AOAC RI ERP EBOOK FOR FERTILIZERS

Table-4 Analyses of Urea using Different Sources of Urease

% Hydrolysis of urea from the amount used

Urease Source

Urease Amount

0.1 g

0.3 g

0.6 g

1.0 g

100.1

99.7 99.7 99.7 97.6 83.2 69.3 99.7 99.9

99.9 69.9 99.4 62.7 52.6 45.5 99.5 99.6

100.1

1 2 2 3 4 5 6 7

20 ml (1%Sol.)

99.9 99.9 99.6 99.3 100 99.9

99.7 99.7 99.7 99.7 98.6 99.9

10 ml Glycerol Ext. 20 ml Glycerol Ext.

20 ml (1%Sol.) 20 ml (1%Sol.)

2 tablets

20 ml Glycerol Ext.

100.3

100.1

20 ml (1%Sol.)

In the amount used, all seven urease sourceshydrolyzed up to about 0.3 grams, thus exceeding the AOAC method requirements. However the urease tablets (source No. 5) begins to fade in effectiveness at 0.3 grams urea, with only 98.6% hydrolyzed, and two urease powders (No. 3and No. 4) begin to fail at about 0.6 grams of urea. The two other powder samples (No. 1 and No. 7) and the two urease glycerol extracts (No. 2 and No. 6) in 20 ml increments all hydrolyzed up to 1.0 gram of urea. Powder No.7 was known from the manufacturer’s label to have a very high urease activity. From the results, it appears that sources 1, 2, and 6 were high activity urease samples. These data show that the AOAC Official Method 959.03 works well for pure urea when applied within the limits of the method. However, if the amount of urea exceeds the recommended amount in the method, there are inconsistencies with the results and some urease sources work better than others. The inconsistencies with the Urease Method are obvious when the method applies to the more complex fertilizer samples, namely, the Urea-Triazone liquid solution containing mixed compositions of urea- forms and triazone moieties (Table 2).

More tests were done to further illustrate these inconsistencies even further.

In the following sets of experiments, five Urea-Triazone fertilizers from four different manufacturers were analyzed for their claim of %SRN. The percentage of free and unreacted urea in these samples was analyzed by the HPLC and by the urease methodusing powdered urease enzyme from Jack bean meal. The results are shown in Table 5.