156 / 172
Table 5
Comparisons of the Results by HPLC (2003-14) and by Urease
Method(Powdered Sample)
Product %
Total N
%SRN
Claimed
SRN Determination
Method
HPLC (2003-14)
Urease
28-0-0
72
72
88
30-0-0
60
60
77
30-0-0
50
50
68
28-0-0
75
50
70
30-0-0
85
46
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.