Dear Peerzada Ovais Hamid

Low concentrations of urea can simulate number and dry weight of created nodes on plant roots, but high levels of urea fertilizer have inhibitory effect on Rhizobium bacteria and prevent potential activity of this bacterium. Generally, the use of nitrogen fertilizers such as urea is necessary until formation of nodes and starting their activities.

best wishes

I full agree with all comments above. Yes, until the commencement of N-fixation by Rhizobia-Plant (legume association), small amount of N which known as starter dose of N in the form of urea or other forms of N-fertilizer is needed. However, the amount of N fertilizer that needed to be applied per unit area depends on the N-content of the soil. If there is high amount of soil N, you many not need to apply starter N fertilizer. Thus, the start dose of N to be applied in specific soil should be determined experimentally. This is one aspect and the other aspect is that if high amount of N fertilizer as urea or any other form is applied, it will depress biological  N-fixation. This is due to the fact that if high amount of N present in the soil, the plant sense and will secret signal compounds needed to initiate nodulation processes. You can read the details in any standard soil microbiology books, please.

I also fully agree with the above facts. Again I want to add that urea the starter dose to be applied in split doses so that each and every molecule can be utilized by the crop plant and no amount of urea will be leached out.  

Correction!

Dear Readers, I have corrected the last four lines of my answer yesterday and should read as follows, please:

"This is due to the fact that if high amount of N present in the soil, the plants will  sense and will not secret signal compounds needed to initiate nodulation processes. You can read the details in any standard soil microbiology books, please". 

Interesting discussion. Why should we invariably consider , exogenous application of urea  will adversely affect the efficacy of inoculated rhizobium with regard to nitrogen fixation . If soil N-test value is sub-critical , any rhizobium species will respond to urea as a starter nutrient . however , this will not be the case when soil N-test value is above optimum. 

Please take a look at the following link.

http://www.fspublishers.org/published_papers/41084_..pdf