Certain conditions are required to make sure that the reversible reaction goes in the right direction.

These conditions are:

Moderately high temperatures - Around 450°C
High pressure - About 200 atmospheres
An iron catalyst

Why are these conditions most suitable?

Because the reaction involves combining one mole of nitrogen with three moles of hydrogen ( a total of four moles of reactants) to produce two moles of ammonia (a total of two moles of products), high pressure will always favour the the forwards reaction, because its products take up less space. The pressure is not any higher than this because it is very expensive to run a plant that operates under high pressure conditions.
High temperature favours the backwards reaction (i.e. producing nitrogen and hydrogen from ammonia) . Therefore, one would think that low temperatures would be most suitable. However, if the reaction was carried out under low temperatures, the rate of reaction would be extremely slow and this would not be economical. Also, the catalyst only works effectively above about 400°C. Therefore, the temperature of 450°C is a compromise between these two factors.
The iron catalysts speeds up the rate of reaction by lowering the activation energy of the reaction. Osmium or uranium can also be used, but these are only available in small quantities. Ruthenium would allow the process to be carried out at lower temperatures, but it is expensive. The catalyst does not change the percentage yield.

These conditions give about a 15% yield of ammonia. The ammonia is liquified and removed. Unreacted hydrogen and nitrogen are passed through the system again and again so that in the end the overall yield is about 97%. This minimises the cost of raw materials.

The raw materials used in an industrial process are known as the feedstocks. Therefore, the feedstocks for the Haber process are air, natural and steam.

The hydrogen used in the Haber process is either producing as a by-product from catalytic cracking or by the steam reformation of natural gas.

Haber Process