Process for the preparation in a batch reactor of at least one Ton, advantageously at least 5 Tons, preferably at least 10 Tons of a liquid nitrogen fertilizer with a nitrogen weight content from 20 to 32%, said process comprising at least the following steps operated at atmospheric pressure: Step A: An aqueous urea formaldehyde solution is prepared in a batch reactor by mixing together at least compounds selected from the group consisting of water, formaldehyde, urea, UFC and mixtures thereof and at least a base, the so prepared aqueous urea formaldehyde solution having an urea / formaldehyde molar ratio comprised between 1.05 and 1.45, advantageously between 1.2 and 1.4; Step B: An aqueous ammonia solution with an ammonia content of 20 to 32% by weight is added to the aqueous urea formaldehyde solution prepared in Step A in the batch reactor, possibly some base being added to control the pH; Step C: The solution prepared in Step B is heated and maintained at a temperature comprised between 80-100°C for at least 30 minutes, but advantageously less than 75 minutes, while controlling the pH between 8 and 10.5 by adding base, the amount of ammonia solution added is such that the molar ratio NH 3 /Urea is comprised between 0.2 and 0.4, advantageously between 0.25 and 0.30; Step D: The solution prepared in Step C is cooled at a temperature of less than 40°C in or outside the batch reactor; said process being further characterized in that: - The solution prepared in Step A is heated at a temperature of 55 to 65°C for at least 5 minutes; - When the temperature of the solution prepared in step A is still above at least 40°C, Step B is started by adding aqueous ammonia solution progressively during a period of at least 15 minutes, so as to control the progressive increase of temperature of the solution up to 80 - 95°C, while controlling the pH above 8 with or without adding some base; - When the temperature of the solution prepared in Step B is still above 75°C, Step C is started, and - in Step B, the progressive increase of temperature of the solution is such that the increase of temperature follows substantially a linear heating increase pattern with an error margin comprised between - 2°C and +2°C. Step A: An aqueous urea formaldehyde solution is prepared in a batch reactor by mixing together at least compounds selected from the group consisting of water, formaldehyde, urea, UFC and mixtures thereof and at least a base, the so prepared aqueous urea formaldehyde solution having an urea / formaldehyde molar ratio comprised between 1.05 and 1.45, advantageously between 1.2 and 1.4; Step B: An aqueous ammonia solution with an ammonia content of 20 to 32% by weight is added to the aqueous urea formaldehyde solution prepared in Step A in the batch reactor, possibly some base being added to control the pH; Step C: The solution prepared in Step B is heated and maintained at a temperature comprised between 80-100°C for at least 30 minutes, but advantageously less than 75 minutes, while controlling the pH between 8 and 10.5 by adding base, the amount of ammonia solution added is such that the molar ratio NH 3 /Urea is comprised between 0.2 and 0.4, advantageously between 0.25 and 0.30; Step D: The solution prepared in Step C is cooled at a temperature of less than 40°C in or outside the batch reactor; said process being further characterized in that: - The solution prepared in Step A is heated at a temperature of 55 to 65°C for at least 5 minutes; - When the temperature of the solution prepared in step A is still above at least 40°C, Step B is started by adding aqueous ammonia solution progressively during a period of at least 15 minutes, so as to control the progressive increase of temperature of the solution up to 80 - 95°C, while controlling the pH above 8 with or without adding some base; - When the temperature of the solution prepared in Step B is still above 75°C, Step C is started, and - in Step B, the progressive increase of temperature of the solution is such that the increase of temperature follows substantially a linear heating increase pattern with an error margin comprised between - 2°C and +2°C. - The solution prepared in Step A is heated at a temperature of 55 to 65°C for at least 5 minutes; - When the temperature of the solution prepared in step A is still above at least 40°C, Step B is started by adding aqueous ammonia solution progressively during a period of at least 15 minutes, so as to control the progressive increase of temperature of the solution up to 80 - 95°C, while controlling the pH above 8 with or without adding some base; - When the temperature of the solution prepared in Step B is still above 75°C, Step C is started, and - in Step B, the progressive increase of temperature of the solution is such that the increase of temperature follows substantially a linear heating increase pattern with an error margin comprised between - 2°C and +2°C.