Hmm, well it looks like the most prevalent theme is how American slaves from the 19th century couldn't "reap what they've harvested".
For imagery, this section caught my eye:
"I planted deep, within my heart the fear
that wind or fowl would take the grain away."
"Wind or fowl" are problems that cannot be controlled/suppressed, so if you farm crops you instill that fear and hope for the best. You could say that this represents a lack of freedom; the author mentions "planting" a fear, which can be interpreted as deeply internalizing a devotion to something that might cause you pain.
Later on, the poem says:
"I scattered seed enough to plant the land
in rows from Canada to Mexico"
This shows the extent of that devotion; the speaker has done so much to protect themselves, to ensure their survival.
Then there's this part:
"but for my reaping only what the hand
can hold at once is all that I can show."
The contrast in imagery in the first and second half of that stanza are huge; the speaker has grown so much grain to sustain himself, but is only able to reap what he can hold in his hand.
This represents a lack of freedom; working so hard only for wealthy slave owners to reap what others have grown for them.
I hope this makes at least some sense lol.