and+it+is+not+foureen+lines+like+furrows+in+a+small,+carefully+plowed+field

I think Collins is comparing the American sonnet to furrows in a small carefully plowed field and how they differ. I think Collins is saying that the American Sonnet is not well thoughtout and it is just thrown together only to serve a purpose not to enlighten. Such as a greeting card that has a generalized greeting to express a thought or feelings to not one individual, but instead to thousands.