According to this: http://www.locallingo.com/czech/grammar/adjectives.html
(and the Tips & Notes in Masculine, too)
Czech adjectives are divided into two groups:
1) Adjectives with a hard ending in the nominative singular -ý (masculine)
2) Adjectives with a soft ending in the nominative singular
-í (masculine, feminine and neuter)
All Czech adjectives end with a long vowel.
It seems n is considered a soft sound.