AOAC ERP Fertilizers - December 2017

where A can be N, P, or K. 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 as determined in F above, where A can be N, P, or K. %AR (ex x ) = % Nutrient released during extraction x where A can be N, P, or K; 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.)

mixture. Note : Quick-release N must be limited to 600 mg N/test portion to prevent ammonia buildup in the column (thus preventing an active biological system); however, when doing so, replicates must be used to cumulatively measure at least 3.0 g total test portion mass and averaged to generate a single result. If soluble N is not limiting, 5–6 g unground fertilizer should be used for the test portion. E. Procedure Test portions from each material to be tested are placed in incubation columns held at room temperature (20–25°C). The column preparation sequence is as follows: fiberglass mat, 100 g sand, then a mixture of remaining sand, soil, and test portion followed by placement of an acid trap. The sand–soil–test portion mixture is brought to 10% gravimetric moisture by adding 180 mL 0.01% citric acid. A 50 mL beaker containing 20 mL 0.2 M H 2 SO 4 is placed in the headspace of the column as an ammonia trap. The solution in the ammonia trap is replaced and analyzed for NH 4 -N by titration every 7 days. After 7, 14, 28, 56, 84, 112, 140, and 180 days, each column is leached at the same time of day with one pore volume (500 mL) 0.01% citric acid using a vacuum manifold. Vacuum is pulled for 2 min at 20–25″ Hg vacuum (1.3 cfm) to ensure all free extraction solution is removed. Mix well and transfer to a 250 mL graduated cylinder. Record the leachate volume and remove aliquots to test for total N. In addition, measure the pH and electrical conductivity of the leachate. Retain the remaining leachate in reserve in case an additional or recheck analysis is required. Store in dark bottles and freeze if retained for more than 7 days. ( Note : If no volatile N is detected in the ammonia trap during the first two sampling periods, the NH 4 trap can be removed and analysis for volatile N discontinued.) F. Analytical Determinations ( a ) Determine total N in each of the extracts obtained using 993.13 (combustion), or 978.02 (modified comprehensive), or an equivalent applicable method validated in your laboratory. Use an applicable method-matched reference material in each run. Use at least three standards appropriate for the range of extract concentrations. Typically a combination of 10, 100, 1000, and 10 000 mg N/L cover the range of N in the extracts. ( b ) Determine total phosphate (as P 2 O 5 ) using AOAC 962.02 (gravimetric quinolinium) or 978.01 (automated spectrophotometric) or an equivalent applicable method validated in your laboratory. Use an applicable method-matched reference material in each run. Use internal reference standards appropriate for the range of the sample extracts; typically 10, 100, 1000, and 10000 mg P 2 O 5 /L will cover the full range of P 2 O 5 concentrations in the extracts. ( c ) Determine soluble potash (as K 2 O) using 958.02 (sodium tetraphenylboron method; STPB) or 983.02 (flame photometry) or an equivalent applicable method validated in your laboratory. Use an applicable method-matched reference material in each run of samples. Use internal reference standards appropriate for the range of the sample extracts; typically 10, 100, 1000, and 10000 mg K 2 O/L will cover the full range of K 2 O concentrations. Nomenclature for extraction calculation equations.— Time is measured in days and is expressed in the extract identifications as days; e.g., ex7 is the extract removed on the 7th day of incubation. A (t) = % Total nutrient/analyte

mg L

1 g 1000 mg

×

×

AC , ( ) x ex

V, mL 1000 mL

=

×

%AR

100

x ( ) ex

g 100 g

×

W, g A ,

t ()

G. Expression of Results Results for each extraction are presented as cumulative percentage of total nutrient. Extraction 1 (7 days) 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 1. 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 . (Example calculations are provided in ref. 1.) . Alternative B: Accelerated 74 h Extraction at 25–60°C H. Apparatus ( a ) Covered water bath capable of maintaining a temperature of up to 60°C for extended periods. Ensure the mean temperature in the system is 50.0, 55.0, or 60.0 ± 1.0°C by monitoring incoming and exit temperatures to the manifold at comparable locations. Before Extractions 2–4 begin, it is necessary to preheat the bath several degrees ( see Extraction section below) above the desired temperature to account for initial heat exchange and temperature equilibration with manifold and columns. The bath should be stabilized at the desired temperature within 10 min. ( b ) Reversible peristaltic pump capable of delivering 4.0 (±0.1) mL/min continuously for 54 h. Pump heads capable of using

Figure 2015.15B. Example release plot showing % N released over 180 days.

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