I'll tell you what, you dig post holes and I'll do heavy squats and lets see who's added more muscle to their legs at the end of a year.
I don't doubt you'll add more muscle mass. It depends on whether you're wanting to build fast or slow twitch muscle. If you're just after bulk, then you want fast-twitch.
I am a farmer, and I guarantee he knows what he is talking about on posthole diggers.
And it actually does something constructive, not just exercising.