This reaction is
(a) reversible,
(b) exothermic, and
(c) proceeds with a decrease in volume.
According to the Le Chatelier's principle, the favorable conditions for the formation of ammonia are,
The temperature should be remain as low as possible, (although at unusually low temperatures, the rate of reaction becomes slow). It has been found that the temperature, which optimizes the yield of ammonia for the reaction, is maximum at about 500°C.
Since Haber's process proceeds with a decrease in volume, it is favored by high pressure. In actual practice, a pressure of 200 - 900 atmospheres is employed.