Note the written prologue states that the film will show what "we" (European settlers) "did" with a half-million square miles of high plains once the swath was "cleared" of Indians and buffalo. Now, the visuals start with waving seas of native grassland, and since the narrative follows an historical timeline, this is a depiction of the land before European settlers arrived. The land may have been harsh and dry—"treeless, windblown, and without rivers or streams"—but it did support rich fields of native grass. In contrast, the visuals end with a stark depiction of the 1930's dustbowl— great clouds of topsoil swept up from a land stripped bare by drought and plowing away of the native grass cover. The images are bleak, searing, and unforgettable.
I call attention to this because a literal reading of the prologue matched against these opening and closing shots is hardly a tale of triumph. The plow that broke the plains really did break them, it appears-- at least to this point in 1936. Hopefully, an improved agronomy has prevented these latter scenes from repeating.
Nonetheless, the documentary itself represents a triumph of artistic imagery (Lorenz) and musical score (Thompson)— and a tribute to its New Deal sponsors. From the first lone rider to the great cattle herds to the mighty plows to soaring WWI demand and finally to the dustbowl and its refugees, the story is elegantly related. I agree that the narration too often goes over the top, but the basic idea works. And I really like that last shot of the lone tree skeleton with its tiny bird's nest looking hopefully to the future. All in all, the 25 minutes adds up to a powerful work of documentary art.
(In passing—I think there are two ways of construing the rather puzzling shots of British WWI tanks plowing forward. In context, the tank armies are juxtaposed with armies of tractors plowing over the plains. Thus, we might view each army as subduing a resistant foe, in the latter case, a difficult land. Or, possibly, the tractors can be taken as a mechanized army of harvesters supplying foodstuffs for a mechanized army of tanks. And even though the two construals may be taken as odes to the power of mechanization, I detect a dark undercurrent to the film as a whole that hardly coincides with the usual tales of "the winning of the West".)