Ask me what soil is and the short answer comes first. It is rock, worn down. Then I spoil the tidy version, because rock is only about 45 percent of the volume of a decent garden soil. The rest is organic matter, air and water, and the balance between those four decides what will grow.
Two of the four you cannot change on any practical timescale. Sand, silt and clay are whatever the rock underneath and the ice or rivers that moved it left behind. Air and water share the pore space in between, and they swap places constantly.
That leaves organic matter, the smallest slice by weight and the part you control most. It is also the part most gardeners misread. People tell me their soil needs feeding when what it needs is structure. Those are two different jobs.
What soil is made of, in numbers
The standard breakdown of a productive soil is half solids and half pore space. Inside that pore space, about a quarter of the total volume is air and a quarter is water when conditions suit roots. Virginia Cooperative Extension’s gardener handbook gives the mineral component as 45 to 48 percent of volume, air and water at around 25 percent each, and organic matter at 2 to 5 percent.
Few garden soils measure out that way. Dig into ground cropped and rotovated for thirty years and the organic matter sits nearer 1 percent. I have done that on my own plot. The colour tells you before any test does. Good topsoil is dark. Worn topsoil is pale grey and sets like brick when it dries.
Roots need oxygen, and they take it from the air-filled pores. Squash the pore space and water cannot drain. Roots then sit in stale, airless ground. Plants in a waterlogged bed look exactly like plants in a drought. Wilting is wilting, whatever the cause.
Pore space also sets how fast water moves. Virginia Tech puts total pore space for most soils at 35 to 55 percent of volume. Sand has fewer pores, and they are large. Clay has many, and they are tiny, so water clings to them and drains slowly.
Sand, silt and clay: three particle sizes, twelve soil names
Soil scientists sort the mineral fraction by particle diameter. Sand is the coarse end, 0.05 to 2.0 millimetres. Silt sits in the middle at 0.002 to 0.05 millimetres. Clay is everything under 0.002 millimetres. The RHS uses the same boundaries, and anything over 2 millimetres is a stone rather than a soil particle.
I explain it to people like this. Sand is a bag of marbles. Silt is a bag of flour. Clay is a bag of talcum powder carrying a charge.
| Fraction | Diameter | Behaviour |
|---|---|---|
| Sand | 0.05–2.0 mm | Drains freely, warms early, holds little water or nutrients |
| Silt | 0.002–0.05 mm | Retains moisture, feels silky, compacts easily |
| Clay | Under 0.002 mm | Holds water and nutrients, drains slowly, sticky when wet |
| Coarse fragments | Over 2 mm | Left out of texture, still affect drainage and root room |
Once you know those percentages, you read the name off the USDA textural triangle. Twelve classes come out of it, from sand through the loams to heavy clay. Virginia Cooperative Extension gives loam as roughly 7 to 27 percent clay, 28 to 50 percent silt and 23 to 50 percent sand. There is a lot of room inside one word.
Your hands are a decent substitute for a laboratory. Moisten a handful and squeeze it out between thumb and forefinger. A ribbon past 5 centimetres means plenty of clay. A gritty sample that will not form a ribbon is sand. Silt feels like dry flour and goes slick the moment it is wet. Maryland Extension and the RHS describe the same test.
Texture you inherit, structure you can build
Texture and structure get used as if they were the same word. They are not. Texture is the proportion of sand, silt and clay, and it is effectively permanent. No amount of sand will change the class of a clay soil in a human lifetime.
The RHS worked out what it would take. Reducing the clay content of a topsoil that is 40 percent clay by half requires about 250 kilograms of grit or gravel per square metre. That is not a garden plan. It is a lorry. I have watched people fork a few bags of builder’s sand into a clay bed and then repeat the mistake twice in one season. Sand plus clay plus compaction gives you something closer to concrete than to loam.
Structure is a different conversation. Structure is how particles clump into crumbs and blocks, and it responds to management within a couple of seasons. Virginia Tech lists six structural forms, among them platy, blocky, prismatic and granular. Granular is the one gardeners want, a crumb with pores between the aggregates and inside them.
Organic matter is the glue. Root exudates, fungal threads and bacterial slime bind particles into aggregates. Earthworm casts pass through the soil as ready-made crumbs.
What wrecks structure is predictable. Working soil when it is wet, walking on beds, and tillage. Every pass of a rotovator breaks aggregates apart and burns off carbon. I gave up deep digging on my vegetable beds years ago. My back has thanked me since.
What organic matter actually does
Organic matter is a small slice of the weight of a soil, and its influence runs out of all proportion to that. Maryland Extension puts organic matter at 1 to 5 percent by weight on most soils, and reports that a teaspoon of soil can hold about a billion microorganisms.
Ohio State’s factsheet on soil microbes goes further. There are more microbes in a teaspoon of soil than people on the planet. Their counts run to 100 million or a billion bacteria per gram. An acre of decent topsoil holds tons of living organisms. That number still stops me.
The water effect is what gardeners notice first. Organic matter behaves like a sponge made of charged crumbs, holding water that would otherwise drain past the roots. Nebraska Extension reports that available water-holding capacity can double when organic matter rises by 1 percent of soil weight. On sand, that is the difference between watering every second day and watering twice a week.
Nutrients travel with it too. Organic matter holds calcium, potassium and magnesium on exchange sites, and it releases nitrogen slowly as microbes work through it. Ohio State’s decomposition figures are worth knowing. A hundred pounds of dead plant residue becomes 60 to 80 pounds of carbon dioxide, 3 to 8 pounds of living microorganisms and 10 to 30 pounds of stable humus. Most of the carbon leaves as gas. Only the humus stays.
How far you can push organic matter
Every extension account I have read says the same thing. Building organic matter is slow, and the ceiling depends on texture. Michigan State Extension puts the practical maximum for sandy soil at around 2 percent. Fine-textured soil with plenty of clay can be pushed past 4 percent, because clay protects organic matter from decomposition. The same publication warns that five years may pass before a change is detectable.
Oregon State Extension is blunt about expectations. For most soils, the maximum change to expect from a shift in management is under 2 percentage points. In a Nebraska trial with annual manure applications, organic matter moved from 1.6 to 2.6 percent over four years, a gain of 0.25 percent a year. Then it stopped rising. The manure kept going on.
That is the useful part. You are not chasing a number. You are holding a level.
Two things hold a level up. Carbon going in, and carbon staying put. Compost, manure, mulch and cover crops all add carbon. Low-till growing and keeping the soil covered slow the losses. Tillage does the opposite. It flushes the soil with oxygen, microbes burn through stored carbon, and the organic matter you spent years building leaves as carbon dioxide.
Sandy soils are the exception. They burn through organic matter faster because little clay protects it, which is why a sandy plot wants compost every year instead of every third.
Reading your own soil without a laboratory
You can learn plenty about your soil with a jam jar, a spade and twenty minutes. I run the jar test on every new plot before I plan anything.
The jar test
Half fill a straight-sided jar with soil, top it up with water, add a pinch of dishwasher powder, and shake hard. Then leave it alone. Sand settles in a minute or two. Silt takes a couple of hours. Clay can take a day or more. Oregon State’s extension guide describes the same test, and it costs nothing.
The slake test
The slake test tells you about structure rather than texture. Drop a dry clod of topsoil into a glass of water and watch it. A well-structured soil keeps its shape for minutes. A poor one collapses in seconds and clouds the water. Oregon State publishes it under that name, and it measures how much glue your soil has left.
Then find out what you are standing on. The USDA’s Web Soil Survey names the soil series under any address in the United States. My own plot turned out to be a heavy clay loam farmed for a century, which explained a lot about the first three years I spent on it. If you are still deciding where the beds should go, the beginner’s guide to starting a garden covers that stage.
For numbers rather than guesses, send a sample to a laboratory. Clemson’s soil testing service and most state extension labs report pH, phosphorus, potassium and organic matter for a modest fee. Take cores from eight or ten spots and mix them in a bucket. Most soil tests I read for friends come back with a pH problem and a potassium shortfall.
Where compost fits, and where it does not
Compost is the most reliable way a home gardener adds organic matter. It is also the most over-sold material in gardening writing. Compost is a soil conditioner first. Its nutrients sit mostly in organic forms that plants cannot use straight away, and its value is in structure, water holding and microbial life.
Apply it as a surface mulch, or fork it into the top few inches. Do not bury it. Compost below the root zone is compost doing very little, because the organisms that break it down need oxygen.
Oregon State’s garden compost work includes one figure I quote often. About half the organic matter in fresh material is lost as carbon dioxide within 60 days of incorporation. Composted material lasts longer, with roughly half gone over one to two years. Apply modest amounts regularly rather than a heroic quantity once.
Two caveats. The carbon-to-nitrogen ratio of what you spread matters, because soft, nitrogen-rich material can tie up soil nitrogen while it breaks down. Half-finished compost belongs back on the heap, not in the bed. And compost is not a fertiliser. If a soil test shows a phosphorus or potassium shortfall, no sensible rate of compost will fix it. My guide to composting at home works through the ratio and the usual failures.
The habit that has served me best is a permanent mulch of compost or leaves on every bed, topped up once a year. I keep two heaps, one working and one finishing. It does more for structure than any digging I have done, and it holds moisture through August.
Sources and further reading
- University of Maryland Extension, Soil Basics: the four components, particle sizes and the twelve textural classes.
- University of Maryland Extension, Organic Matter and Soil Amendments: organic matter at 1 to 5 percent by weight.
- Oregon State University Extension, Soil: The Dirty Secrets of a Living Soil (EM 9304): the 45 / 25 / 25 / 5 volume split, the jar test and the slake test.
- Oregon State University Extension, Soil Organic Matter as a Soil Health Indicator (EM 9251): the ceiling on organic matter change and the manure trial figures.
- Virginia Cooperative Extension, Soil and Soil Water Relationships (BSE-194): pore space at 35 to 55 percent of volume and the six structural classes.
- Virginia Cooperative Extension Gardener Handbook: the composition of a productive soil and the ranges that define loam.
- Nebraska Extension, Soil Management for Increased Soil Organic Matter (G2283): the doubling of water-holding capacity with a 1 percent rise in organic matter.
- Michigan State University Extension, Building Soil for Organic and Sustainable Farms: organic matter ceilings by texture and the timescale for change.
- Ohio State University Extension, Understanding Soil Microbes and Nutrient Recycling (SAG-16): microbial numbers per gram and residue breakdown into gas, microbes and humus.
- Royal Horticultural Society, Soil Types Explained: particle size boundaries and the behaviour of clay, sandy, silty and loam soils.
- Royal Horticultural Society, Clay Soils: the 250 kilograms of grit per square metre needed to dilute clay.
- Oregon State University Extension, Improving Garden Soils with Organic Matter (EC 1561): the loss of half of fresh organic matter in 60 days.
- Clemson University Home and Garden Information Center, Soil Testing: what a routine soil test measures and how to sample.
- USDA Natural Resources Conservation Service, Web Soil Survey: the named soil series for any address.