The ploughman, a "young Negro at dawn in straw hat and overalls," is seen as the "emblem" or "engraving" that emerges "out of the turmoil." This ploughman is the starting point, and gives the metaphot further direction. The scene described is that of an African American slave harvesting cotton. However, from there the imagery becomes more complex. The ploughman is an "emblem of impossible prophecy," a paradox that seems to refer to Barack Obama (who is of course the first African American president in our history. The term "president" is even used in the line following the next line. The crowd divides for their "president," and we see that the crowd is likened to a field.
This field is forty acres wide: this demonstrates, for the ploughman, his terribly journey to where we are today. Forty acres and a mule (the mule was mentioned earlier in the poem) were promised to freed slaves who fought in the Civil War. However, this promise was revoked by Lincoln's successor, President Andrew Johnson, and thus the phrase "forty acres" has come to symbolize the failure of Reconstruction. Thus, "forty acres" is a metaphor for broken promises and adversity, through which the ploughman must have traveled. Included in this field are the "crows of predictable omens" which could be an allusion to the Jim Crow laws, rules that legally enforced the Plessy vs. Ferguson "separate but equal" philosophy. However, the ploughman, which at this time seems to me to refer to the entire African American race, "ignores for his unforgotten cotton-haired ancestors." The ploughman is also faced with the "tense court of bespectacled owls" and the "gesticulating scarecrow" which might refer to discriminatory judicial boards and racism, respectively. The cotton hair is of course a reference to slavery; thus, at the time of the forty acres and Jim Crow law controversies, the African American people had to ignore their troubles and push on in order to make possible what is now today possible: an African American resides in the White House.
"The small plough continues on this lined page/beyond the moaning ground..." There is more symbolism here, but it seems to continue what has already been started in the previous lines. The "moaning ground" could be the ground where dead black soldiers laid in their final rest. The "lynching tree" is a more obvious image. However, the young ploughman "feels the change in his veins" which could point explicitly towards Barack Obama, whose platform rests on the hope of change.
The most powerful image for me was the last line. And "furrows wait for the sower." A sower plants seeds; he does not harvest them. The ploughman has only changed the ground so that a sower could plant seeds. What we see is a tradition going way back. Obama himself is the next sower. That is so powerful. Obama is only picking up the work of his ancestors. He is not the epitome of black rights or of civil rights. He is not the final goal. Barack Obama's presidency is the penultimate step in the ploughman's journey. Now he will "sow the seeds" and wait for the next ploughman in his line to take up the plough.