WHILE CLBG FM A GO-ARND AT ABT 400' AGL, THE INSTRUCTOR (CFI) SHUT DWN THE L ENG BY CLOSING THE MIXTURE & THE STUDENT (PVT PLT) FEATHERED THE LEFT PROP. THE CFI THEN MOVED THE PROP CTL OUT OF THE FEATHER PSN, BUT THE PROP RMND FEATHERED. THE CFI WAS UNABLE TO RESTART THE L ENG & THE ACFT WOULD NOT CLIMB AT THE BEST SINGLE-ENG RATE OF CLB SPD. IT WOULD ONLY MAINT A MIN RATE OF DSCNT OF 50' PER MIN. SUBSEQUENTLY, THE ACFT WAS DMGD DRG A WHEELS-UP LNDG ACROSS FURROWS IN AN ALFALFA FLD. THE FLT MANUAL STATED: DO NOT FEATHER A PROP IF THERE IS REASON TO SUSPECT THAT THE STARTING CHARACTER- ISTICS OF THE ENG ARE NOT NORMAL OR THAT RESTARTING IN THE AIR MAY BE DIFFICULT OR IMPOSSIBLE; DO NOT FEATHER A PROP IN CONDITIONS OF TEMPERATURE, ALTITUDE, WEIGHT OR TURBULENCE WHICH MAY PREVENT SINGLE ENG FLT AT ALTITUDES WELL ABOVE THE LOCAL GROUND ELEVATION; DO NOT FEATHER A PROP AT ANY TIME WHEN CONDITIONS OF TERRAIN OR OTHER CONDITIONS MAY PREVENT THE AIRPLANE FROM REACHING AN AIRPORT, IN CASE THE DEAD ENG CANNOT BE RESTARTED.