209 / 363
AOAC Official Method 2015.15
Nitrogen, Phosphorus and Potassium Release Patterns
of Slow- and Controlled-Release Fertilizers
First Action 2015
A. Scope
This method is applicable for the determination of N, P and K in slow-release and controlled-
release fertilizers.
B. Principle
In Alternative A, a representative unground test portion is exposed to ambient temperature
extractions of a solvent in a biologically active sandy soil medium. In Alternative B,
a
representative unground test portion is exposed to increasingly aggressive solvent extractions.
Extractions are designed to extract and isolate nutrients becoming available over time.
Each extract is analyzed by AOAC procedures for the nutrient of interest (total N, P and K).
Along with analysis of total nutrients and reference materials, data are used to develop
information specific to the cumulative percentage of nutrient released over time.
Alternative A: 180-Day Extraction at Ambient Temperature
C. Apparatus
(a) Extraction columns (incubation lysimeters;
see
Figure
2015.15A
) are constructed of PVC pipe
(30 × 7.5 cm) fittedwith a fiberglass mat in the bottom held in place by a 7.5 id PVC cap. The cap
is fitted with a barbed plastic fitting, and vacuum tubing attached for leachate collection. A PVC
cap is used on the top with no hole, but with a coating of stopcock grease to cap the lysimeter;
all columns are supported on wood frame.
(b) A 50 mL beaker placed in the headspace of each incubation lysimeter.
(c) Filtering flasks with a one-hole stopper is placed beneath the leaching columns and
attached to the vacuum tubing. A pinch clamp is used to prevent leaks when filtration and
leachate collection is complete.
(d) Vacuum manifold and tubing connecting each flask to a
standard laboratory vacuum pump.
(e) Riffle, gated or rotary.
D. Reagents and Reference Materials
Candidates for 2016 Method of the Year
208