Moss does not need feeding the way a border plant or a vegetable does, and most of the times I have tried to help it along with fertiliser I have made things worse. A moss patch wants moisture, the right light and to be left lean; a dose of plant food usually grows everything except the moss. If you take one thing from this page, let it be that a hungry-looking mossy corner is very rarely actually hungry.
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How moss gathers what it needs
With no roots to draw on a reservoir of soil, moss takes its water and its scant minerals straight across its surface, from rain, from dew, from the dust and grit that wash over it. A little nitrogen even arrives dissolved in rainfall. This is why the plant is at home on the poorest ground going, on bare rock, on a concrete path, on a baked wall top where nothing else will root. Those places suit it precisely because they are impoverished: moss is a poor competitor, quick to be crowded out wherever the soil is rich enough for grasses and weeds to romp away. Feed the ground and you tip the balance against the very plant you meant to help.
Why fertiliser usually backfires
Reach for a general plant food and you mostly feed the moss's rivals. Nitrogen sends up grass, seedlings and, on a damp surface, a slick of algae, and any of these will smother a moss carpet within a season. There is a sharper irony with lawns. The autumn and spring lawn feeds sold to green up grass often contain ferrous sulphate, which is the same iron salt marketed separately as a moss killer; scatter it hoping to strengthen a mossy lawn and you scorch the moss black instead. I learned the lesson on a shaded wall I was trying to green with a slurry: keen to speed it up, I mixed in a splash of dilute houseplant feed, and within weeks the joints had grown a fur of algae while the moss itself sulked. I have not repeated the experiment.
The buttermilk question
People often assume the buttermilk or yoghurt in a moss slurry is food, a sort of nourishing smoothie for the plant. It is not really doing that job. The dairy works mainly as a mild glue and an acidifier, helping the fragments cling to a wall or a pot while they take hold and nudging the surface towards the acidity most mosses prefer. The moss is not digesting the milk; it is using it as scaffolding for a few weeks. That is a different matter from feeding, and it is covered as a technique in attaching moss to wood or rock. Once the fragments have gripped, the dairy has done its work and the moss carries on as it always would, living off rain and air.
The rare times a little help is fair
There are a couple of honest exceptions, both small. In a sealed terrarium that has run for a year or two, the closed system can genuinely run short of minerals, and a very weak misting, perhaps a quarter-strength orchid feed once or twice a year, sometimes brings a tired carpet back without waking a plague of algae; go gentle and stop at the first sign of green film on the glass. Outdoors, the thing that most often reads as hunger is nothing of the sort, it is thirst or the wrong water, and the fix is rainwater rather than feed. Hard tap water leaves a chalky, alkaline residue that many mosses resent, as I explain in watering moss. Sort the water and the light and the supposed deficiency usually vanishes.
What moss actually wants instead
Put the feed away and give it the four things it does ask for. Steady moisture matters most, ideally soft rainwater rather than hard tap. Shade or gentle light suits the great majority of garden and woodland mosses, though a few wall specialists take full sun. A slightly acidic, firm surface holds it better than loose rich soil, which is one reason a mossy lawn does best when you stop feeding and liming the grass, a shift I go into in making a moss lawn. And then patience, because moss keeps its own unhurried clock. Leave it lean, keep it damp, and it will thicken on its own far more reliably than any bottle of fertiliser will persuade it to.