Read on the surface, Fertiliser is about a flower close to death. The speaker comes across a brown, scraggly plant with crispy leaves, and watches them fall one by one. The title and the way the poem talks about "elements" that help something "thrive" suggest the flower may stand for a person, someone the speaker met when they were struggling and barely holding on. That reading is an inference, since the poem never says so directly.
What the speaker feels is a mix of worry and determination. Watching the leaves drop is painful, and it pushes the speaker to make a vow rather than just look on. The word "vow" makes the promise feel serious, almost like a pledge of love or loyalty. The mood begins bleak and fragile, with images of decay, but it turns hopeful by the end. The poem stops at the promise, so we never see whether the flower recovers. That leaves a quiet, tender feeling of care and possibility.