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The key reaction N (+)+ H2 yields NH (+)+ H (1) for the gas phase synthesis of ammonia has a 170 K barrier if H2 is in its lowest vibrational and rotational state. It is shown that this barrier has no influence on the formation of NH3 as soon as the ortho to para (O/P) ratio of H2 is greater than a few 0.0001. This value is not model dependent. Furthermore, it is shown that even in the coldest clouds, reprocessing of H2 by the chemistry is fast enough even in steady state so that LTE is never reached and the O/P ratio stays above this critical value. The consequence is that no other process is needed to explain the formation of NH3 in dark clouds, even at 10 K.