Compost goes on a vegetable bed at 1 to 2 inches, worked into the top 6 inches, and no other bulk amendment on this list takes that same rate, because they don't do the same job. Aged manure is a nutrient amendment, not a structural one, and it goes on at roughly an inch, in the fall, months ahead of anything you plan to eat. Leaf mold is the springy, fungal, water-holding one and can sit 2 to 3 inches deep on the surface. Worm castings run a quarter inch or less, used as a topdress or as 10 to 20 percent of a potting mix. Peat and coir are potting-mix ingredients first; on an open bed they earn an inch at most, and only where water holding is the real problem.
The stick with two lines on it
The bed I'm standing at is four feet by eight, and it is the last week of October. The tomato stakes lean against the fence behind me, the soil is bare and gone gray-brown, and there's a wheelbarrow of sifted compost on the path with a thread of steam coming off it in the cold air. Flat spade, garden fork, rake, one five-gallon bucket.
I keep a length of lath jammed into the corner of that bed with two wraps of masking tape around it, one at an inch and one at two. I lay the stick flat on the soil before I spread anything, because my eye is a liar. Ask me to put down "about an inch" and I will put down two, then add a little more to even out the thin spots.
Here's the arithmetic that made inches the only unit I bother with. A five-gallon bucket holds about two-thirds of a cubic foot, which spreads to roughly eight square feet at one inch deep. My bed is thirty-two square feet, so one inch is four buckets and two inches is eight. That's the whole conversion. Once I know the depth I want, the bucket count falls out on its own, and I stop caring how many trips to the pile it takes or what size the packaging happens to be.
Compost: one to two inches, and why not four
I spread the first inch with the back of a rake, then fork it into the top six inches, which is the full depth of my fork's tines. That's the whole operation, about five minutes a bed. Two inches goes on a bed that's new, sandy, or has been worked hard and is low in organic matter.
Past two inches is where I learned something. One fall I had more compost than beds and heaped close to four inches onto a single plot. The next spring I did not get four times the growth. I got a bed that crusted when it dried, beans with scorched leaf margins, and a soil test that came back with higher soluble salts than the bed next to it that had received an inch. Compost carries salts. Doubling the rate doesn't double the benefit. It just stacks up what the roots can't take.
I also only spread compost that's finished. Cool in the middle, smells like a forest floor after rain, doesn't make my eyes water. If the pile still has heat in it, or smells like ammonia or vinegar, it's still cooking, and it will keep cooking in the bed. Unstable compost ties up nitrogen right at the root zone and can burn tender roots at the transplant hole. The number people cite for finished is a carbon-to-nitrogen ratio somewhere in the 15 to 25 to 1 range, and that is the right target. But temperature and smell are what I actually check, because I can check those with my hands in four seconds.
The other five, at their own depths
Aged manure
About an inch, in October, on the bed that will carry the heavy feeders. Manure is a nutrient amendment. It is not a structure amendment. Composted manure has already broken down fine, and when I dig it in it goes a little gelatinous; a bed that is mostly manure will crust and slump. Fresh manure is a different thing entirely, with salts and pathogens, and it does not belong anywhere near a crop you are about to pick. I put it down in fall so that winter freezing and thawing, worms, and the first weeks of spring do the work long before a seed goes in.
Leaf mold
Two to three inches, which makes it the thickest application on this list. Leaf mold takes longer to make than compost: a wire bin, shredded leaves, kept damp, turned once or twice, and a year or two before it looks like crumbly dark tea leaves and smells like the floor of a woodlot. It's high in lignin, which is fungal food, and it holds water like a sponge without supplying much nitrogen at all. So I don't count it as fertilizer. I put two or three inches on top of beds going into winter, and I dig it into the sandy corner where water runs straight through. In a bed that already has its nitrogen from compost or manure, leaf mold is often what separates soil that sheds water from soil that drinks it.
Worm castings
A quarter inch or less, and I mean less. I put castings where the plant is, not across a whole bed: a band a few inches wide around the base of a transplant, a light scatter down a seed row, or 10 to 20 percent of the volume of a potting mix. They're concentrated, and by volume they're the most expensive thing on this list, which is exactly why the rate is a quarter inch instead of an inch. Spread bed-wide, they're a waste of material and can leave a mucky, waterlogged surface, because wet castings hold a great deal of water and pack down.
Peat and coir
Both are container ingredients first for me. Peat holds water well and lowers pH slightly, and it is nutrient-poor. Its worst habit is going hydrophobic once it dries: a dry layer of peat on a bed sheds water instead of absorbing it. If I use it, I soak it first and mix it into the top six inches. Coir holds comparable water at nearly neutral pH and rewets more easily, which is the main reason I reach for it over peat in a mix. On an open bed, either one gets an inch at most, and only in a spot that drains too fast. Laid on thicker, they feed nothing and dilute the soil you spent years building.
Why the source matters more than the type
Herbicide carryover is the reason I ask questions before I accept a load. Aminopyralid and clopyralid survive composting, and they survive an animal's digestive tract. Grass clippings from a treated lawn, hay from a treated field, or manure from an animal that ate that hay can carry residues into the pile and out into the beds. The signature is new growth that comes out cupped, twisted, and fern-like, with tomato stems curling in on themselves. It isn't a disease and it doesn't respond to anything you spray. It has to grow out, and by then the season is gone.
So I do two things. I ask what the animals ate and whether the hay or pasture was sprayed. And before I spread a whole load, I run a test: fill a couple of pots with the material, plant a bean or a pea in each, and give it three weeks. Beans are sensitive enough that a distorted first true leaf tells you what you need to know.
The number that takes years
Soil organic matter in a productive vegetable bed sits around 4 to 6 percent, and you get there with repeated annual additions, not one large one. An inch a year on an established bed that has decent structure. Two on a bed that's new or depleted. Then again next year, and the year after that. The inches you put down in October are a fraction of that volume by June, because soil biology has already eaten most of it and turned it into the crumb and the water holding you were actually after.
I still soil test every few years. None of this tells me my phosphorus level or my pH, and no amount of compost substitutes for knowing those numbers.
That's why the stick with the tape lines stays in the corner of the bed. Compost is the easy part. Getting the depth right is the part I keep coming back to.
