Methods for synthesizing nano-urea particles are described. The methods involve using a spinning cone reactor, a nano-channel reactor, combustion synthesis or spray drying. Ammonia and carbon dioxide are used as raw materials for producing urea. The methods allow for the production of urea nanoparticles at high conversion rates. The methods allow for better control over the hydrolysis rate of the nano-urea particles. The nano-urea particles can be used as fertilizers and provide for sustained release of ammonia and its conversion to nitrates in the soil. The nano-urea particles have low volatilization rates, low moisture absorption rates and low biuret concentration. The nano-urea particles can be applied to the soil via fertigation.