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Currently, there are three AOAC Official Methods available for the determination of the free and

unreacted urea in liquid and water-soluble urea containing fertilizers.

The AOAC Official Method 959.03 is the most well-known and the oldest method for urea determination

in fertilizers. This method, which was introduced in 1959, is based on the quantitative hydrolysis of urea

to ammonia by means of urease enzyme. The amount of ammonia generated by hydrolysis is

subsequently determined and reported as equivalent urea or urea N by a simple titration. With most

simple and conventional (i.e. non-SRN-containing) liquid fertilizers, the urease method works very well.

Urea-based methods are well accepted and usually quite suitable for urea determinations in most urea-

containing fertilizers, however, they have been found to demonstrate significant problems, particularly

with incomplete recovery of urea, when applied to such determinations in Urea-Formaldehyde (UF)

fertilizer solutions. Comparisons of the HPLC methods, particularly the AOAC Official Method 2003.14

with results from the urease method, frequently produced results that were not in agreement with each

other. Further study also showed that urea results from the urease methods often were not

reproducible. Thus a controlled study, which is the principle subject of this article, was carried out to

confirm and elaborate upon these observations, and to suggest some likely reasons for the reduced

effectiveness of the urease method with UF fertilizers.

Experimental

Materials

Twelve of the most commonly available commercial liquid urea-based fertilizers were selected for the

determination of their free urea contents using the three aforementioned methods. These commercial

samples were:

1.

LF3060 (30-0-0), Koch Agronomic Services, LLC, 4111 E. 37

th

St. N, Wichita, KS 67220, USA

2.

CoRoN (28-0-0), Helena Chemical Company, 225 Schilling Blvd., Collierville, TN 38017

3.

Arclin 28-0-0, Arclin Resins, Research & Technology, 4754 28

th

St., Springfield, OR 97477

4.

Greenfeed 27-0-0 (light Green liquid), Plant Food Company, 38 Hightstown-Cranbury Station

Road, Cranbury, NJ 08512

5.

Greenfeed 27-0-0 (Dark Green liquid), Plant Food Company, 38 Hightstown-Cranbury Station

Road, Cranbury, NJ 08512

6.

NDemand 30-0-0, Wilbur-Ellis Agribusiness, 3300 S. Parker Road, Suite 500, Aurora, CO 80014,

USA

7.

NDemand 30L, Wilbur-Ellis Agribusiness, 3300 S. Parker Road, Suite 500, Aurora, CO 80014, USA

8.

Gradual N (30-0-0), Winfield Solutions, LLC, 1080 County Road West, Shoreview, MN 55126

9.

N-28 clear Fertilizer (28-0-0), Advachem, Route de Wallonie, Darse d’Hautrage, BE 7334

Hautrage

10.

N-Sure (28-0-0), Tessenderlo Kerley, Inc., 2255 N. 44

th

St., Suite 300, Phoenix, AZ

11.

Trisert NB (26-0-0), Tessenderlo Kerley, Inc., 2255 N. 44

th

St., Suite 300, Phoenix, AZ

12.

Formolene Plus (30-0-0), Tessenderlo Kerley, Inc., 2255 N. 44

th

St., Suite 300, Phoenix, AZ