Sorry , Yes RCM is applicable on cotton fro agriculturist. Raw Cotton under RCM from 15 Nov. 2017: CBEC Notification 43/2017 Central Tax Rate dt. 14 Nov
As per 26th GST council meeting held on 10.03.2018, Reverse charge mechanism in case supplies made by unregistered persons to registered persons delayed till 1st july.
Raw Cotton is 'agriculural produce' within the definition of Noti. 12/2017 and that too if purchased from Agriculturist is sure exempted service. Hence RCM is not applicable.