Student or Learner
The rich soil crumbles through the yeoman’s fingers. As the pearl diver murmurs, ‘I am home’ as he moves dimly in strange water-lights, and as the painter mutters, ‘I am me’ on his lone raft of floorboards, so the slow landsman on his acre’d marl – says with dark Fuchsia on her twisting staircase, ‘I am home.’
(M. Peake; Titus Groan)
Why not 'acred'?
You might well say that.
Go for the music, the vision, and the beauty of Peake's language rather than trying to squeeze it bone dry for meaning. He's a one-off and doesn't have to follow rules. What he's doing here is common in poetry and he's simply lopping off a syllable to make the prose sound better, even though the syllable doesn't exist. Just relax and enjoy the ride- he may ignore a lot of grammar rules, but he does English and does it very well. He doesn't do Murphy, but he does literature.
An "acre" (noun) is a measure of land area. Peake has used the word as an adjective by adding "ed". Since the word ends in "e" he has replaced the second vowel with an apostrophe.
"Marl" is a soil type composed mainly of clay. It is also a compacted, impure limestone.
So, in two words, Peake has described an expanse (a large area) of farm land composed of a clay soil. (Typical of East Anglia).
Remember, Peake was also a poet and he often uses writing conventions from that discipline. Conventions which include invention of words with the desired metre. Try reading him aloud and see how his prose roll!