1. Mechanisms of urea adsorption by soils Physically adsorbed urea may be easily desorbed by dilution. Chemisorbed urea existing primarily in the form of relatively stable soil organic matter-urea complexes may, in part, be slowly dissociated upon dilution.

Differences between soil types with respect to urea adsorption were primarily related to organic matter contents; the effects of CEC, pH, and clay mineral content were found to be insignificant. Wet soils had a lower capacity for urea adsorption than dry soils. Considering the amounts of urea adsorbed it appears that soils have a weak affinity for the urea molecule.

1. Mechanisms of urea hydrolysis and volatilization in soils The biological or catalytic hydrolysis of urea is rapid and can be related to soil microbial activities. The reaction rate of the chemical hydrolysis of urea is very slow and insignificant in comparison with the biological or catalytic hydrolysis. Urea-nitrogen loss through ammonia volatilization may immediately follow urea hydrolysis and proceeds rapidly. Urea hydrolysis and ammonia volatilization from ammonium carbonate are first order reactions.

2. Mechanism of foliar absorption and subsequent utilization of urea The mechanism of foliar absorption of urea solution is suggested to be a simple physical diffusion phenomenon. The absorbing capacity of the foliage for urea solution is relatively lover during the day than during the night. It is suggested that rapid evaporation at high temperature and low humidity conditions may increase the concentration of the urea solution on the leaf thereby decreasing the rate of diffusion of foliar applied urea into the leaf. The absorbing capacity of the leaf for urea solution is independent of its position, but the younger leaves have a higher capacity to metabolize the absorbed urea.

Urea in foliage cannot be translocated, but more than 90%of the absorbed urea can be rapidly and directly transformed into other soluble nitrogenous compound(s). The mechanism of the biochemical reactions and their related enzymatic systems are not yet known.

Sucrose does not influence the urease activity, and no urease activity was detected in the tobacco leaves used. The reduction of urea injury from foliar applied urea solutions containing sucrose was associated with decreased urea absorption and increased urea utilization. A relationship was found to exist between the urea concentration in the leaf and the incidence of injury. Plasmolysis resulting from increased urea concentrations in the leaf may be the cause of foliage burning resulting from foliar application of urea.

1. Methodology The principle of the newly developed methods for the determination of urea in fertilizers, urine, and blood and the estimation of urease activity is based on the difference in electric conductivity of urea and ammonium carbonate produced from urea by urease in solution. These methods are rapid, simple and accurate. They have very wide testing ranges, and the values of the standard curves are constant at a specified temperature. Interference due to the presence of colored impurities are eliminated. However, these methods will not be applicable if heavy metal ions such as Ag, Hg, and the protein-destroying substances are present in the testing solution.