AOAC ERP Fertilizers - December 2017

354 T 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:

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. (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. C. Reagents and Reference Materials

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

Figure 2015.15A. Incubation lysimeters.