Better Farming - November 2016

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44 FarmNews First > Better Farming November 2016
offer no value in neutralizing acidity.
Therefore, coarse lime is discounted
in the AI calculation. Any particle
greater than a #10 sieve has zero
neutralizing power. The above table
illustrates the calculations to deter-
mine the fineness rating.
The Agricultural Index is deter-
mined by multiplying the neutraliz-
ing value by the fineness rating.
Assuming a lab determination of 99
per cent neutralizing value multiplied
by the fineness rating of 66 from the
table above results in an AI of 65. (99
x 66/100)
To calculate the rate adjustment
for limestone with differing AI, use
the following equation:
Rate of application of limestone
= limestone application rate from
soil text x (75 ÷ Agricultural Index
of limestone)
The adjustment that needs to be
made to the rate from the previous
example is calculated as follows: 5 x
(75 ÷ 65) = 5.8 MT/ha. An applica-
tion of 5.8 MT of AI 65 limestone is
necessary to have the same neutraliz-
ing power as 5 MT of AI 75 lime. This
adjustment assures we are applying a
sufficient quantity of limestone to
make the necessary adjustment to the
soil pH.
This adjustment is more important
with limestone with a lower AI. If the
limestone being used only has an AI
of 45, the adjustment becomes 5 x (75
÷ 45) = 8.3 MT/ha. This rate change
is required for proper pH adjustment
but the higher application rate
needed adds to the application cost.
Choose limestone sources careful-
ly, obtain a lab analysis of the Agri-
cultural Index and be aware of
necessary rate changes if the AI is
different than 75.
It is also worth noting that if the
AI was 85, the rate adjustment would
be 5 x (75 ÷ 85) = 4.4 MT/ha. Rate
adjustments can lead to either
increased or decreased application
amounts.
I have heard it said many times by
farmers and industry personnel that
limestone is limestone. I trust you
now see there is more involved in
determining the proper application of
limestone. The goal is to adjust soil
pH when the soil test indicates it is
necessary using the correct type of
limestone and the right application
rate. BF
Dale Cowan is a senior agronomist with
AGRIS Co-operative Ltd. and Wanstead
Farmers Co-operative Ltd.
CROPS: YIELDMAT TER$
Particle Size % of Sample Fineness Factor
Coarser than No.10 sieve1 10 x 0 = 0
No.10 to No. 60 sieve2 40 x 0.4 = 16
Passing through No. 60 sieve 50 x 1.0 = 50
Fitness Rating = 66
Example Calculation of Fineness Rating of a Limestone
1 A #10 Tyler sieve has wires spaced 2.0 mm apart.
2A #60 Tyler sieve has wires spaced 0.25 mm apart.
Source: Ontario Ministry of Agriculture, Food and Rural Affairs. Agronomy Guide for Field Crops – Publication 811.
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