Can You Afford To Cut Back On Fertility?
CARBONDALE, ILL.
Cost of production for
2009 has been the main
topic of discussion with
farmers these days. After
doing a wheat production
meeting a few weeks ago, I can
see why. Every scenario that I
tried for production costs
showed that 70-bushel-peracre
wheat would lose money. Adding double
crop soybeans helped; but at the prices for next
year, it was still close. So the question everyone
is asking is this: “Can I cut back on fertility?”
For wheat, there is a good chance of a yield
loss unless fertility levels are high – especially
phosphorus, which should be a P1 test of 60 or
better. But, what about corn and soybeans?
Looking at the basics of fertility, it takes 19.6
pounds of DAP to change the soil test for phosphorus
1 point. Then for 140-bushel corn, not
fertilizing would remove about 130 pounds of
DAP or would reduce your soil test by 6.7 points.
Not fertilizing with potash for 140-bushel corn
(takes 6.7 pounds of 0-0-60 per point) removes
65 pounds of 0-0-60 or would reduce your soil
test by 9.7 points. Soybeans would be a little different;
40-bushel soybeans would reduce your
soil test by 3.3 points for phosphorus and 13
points for potassium.
The big question is whether it would hurt
yields. University of Illinois Extension recommendations
for southern Illinois soils state that
to get 95 percent of your yield, the soil test for
phosphorus should be 28 to 35 pounds per acre
and that potassium should be around 230
pounds per acre. But, this is assuming averages.
Soil tests are not that accurate; fields are not
that uniform; cation exchange capacity varies;
and soils act differently with different tillage, organic
matter levels and rotations. That is why
we normally recommend higher soil tests.
So, do you have a reasonably current representative
soil test? Can you afford to pull that
soil test down? Extension has always said that
good soil tests are like money in the bank; is it
time for a withdrawal? Δ
Mike Plumer is Extension Educator, Natural Resources
Management, with the University of Illinois
at the Carbondale Extension Center.