Before the poem mentions a diary, it makes us smell something. "A rank morning" is only three words, but "rank" does a lot of work. It means foul-smelling, yet it also describes vegetation grown thick and overripe. A rank morning, then, is one where things are rotting and growing at the same time. That double sense of stench and fertility is the poem in miniature.
The next lines turn on two dashes. Remove them and the sentence reads: "I batten on ash and rot, break down the day." The speaker sounds like a creature, a beetle or a fungus or some small scavenger at work in the leaf litter. The dashes then slip in the human identifications almost like confessions: the ash is "my wife," the rot is "my diary." The shock comes from setting a person and a notebook beside such unlovely words, and from the plain, matter-of-fact tone in which it is done.
The poem never says what the ash means, and the ambiguity is useful. Ash is what remains after a fire, grey and spent, so it could suggest a worn-out marriage, tiredness, or loss. But ash is also something gardeners work into soil. What the poem does make clear is the speaker's behaviour. To "batten on" something is to feed greedily and grow fat at its expense. The verb is unflattering, and the speaker aims it at himself. Judging by the imagery, the poem is less about the wife than about what a writer does with the people close to him.
The diary fares no better. A diary is supposed to preserve the day, yet here it is "rot," organic matter that is already decaying. The word "diary" comes from the Latin for "day," and the poem's last word returns to that root. The diarist sets out to keep the day and ends up taking it apart. The repeated "my" in "my wife" and "my diary" is quietly telling, because it puts the two on the same footing, as things owned and used.
The final line, "break down the day," gathers the poem's meanings. In compost, to break down is to decompose, and "ash," "rot" and "break down" belong to the same vocabulary. It also means to analyse, which is what a diarist does with an afternoon, splitting it into incidents and moods. And it hints at emotional collapse, the morning that has defeated someone before it has properly started. The grammar adds to the effect. "Rot" could be read as a second noun or as a verb, as if the speaker rots along with his material. "Break" could continue the speaker's own actions or work as an order he gives himself. The poem doesn't settle either reading.
The form is as stripped down as the language. There is no rhyme and no regular metre. The lines run roughly four, seven, six and four syllables, swelling in the middle where the two dashed interruptions sit and then tightening to the end. The sounds help. The short, clipped "a" of "rank," "batten" and "ash" feels like chewing. Then "break" and "day" share a long vowel, and that echo closes the poem like a click. It acts as a one-line rhyme with no partner line needed.
This is why the poem matters despite its size. Writers often present the diary as a place of honesty and self-knowledge. Here the diarist admits that writing is a kind of consumption, and that turning life into text means feeding on it and letting it decay on the page. The poem does not flatter the speaker, and it does not end in despair either. Decomposition is also how soil is made, and "break down" is a verb of process rather than of ending. Whether anything grows from this rank morning is left for the reader to decide.