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hiex
:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
o
.
2, 2016
release rates. Ruggedness testing was also performed using a
fractional multifactorial design. Fertilizer materials used for the
optimization experiments were polymer-coated urea, three types
of ureaform, and two types of polymer-coated NPK. Fertilizer
materials used for the ruggedness testing were polymer-coated
urea, polymer-sulfur-coated urea, polyolefin-coated NPK,
reactive layer-coated urea, and isobutylidenediurea,
Optimization experiments indicated that temperature was the
only factor found to substantially influence nutrient-release rates
from the materials studied. The optimal extraction temperature
sequence that produced the most consistent and highly
correlated N, P, and K release rates and showed no abnormal
nutrient release due to coating deformation or fertilizer caking
was determined to be:
Extraction 1
.—2 h at 25°C
Extraction 2
.—2 h at 50°C
Extraction 3
.—20 h at 55°C
Extraction 4
.—50 h at 60°C
Overall, the optimized method proved to be rugged for
measuring N release rates of CRFs. The release of P and K were
also studied, but at fewer replications than for N.
Method Summary
With the 180 day soil method, a fertilizer test portion is exposed
to ambient temperature extractions with 0.01% citric acid in a
biologically active sandy soil medium. Extractions are designed to
extract and isolate nutrients becoming available over time (e.g., 7,
14, 28, 56, 140, and 180 days). Each extract is analyzed using an
appropriate AOAC method (or comparable validated method) for
the nutrient of interest. Cumulative nutrient released over time is
calculated and release plots are graphed.
The alternative 74 h accelerated method provides an
estimate of the 180 day method cumulative nutrient-release
and nutrient-release plot within a time frame amenable to
laboratory testing for manufacturing process control and
regulatory testing to verify manufacturer label claims.
Extractions are made with 0.2% citric acid and temperatures
increased in a step-wise manner to accelerate the release of
nutrients: 2 h at 25°C, 2 h at 50°C, 20 h at 55°C, 50 h at 60°C,
and if needed, 94 h at 60°C.
AOAC Official Method 2015.15
Nitrogen, Phosphorus, and Potassium
Release Rates of Slow- and Controlled-
Release Fertilizers
First Action 2015
(Applicable for the determination of extractable N, P (as
P
2
O
5
), and K (as K
2
O) and cumulative N, P or K release in
slow release fertilizers (SRFs) and controlled release fertilizers
(CRFs).)
A. 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 temperature extractions. Extractions are
designed to extract and isolate nutrients becoming available
over time. Each extract is analyzed byAOAC procedures for the
nutrient of interest (total N, P, and K). Along with analyses 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
B. Apparatus
(a)
Extraction columns
.—Extraction columns (incubation
lysimeters;
see
Figure
2015.15A
) are constructed of PVC
pipe (30 × 7.5 cm) fitted with a fiberglass mat in the bottom
held in place by a 7.5 in. 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 a wood frame.
(b)
Beaker
.—A 50 mL beaker is placed in the headspace of
each incubation lysimeter.
(c)
Filtering flasks
.—Filtering flasks with a one-hole stopper
are 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
.—Vacuum manifold and tubing
connecting each flask to a standard laboratory vacuum pump.
(e)
Riffle.—
gated or rotary.
C. Reagents and Reference Materials
(a)
Extraction solution
.—0.01% (w/v) citric acid [2 g/20 L
deionized water (DI)] prepared from reagent-grade citric acid.
(b)
Ammonia trap solution
.—0.2 M H
2
SO
4
solution.
(c)
Loamy, siliceous, hyperthermic, Grossarenic Paleudult
soil
.—Arredondo fine sand. Particle size analysis is shown in
Table
2015.15A
.
(d)
Uncoated quartz sand United States Golf Association
Greens
(
USGA Mix)
.—Topdress sand (noncoated quartz), 20/30
silica sand. Available from Edgar Minerals Inc. (Edgar, FL) and
Standard Sand and Silica Co. (Lynne, FL). Particle size analysis
is shown in Table
2015.15B
.
(e)
Soil media
.—Mixture of 1710 g uncoated quartz sand,
C(d)
, and 90 g loamy siliceous, hyperthermic, Grossarenic
Paleudult soil,
C(c)
, or similar type of local soil acting as a
microbial inoculum.
Figure 2015.15A. Incubation lysimeters.