sod have 65 percent
calcium saturation, 10
percent magnesium
saturation and five percent
potassium saturation.
In the late 30’s a Mid
western agronomist com
bined this data with in
formation from a Dutch
research report and used it
to generate figures for what
he considered to be the
optimum ratios between
these three basic plant
nutrients. His BCSR
philosophy calls for
saturations of 63 to 35 per
cent calcium, 6 to 12 percent
magnesium, and 2 to 5
percent potassium.
Commercial laboratories
adopted this philosophy,
with some using 75 percent
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calcium saturation, 10 to 13
percent magnesium
saturation, and 2 to 5 percent
potassium saturation.
Since the concept hasn’t
been questioned in 30 years,
Liebhardt assumes it’s
worked on the highly buf
fered soils of the Midwest
which can tolerate a pH of
6.5 or 6.7. It may not have
caused yield decreases, he
said, but it probably has
resulted in the addition of
unnecessary nutrients just to
balance up these ratios.
On the poorly buffered,
low organic, sandy soils of
the Atlantic Coastal Plain,
these saturation levels result
in excessive soil pH and
drastically reduce yields. In
its present form, Liebhardt
said, the theory simply
doesn’t work on these soils.
The soil scientist based his
comments on results of a 10-
year study at the university
Substation farm near
Georgetown, Delaware. Soil
in the research field was a
typical, sandy coastal soil
with a low cation exchange
and very little organic
matter.
Plots in the test received
one and four tons of calcitic
lime and dolomitic lime, and
a one to one mixture of the
two, twice during the study
period - in 1970 and again m
1973. No lime has been added
smce then.
Potassium was also added
at the rates of zero, 60, and
120 pounds per acre an
nually.
The calcitic lime had a
calcium-magnesium ratio of
18 to 1. The dolomitic lime
had a one to one ratio which
resulted m wide variations in
soil calcium and
magnesium. The BCSR
concept calls for a ratio of
about six to one and a
precise saturation level for
each of these elements.
“Trying to achieve this
ratio and saturation level on
our sandy sods results in
overtiming and reduced
yields,” said Liebhardt. “If
we go beyond a pH of 6 we
run into a manganese
deficiency and yields drop.
The highest corn yield in the
study was 195 bushels at a
pH of 5.6. At a pH of 6.4, yield
declined between 20 and 35
bushels an acre. (All plots in
the study were irrigated.)
“When we looked at the
correlation between
manganese concentrations
m earl leaves at silking and
various other plant and soil
factors,” he continued, “we
found that it really boils
down to one thing.
Manganese deficiency is a
soil-related factor. When the
pH is too high, the
manganese is tied up and the
com plant can’t use it.”
When soybeans were
grown on the same plot,
there was no significant
difference m yield between
treatments, even though pH
ranged all the way from 5.3
to 6.4. However, there was a
trend toward lower yields as
pH increased. Calcium and
magnesium saturation
levels in the soil also varied
widely but had no statistical
effect on yield.
These research results
were supported by findings
on commercially grown corn
around the state.
Last summer Liebhardt
compared results of soil
tests and leaf tissue analysis
from fields of farmers
participating m a statewide
extension irrigation
program.
The theory calls for
potassium cation saturation
levels between 2 to 5 percent.
Eighty-eight percent of soils
tested m the program had
potassium cation levels well
above this.
Most Delaware fields also
had much higher
magnesium saturation
levels than the 6 to 12 percent
called for. Ninety-nine
percent of the growers’ soils
were above the 6 to 12 per
cent soil magnesium con
sidered optimal by the BCSR
concept. Levels of this
nutrient ranged anywhere
from seven percent to
greater than 30 percent, with
all the low readings coming
from one farm.
(Turn to Page D 27)