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(b) Determine total phosphate (as P2 O 5 ) using AOAC Method
962.02
or AOAC Method
978.01
(1)
or equivalent, applicable methods validated in your laboratory. Utilize applicable method-
matched reference material in each run. Utilize internal reference standard appropriate for the
range of the sample extracts; typically 100, 1000, and 10000 mg P 2 O5 /L will cover the full range
of P 2 O5 concentrations.
(c) Determine soluble potash (as K 2 O) using AOAC Method
958.02
(STPB) or AOAC Method
983.02
(flame photometry; 1) or equivalent, applicable methods validated in your laboratory.
Utilize applicablemethod-matched referencematerial in each run of samples. Utilize internal
reference standard appropriate for the
range of the sample extracts; typically 100, 1000, and 10
000 mg
.
K2 O/L will cover the full range of K2 O concentrations.
Nomenclature for Extraction Calculation Equations
Time is measured in days and is expressed in the extract identifications as days, e.g., ex 7 is
the extract removed on the 7th day of incubation.
A (t) = % total nutrient/analyte where A is N.
ex x = An extract collected on a specific day (7, 14, 28, 56, 84, 140, or 180 days).
AC (ex x) = Analyte concentration (in mg/L) in extract x determined as in G above; where A is N.
%AR (ex x) = % nutrient released during extraction x where A is N.
V = Volume (in mL) of respective extract collected.
W = Total unground test portion weight in g.
Calculations
(An example calculation is provided in Ref. 1)
%
( )
= �
( ),
1
1000
/1000
, ( , 100 )
� 100
H. Expression of Results
Results for each extraction are presented as cumulative percentage of total nutrient.
Extraction 7 is considered water-soluble and not an SRF. However, slowly available water-
soluble materials (low MW urea formaldehydes and methylene ureas) may be present. These
materials can be analyzed directly from extract 7.
Graphing release plots.--
Plot the cumulative % of analyte (nutrient) released on the y-axis
versus days of extraction on the x-axis as in Figure
2015.15B
. (The example calculations are
provided in ref. 1.)
Candidates for 2016 Method of the Year
210