with usura, sin against nature, is they bread ever more of stale rags (14-15)
Well he ain't exactly being subtle here. Pound calls usura a sin against nature, claiming that it turns our bread into stale rags. He's probably talking about the Christian symbol of the bread of life, meaning the stuff that our souls need to feed on in order to be healthy. But our spiritual bread of life is turned to rags by usura, and so all of us feel a quiet despair about the future, whether we know it or not.
with usura is no clear demarcation and no man can find site for his dwelling (19-20)
Again, Pound comes at us full bore with Biblical words like "dwelling" to talk about what people lose when they start engaging in moneylending and speculation. According to Pound, people somehow lose the ability to draw clear boundary lines and build their houses. It's not quite clear how this is the case, but Pound might mean it symbolically to suggest that no one can have a clear sense of "home" in a world constantly turned upside-down by money and numbers.
Usura is a murrain (26)
Many of us have probably never read the word "murrain" before. But it's basically just a synonym for "plague" or "sickness." Saying that usury is a sickness allows Pound to connect the practice of usury with having a soul that isn't healthy.
Came no church of cut stone signed: Adamo me fecit (34)
One of the first things to suffer from usura is churches, since churches aren't built with cheap, efficient budgets like most modern buildings are. Churches are built to be beautiful and to inspire faith in the power and goodness of God. But in a world that cares only about money, these beautiful things don't get built because they aren't cost-effective.
Not by usura St Trophime Not by usura Saint Hilaire (35-36)
Pound mentions two examples of churches that he finds especially beautiful, but only for the sake of saying that these churches can't be built in a world with usura. Nothing beautiful can be built at all. But the fact that it's specifically churches that can't be built allows Pound to make his connection between usura and the failure of religious faith in the modern world.
CONTRA NATURAM (47)
It's in Latin and it's all caps. Hard to miss it when Pound basically yells in our faces the phrase "AGAINST NATURE." Here, he sounds like he's taking on the voice of God, telling us that usura is a sin against nature and implying that we'll all be doomed to hell (or something like it) if we choose to make money without actually contributing anything to the world.