AOAC RI ERP EBOOK FOR FERTILIZERS

Table 5 Comparisons of the Results by HPLC (2003-14) and by Urease Method(Powdered Sample)

SRN Determination Method

Product % Total N

%SRN Claimed

HPLC (2003-14)

Urease

28-0-0 30-0-0 30-0-0 28-0-0 30-0-0

72 60 50 75 85

72 60 50 50 46

88 77 68 70 80

The results by the urease method for the SRN contents (i.e., Total N minus Free Urea-N) of these known fertilizers were always different from the claimed amount, while the results by HPLC were on target for three of five samples and off for two samples. The following results further illustrate the inconsistencies of the Urease Method when applied to this class of fertilizers 7 . Comparisons of the urea results obtained with the different urease sources listed in Table 4 with the HPLC Method 2003.14 are shown in Table6. The results are for four different aqueous Urea-Triazone fertilizers and are shown in three ways, (1) as the absolute weight of urea found in each sample volume by both methods, (2) as the weight percent urea in the fertilizers, and (3) as the percent recovery of urea by the Urease Method relative to 100% recovery by the HPLC Method. These results clearly show that the Urease Method using urease sources 1, 3 and 4 finds less urea in all four fertilizer samples in comparisons with the results from the HPLC Method. The differences are substantial. For example, sometimes less than half as much urea is recovered by the Urease Method. For the source No. 2, it makes a significant difference whether 10 ml of Glycerol extract or 20 ml of glycerol extract was used. Source No. 5 (Urease tablets) showed the poorest recovery results. All of these urease sources should normally be expected to totally hydrolyze the urea in these fertilizers. However, four of these Urease sources (Nos. 1, 3, 4, and 5) plus 10 ml of the source No.2 Glycerol extract significantly failed to hydrolyze all of the urea in any of these four fertilizer samples.