The living layer, and it is thinner than people think
Soil is not one substance, it is layers, and topsoil is the top one. It is darker than what sits underneath because it holds organic matter, it is looser because roots and worms have been working it, and it is where essentially all the biology in the ground lives.
Underneath it is subsoil: paler, often grey or orange, denser, and holding very little organic material. Roots can get into it and they do not thrive there. Below that is whatever the ground is made of.
It is not compost, and the substitution is expensive
This is the mistake worth preventing, because it is made with good intentions and by people spending more rather than less. Compost looks like the richest possible soil: dark, light, full of goodness. So raised beds get filled with it and borders get topped up with it.
The difference is what the material is. Compost is organic matter that has been decomposing, and it carries on decomposing after you have put it down. A bed filled with it can drop by a quarter or more over the first year, and because there is little mineral content there is little holding water, so it dries out much faster than soil does.
Topsoil is mineral: sand, silt and clay, with organic matter as a modest fraction of the whole. That mineral skeleton is what gives it structure and keeps the level where you put it. Compost belongs mixed into topsoil, and bark belongs on top of it as a mulch, which is a third material again with a bulk density nothing like either.
Screened, as-dug, and what a specification signals
Screened soil has been through a mesh, usually stated at something like 10mm, so the stones and root pieces are out. As-dug or unscreened has not. Screened costs more and it is worth it under a lawn or a seed bed, where stones surface for years and a lumpy base becomes a lumpy finish. Under a levelling job that will be covered by something else, as-dug is usually the sensible buy.
British suppliers quote BS 3882, which is the specification for topsoil, and the grade names on a price list come from it. It is a paid standard so the limits are not printed here. What the reference tells you is that the material has been tested against something published rather than loaded off a field, which is a fair question to ask for a lawn and a fairly academic one for backfill.
Judging a load before you tip it
You get about a minute while it is still on the wagon, so it is worth knowing what to look at.Colour first: dark brown is soil, grey or orange is subsoil wearing a soil price.Smell second: earthy is right, and sour or eggy means it has been standing wet.Feel last: a handful should hold together and crumble when you prod it, and if it rolls into a smooth ribbon between your fingers it is heavy clay, which is workable but is not what you thought you bought.
Then the quantity, which is where most of the money is decided. The topsoil calculator covers the part that catches everybody, which is that soil, compost and bark weigh completely different amounts for the same volume, so ordering a tonne of one and a tonne of another gets you very different heaps. If it is going under grass, the turf laying guide is the sequence that soil is the first step of, and the turf calculator does the grass on top.
Common questions
What is the difference between topsoil and compost?
Topsoil is mineral material, sand silt and clay, carrying a modest fraction of organic matter. Compost is almost entirely decomposed organic matter. The practical consequence is that compost keeps decomposing after you have used it, so a bed filled with compost slumps noticeably over the first year and dries out fast because there is little mineral content holding water. Compost is an excellent thing to mix INTO topsoil and a poor thing to use instead of it.
How deep is topsoil normally?
Anywhere from 100mm to 300mm on undisturbed ground, and effectively nothing on a lot of new-build plots, because it gets stripped during construction and does not always come back. That is worth checking before pricing anything: dig a hole in two or three places and look for where the colour changes from dark to pale. What you find decides whether you are topping up an existing layer or importing the whole thing.
What does screened topsoil mean?
It has been passed through a mesh to take out stones, roots and lumps, and the mesh size is normally stated, often around 10mm. As-dug or unscreened soil has not, so it is cheaper and it arrives with stones and root pieces in it. Screened is worth the difference for lawns and seed beds where you want a fine level surface. For filling and levelling under something else, as-dug is usually fine and the saving is real.
What is BS 3882 and does it matter?
It is the British specification for topsoil, and the grade names on a supplier price list come from it. It is a paid standard so the limits are not reproduced here, and the useful thing to know is what quoting it signals: a supplier referencing it is claiming the material has been tested against a published specification rather than dug out of a field. For a lawn or a vegetable bed that is worth asking about. For levelling a patch of ground it matters much less.
How can I tell good topsoil from bad without a test?
Colour, smell and feel, in that order. Good soil is dark brown and smells earthy. Grey, blue or orange material is subsoil, and a sour or sulphurous smell suggests it has been sitting waterlogged. Squeeze a handful: it should hold together and break apart when prodded. If it rolls into a smooth ribbon it is heavy clay, and if it will not hold at all it is mostly sand. Neither is unusable and both change what you should do next.
How much do I need per square metre?
Depth decides it and the arithmetic is straightforward: 100mm across one square metre is 0.1 cubic metres, so a 50 square metre lawn wants 5 cubic metres. What catches people is the unit, because that quantity is roughly seven tonnes, and it is sold by weight as often as by volume. Outside Britain and Australia the unit is the cubic yard, which is 0.765 of a cubic metre, so a yard of soil is about three quarters of what a metric reader is picturing.