Why would the corner of some foreign field become England if Brooke should die?

In his death on a foreign battlefield, the speaker believes that he will figuratively transform part of that field into England.
The speaker asks readers to remember him fondly. If he should die on a foreign battlefield, he says, the earth where his body has fallen will always symbolize England and her greatness. In fact, the speaker believes that, because he is English, his body will turn into "richer dust" than the original dust of the battlefield.
The speaker maintains that his dead body would enrich the earth on the battlefield. After all, his body was formed from the "dust" of England. He tells us that England is full of bucolic beauty. It is a pristine land, filled with the fruits of nature. It is this England that the speaker wants to die for.
Finally, the speaker maintains that he is happy to die for England, for she has given him much in life. To the speaker, England is a land of laughter, peace, and gentleness. By dying for England, he will have given back to his mother country all that she has blessed him with in life.