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Topics: Density, Nitrogen, Fertilizer Pages: 2 (1687 words) Published: September 22, 2014
January 2011

Math Anxiety: Fertilizer Calculations

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OU MAY KNOW people who suffer from math anxiety.
They avoid situations where mathematics and
calculations are required. However, avoiding math
is simply not an option when working with agriculture.
The International Plant Nutrition Institute (IPNI)...
in cooperation with university specialists...has recently
published a “how-to” workbook that presents commonly
used mathematical concepts in agriculture. It begins with
simple arithmetic and advances all the way to complex
modeling. More details are available at the end of this
newsletter.
Most of us do not use sophisticated math on a regular
basis, but a review of commonly performed calculations will
be the subject of this and future INSIGHTS. We’ll start with some of common calculations that are made when dealing
with fertilizers.

Fertilizer Notation
Commercial fertilizers are required to show on their label
the minimum percentage of nutrients that the manufacturer
guarantees to be present. The chemical analysis is composed
of at least three numbers separated by dashes. The first
number indicates the percent N, the second number
indicates the percent P as P2O5, and the third number shows
the percent K as K2O based on weight.
The nutrient content of the fertilizer is indicated
by these three numbers, but the tradition of using the
oxide form of P and K can be a bit confusing and is set in
fertilizer law. From the percent N value on the label, it
is not obvious if the N is present as nitrate, ammonium,
or urea. Similarly, the P in most commercial fertilizers is
chemically present as phosphate (PO4), but this number is
mathematically converted to P2O5 equivalents for display on
the fertilizer label. Potassium fertilizers are never present as K2O, but the K present in the fertilizer is mathematically Dr. Robert Mikkelsen
Western North America Director
International Plant Nutrition
Institute (IPNI)
4125 Sattui Court...
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