In line with Phil's post, "softness" can be caused by (at least)
#1 Unwanted subject motion (a high shutter speed will help with this)
and/or
#2 Unwanted camera motion (a high shutter speed will help with this or make sure your camera is stable)
and/or
#3 Miss focus due to operator error or the camera/lens being inadequate for the subject.
and/or
#4 "junk" on the front of the lens - could be anything from a filter to rain on the lens
and/or
#5 Flare - unwanted light hitting the front of the lens (a lens hood may help or shading the front of the lens)
and/or
#6 atmospherics, including heat haze when shootintg across a hot surface.
and/or
#7 Shooting at very high ISO (noise will tend to make an image soft)
and/or
#8 Failing to sharpen (if needed) when post processing.