The hydrolysis of urea to ammonia is an important pathway of conversion of organic nitrogen to inorganic nitrogen in solls receiving applications of urea 9 and excretions of urine from stock 6. Conrad 8 4 hypSthesised that urea hydrolysis could be catalysed by extracelhflar urease adsorbed in soils, a process quite independent of the catalytic action initiated by viable urease-containing soil micro-organisms. Since then the activity of this enzyme in soils has been examined both as a separate enzyme system 2 14 15 23 and as orte of a number of enzymes in broader studies of the total enzyme activity s 12 la It would appear that urease in soil is found within the eells of metabolizing ureolytic micro-organisms and plant organs, and as an extracellular portion derived from ruptured, moribund cells. Since extracellular urease is adsorbed on clay particles and humified organic marter 4 iß 19 23, soils exhibit levels of urease activity which are not necessarily directly related to the numbers, weights or activity of the viable ureolytic organisms at the time of determination of the urease activity la. Nevertheless, factors influencing the ureolytic population and the production of urease from this source, as well as factors influencing the aetivity and preservation of the enzyme itself, taust together determine the urease activity of a soil.It should be noted that in conformity with other workers 12 la 14 23 the term urease activity as used herein without qualification refers principally to the activity of extracellular urease in the soil, and excludes the urease activity of metabolizing ureolytic micro-organisms. This definition accepts the premises that in the method

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