Better Farming - November 2016

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40 FarmNews First > Better Farming November 2016
Limestone application basics
Learn about the type and rate of limestone application to correctly adjust the pH of your soils.
by DALE COWAN
The fall season is a great time to
soil sample after the completion
of harvest. It allows ample time
for turnaround of soil reports from the
laboratories.
After receiving the sample report,
one of the first results to look at is soil
pH. This single measurement of your
soil indicates whether your soil is
acidic, neutral or basic. The pH scale
is read in the lab on an instrument
with a logarithmic scale that ranges
from 0 to 14. A pH below 7 indicates
increasing acidity, a pH of 7 is neutral
and a pH above 7 indicates increasing
alkalinity.
The importance of knowing this
reading is that the pH controls the
solubility or availability of soil
nutrients, as well as the availability of
applied nutrients from manures, bio
solids and commercial fertilizers. It
can also be used to develop nutrient
management strategies on fertilizer
applications and methods of place-
ment.
As soil pH drops below 6.2, the
soil is beginning to become acidic
enough to warrant adjustments for
most field crops. The adjustment to
raise soil pH is accomplished with an
application of agricultural limestone.
The limestone application rate is
determined by another lab reading
called buffer pH (BpH). When the
soil pH drops below 6.2, labs auto-
matically run the BpH test. (Some
labs run the BpH at 6.5.) BpH
measures the amount of acidity that
needs to be neutralized.
The following steps are required to
determine the application rate of
limestone. First, determine the
desired target pH for the crops to be
grown in the rotation. Next, use the
chart shown on the following page to
locate the BpH and the resulting
recommended rate of limestone. As
an example, the target pH of 6.5 and
BpH of 6.3 results in a lime recom-
mendation of 5 MT/ha or 2.2 short
tons per acre.
Once you have determined the
proper application rate, you must
choose the most appropriate type of
lime: calcitic or dolomitic. Calcitic
contains mostly calcium carbonate
and dolomitic may contain up to 13
per cent magnesium carbonate in
addition to calcium carbonate. If the
soil test value for magnesium is below
100 ppm, dolomitic is the ideal
choice. Not only does this lime
application neutralize the soil acidity
but it is also the most economical way
to add the essential nutrient magne-
sium to your soil.
But we are not done making the
final rate decision just yet. We need to
look at the Agricultural Index (AI).
This is a measure of limestone quality.
The chart below assumes you will use
a limestone with an AI of 75. AI is a
factor based on the neutralizing value
and fineness rating of the lime.
The AI is determined by a lab test.
The lab titrates for neutralizing value
expressed as the calcium carbonate
equivalent as a percentage and runs
the sample through a series of sieves
to get a particle size. Coarse particles
CROPS: YIELD MAT TER$
The adjustment to raise soil pH is accomplished
with an application of agricultural limestone.
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