AOAC RI ERP EBOOK FOR FERTILIZERS

Table-3 Urease Analyses of Twelve Fertilizer Samples & Comparisons with the HPLC Results for Urea % Urea-N %Urea-N % Urea-N

HPLC Method AOAC 2003.14 ( Means from two Labs)

Difference Urease-HPLC ( Mean )

Fertilizer Sample

Urease Method AOAC 959.03

Total-N

1 2 3 4 5 6 7 8 9

29.89 28.54 28.44 28.33 28.47 24.95 30.78 30.06 28.01 28.13 26.36 29.87

+1.02 +1.30 +1.44 +0.88 +1.32 -0.45 +2.19 +1.77 -2.48 +2.10 +0.77 +2.42

13.26

12.25

9.87

8.57 8.63 8.45 8.59

10.06

9.33 9.91

15.44 14.81 14.08

15.89 12.63 12.32 11.37

8.89 8.94

10 11 12

6.84

16.18 11.86

15.42

9.44

In another series of studies supplies of seven different analytical quality urease enzyme sources were obtained from five well-known suppliers of laboratory chemicals which are identified as numbers 1 through 7. It is highly probable that one or more of these common sources are found in a large number of fertilizer analytical laboratories. Represented are four brands of urease powder, one brand of urease tablets and two brands of urease-glycerol extract solution. Only one supplier provided the information regarding the urease activity as part of its product label. Tests using the urease enzymes in this study were carried out in accordance with the AOAC Official Method 959.03, with one exception where anaccommodation for the use of urease tablets or glycerol extract was required. The AOAC urease method specifies the use of a 1% aqueous solution of urease powder or Jack bean meal. According to the AOAC urease method, 20ml of 1% solution of urease should be used and also the urease activity should be such that it will hydrolyze at least 0.1 gram of urea under the conditions specified in the method. The weight of fertilizer sample should also be such that the amount of urea in the final working volume does not exceed 0.1 gram. In one series of tests, pure urea (99.4%) was used. The weights of urea samples used were 0.1g, 0.3g, 0.6 g, and 1.0 gram. These urea samples were analyzed using seven ureasematerials from different sources. The degrees of urea hydrolysis with the seven different urease materials are shown in Table 4.