I planted black seeds

I planted black seeds in the white earth;
its harvest cannot be harvested with the hand,
but only with the mind.

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Scroll down for answer and notes.

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I changed "rice" in the original to "seeds" here, and "knife" to "hand." Now that people don't really do as much handwriting as they used to, this riddle is becoming obsolete, but I like the idea of harvesting with the mind, so I wanted to go ahead and include it. This contrast between black and white is a good one to play with in making riddles about writing.

Writing.