A farmer who rotates rice and soybeans asks, "If I
lime my soil for soybeans, will it hurt next year's rice
crop?"
Not necessarily, according to Nathan Slaton, an agronomist
for the Cooperative Extension Service, University of Arkansas.
Slaton is studying the effect of lime on soybeans and
rice in rotation at the U of A Rice Research and Extension
Center, Stuttgart. The project is financed by grower checkoff
funds through the Arkansas Soybean Promotion Board.
Slaton notes that soybeans like a neutral pH, which
can cause nutritional problems for rice. The Stuttgart
study site has a low soil pH, and both the soybeans and
rice are watered with high quality reservoir water. "When
farmers pump groundwater, they may be pumping calcium
bicarbonate, which becomes lime.
"The low pH here at the Rice Research and Extension
Center contributed to low yields in the university soybean
performance trials during the early '90s."
Slaton says there are farmers across the state with
the same problem -- soil with a pH level too low for optimum
soybean production -- but when they're growing beans and
rice in a one-to-one rotation, they fear that applying
lime will reduce their rice yields, not to mention their
income.
"In years like 1998, when you compare the price you
get for rice and soybeans, and then look at what you can
make yield-wise, generally the rice is going to be more
profitable," says Slaton.
But is lime always bad for rice? Not according to the
study, says Slaton.
"In the fall of 1995, we applied four rates of lime
-- zero, 1 ton, 2 tons and 4 tons per acre -- on soil
with a very low pH of 4.9 to 5.0. With each rate, we applied
two rates of phosphorus and zinc. We were interested in
phosphorus and zinc because their availability is influenced
by soil pH."
Rice and soybeans were planted in alternating strips,
with four replications of each lime rate across the plots.
Two soybean varieties, Holladay and Delsoy 5500, and four
rice varieties were used in the study.
"In 1996, averaging the two soybean varieties together,
we saw a yield of 39 bushels per acre where no lime was
applied," says Slaton. "The average yield was 44 bushels
per acre where 1 ton of lime was applied, 48 bushels where
2 tons were used and 48 bushels where 4 tons of lime were
applied.
"So we saw a 9-bushel increase where 2 tons or more
lime was applied."
He adds, "There was also a trend toward higher rice
yields when lime was applied."
In the 1996 study, the rice yield was 107 bushels per
acre where no lime was applied, 120 bushels where 1 ton
of lime was applied, 118 bushels where 2 tons were used
and 116 bushels per acre where 4 tons of lime were applied.
"That's not at all what we expected," says Slaton.
"We expected the 4-ton rate of lime to hammer the rice."
In 1997, the average soybean yield was 55 bushels per
acre where no lime was used, 57 bushels where 1 ton was
applied, 59 bushels where 2 tons were used and 60 bushels
per acre where 4 tons of lime were applied.
Rice yields in the second year of the study were 148
bushels per acre where no lime was applied, 150 bushels
with the 1-ton rate, 146 bushels where 2 tons of lime
were used and 154 bushels per acre where 4 tons were applied.
"While the trend toward higher soybean yields continued,
statistically there wasn't a significant difference in
rice yields in the second year," says Slaton. "The overall
increase in yields from the first to the second year was
because of management practices. The field was planted
late in 1996."
In 1997, a year after lime was applied, the soil with
an original pH of 4.9 to 5.0 had a pH of 5.1 where no
lime was applied, 5.5 where 1 ton was used, 6.5 where
2 tons were used, and 7.1 where 4 tons of lime were applied.
"The fact that the soil pH was only 7.1 where 4 tons
of lime were applied helps explain why that rate of lime
didn't hurt rice yields," says Slaton. "Where we see problems
in growers' rice fields is when the soil pH is 7.5 to
8."
Slaton says that while rice yields didn't decline in
the lime study, phosphorus and zinc levels in rice tissue
did decrease as the lime rate and pH increased. "Overliming,
getting the pH too high, can cause phosphorus and zinc
deficiencies in rice.
"When we looked at soybean tissue, we found that as
pH increased, the phosphorus concentration increased and
the zinc concentration decreased. Some soybeans varieties
may be more susceptible to zinc deficiency than others."
Soybeans Today January 1999
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