When subroutines and functions in a module are accessed by USE <module_name> in a "consumer" of that module, an interface is already available, so there is no need to provide an interface block. Similarly, any procedure that is in a module has interfaces for other procedures in that module without your having to write interface blocks.
My position is that, with a couple of exceptions, if you're writing an interface block for a Fortran procedure in your own application, you're doing it wrong. Exceptions are declarations of generic procedures and when using submodules. Otherwise, your procedures should be in modules (or contained), in which case, as @mecej4 says, you get the explicit interface automatically.