A LOW TIME PA28 CFI PERFORMS A TKOF ON A CLOSED RWY AT CKN; MN.

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ON 04/THU/02; I OBSERVED AN FBO-OPERATED ACFT STAFFED WITH THEIR PERSONNEL DOING MULTIPLE STOP-AND-GOES ON GRASS RWY 24 AT CKN. ON APR/FRI/02; I SPOKE TO ANOTHER FLT INSTRUCTOR AND QUERIED HER ON WHETHER THERE WERE ANY RESTRS ON THE GRASS RWYS (6/24; 17/35) AT CROOKSTON. SHE STATED THAT THERE WERE NO NOTAMS AND NO RESTRS FOR THE RWYS IN QUESTION. BASED ON MY OBSERVATIONS OF THE LCL ACFT OPERATING ON THE SODS THE PREVIOUS DAY AND THE INFO FROM ANOTHER CFI; I ASSUMED THE GRASS RWYS AT CKN WERE OPEN. UNSURE OF THE GRASS CONDITION OF THE GRASS RWY 35; I LANDED ON THE PAVED RWY 31; AND TAXIED ONTO RWY 35 TO CHK CONDITION OF THE RWY. THE RWY WAS FIRM AND IN GOOD CONDITION. I TOOK OFF FROM RWY 35 AND PROCEEDED TO STAY IN THE PATTERN FOR ANOTHER LNDG. ON FINAL; I HEARD ON THE RADIO THAT RWY 35 WAS CLOSED AND WAS TOLD TO GO AROUND. (CKN IS AN UNCTLED FIELD.) I COMMENCED A GAR AND NEVER LANDED ON THE SOD RWY 35. WHEN I RETURNED HOME; I CHKED THE NOTAMS FOR CKN AND DISCOVERED THAT RWY 17/35; AS WELL AS RWY 6/24; WAS CLOSED. AT NO TIME WAS THERE ANY CONFLICT WITH OTHER ACFT OR DAMAGE TO THE RWY. THIS MISTAKE COULD HAVE BEEN PREVENTED IF I HAD CHKED THE SPECIFIC NOTAMS FOR THE FIELD; INSTEAD OF RELYING ON HEARSAY AND THE ACTIONS OF OTHERS. ALSO; A WAY TO ENSURE THAT THIS DOES NOT HAPPEN IN THE FUTURE IS TO ADD THE NOTICE OF RWY CLOSURES ONTO THE LCL AWOS FREQ.