We spend a great deal of time thinking about what grows above the ground. The roses, trees and herbaceous borders naturally command our attention, while the soil beneath them is rather easier to overlook.
Yet much of what makes a garden flourish begins below the surface.
Healthy soil is not simply earth for anchoring roots. It is a complex, living environment containing minerals, organic matter, fungi, bacteria, earthworms and countless other organisms. Between its particles are tiny spaces that hold both air and water – two things every healthy root system needs.
When that balance is right, plants have a remarkably good foundation from which to grow.
The difficulty is that garden soil doesn’t always begin in ideal condition. Building work may have left it compacted. Heavy clay can become sticky in winter and hard in summer. Very sandy soil may drain so quickly that plants struggle for moisture. Years of cultivation can also leave soil short of organic matter.
Fortunately, soil can be improved. Not by trying to turn it into something entirely different, but by understanding what you have and working with it.
What is soil structure?
Soil structure describes the way individual particles of sand, silt and clay come together to form small clusters, or aggregates.
Between these aggregates are spaces through which water can drain, air can circulate and roots can grow. Good structure also creates the conditions in which earthworms, fungi and other soil organisms can flourish.
Pick up a handful of healthy garden soil and you can often see and feel the difference. It tends to crumble rather than forming a solid lump and has a loose, open quality that allows roots to find their way through it.
Compacted soil has far fewer of these useful spaces. Water may sit on its surface rather than draining away, roots can struggle to penetrate it and oxygen becomes limited.
At the other extreme, very sandy soil may have plenty of air spaces but struggle to retain sufficient moisture and nutrients.
The aim isn’t to create some imaginary perfect soil. It is to help the soil you already have function as well as possible.
First, get to know your soil
Before attempting to improve it, spend a little time understanding it.
Take a handful of moist soil and rub it between your fingers. Sandy soil feels gritty and falls apart readily. Clay soil feels smooth and sticky and can often be moulded into a shape. Loam contains a more balanced mixture of particles and usually feels crumbly and workable.
It is also worth noticing what happens after heavy rain. Does water disappear readily, remain in puddles or run across the surface? In summer, does the ground become hard and cracked, or does it dry almost as soon as the rain stops?
Your existing plants can provide clues too. Some thrive in free-draining ground; others naturally favour heavier, moisture-retentive soils.
Understanding these characteristics means you can improve the soil appropriately rather than simply adding products in the hope that something will work.
Organic matter: the gardener’s great ally
For most garden soils, one of the best ways to improve structure is to add organic matter.
Garden compost, leaf mould and well-rotted manure can all play a part.
On heavy soils, organic matter helps create a more open structure, allowing better movement of air and water. On light, sandy ground, it helps the soil hold on to moisture and nutrients for longer.
It also provides food for the extraordinary community of organisms living beneath the surface.
This is important because improving soil isn’t simply a mechanical process. Earthworms pull organic material into the ground and create channels as they move. Fungi extend microscopic networks through the soil, while bacteria and other organisms help decompose organic matter and cycle nutrients.
Feed the soil and much of the work begins to happen naturally.
Make compost part of the garden
A compost heap is one of the most useful features a garden can have.
Grass clippings, suitable soft prunings, annual weeds that haven’t set seed, fallen plant material and uncooked fruit and vegetable scraps can gradually be transformed into rich organic matter.
Once mature, homemade compost can be spread over beds and borders, used as a mulch or incorporated where soil genuinely needs cultivation.
There is no need to dig it deeply into established borders. Spread over the surface, organic matter can gradually be taken down by worms and other soil organisms, causing far less disturbance to the soil beneath.
It is a pleasingly simple cycle: material produced by the garden is returned to the ground to help the next generation of plants grow.
Don’t overlook autumn leaves
Leaves are sometimes treated as an autumn nuisance, gathered into bags and removed as quickly as possible.
In fact, they are an extremely useful resource.
Allowed to decompose, fallen leaves produce leaf mould – a beautiful, dark material that can improve soil structure and moisture retention.
Leaves can be collected into a simple wire enclosure or stored in suitable bags with ventilation and left to break down. Depending on the leaves and conditions, the process can take a year or more, but it requires very little attention.
Nature is in no particular hurry, and good soil rarely is either.
Mulch and let the soil work
Mulching is perhaps one of the easiest ways to care for soil.
A generous layer of garden compost, leaf mould, composted bark or well-rotted manure spread over bare ground helps protect the surface from heavy rain and drying sun.
It also suppresses many annual weeds, reduces moisture loss and provides a steady supply of organic material as it breaks down.
Spring and autumn are both useful times to mulch, although it can be done whenever the soil is moist and conditions are suitable.
Keep mulch away from the crowns of plants and don’t heap it against the trunks of trees and shrubs. A thick collar of damp organic material around woody stems can cause problems rather than prevent them.
Elsewhere, leave the soil organisms to do much of the incorporation for you.
Green manures: plants that work for the soil
Bare ground is vulnerable ground.
In vegetable gardens, allotments and newly prepared areas, green manures can be grown when the soil would otherwise be left empty.
Plants such as clover, phacelia, field beans, vetches and certain mustards can help protect the surface from erosion, suppress weeds and add organic material.
Some legumes also work with bacteria in their root nodules to fix atmospheric nitrogen. The benefit to subsequent crops depends on how that green manure is managed, so it is better to think of these plants as part of a wider soil-care system rather than simply as free fertiliser.
Traditionally, green manures were dug into the ground. They can also be cut down and left on the surface as organic material, avoiding unnecessary disturbance of the soil.
Choose the green manure according to your soil, season and what you intend to grow afterwards – particularly in the vegetable garden, where plant families and crop rotations may matter.
Protect the soil from compaction
Improving soil structure is only half the story. We also need to avoid damaging it.
Walking repeatedly over beds compresses the soil, squeezing out the spaces needed for air, water and roots. The problem is particularly acute when the ground is wet.
One of the simplest improvements you can make is therefore not to walk on cultivated soil unnecessarily.
Paths through larger borders, stepping stones and beds narrow enough to reach from the sides all help. Raised beds can be particularly useful in a productive garden because they make it possible to tend the soil without standing on it.
If the ground is sodden and sticks heavily to your boots, it is usually telling you something: today may not be the day to work it.
Do we need to dig?
Generations of gardeners have dug their plots, and there are still occasions when digging is useful – particularly when creating a new bed, dealing with severe compaction or undertaking major changes.
But established soil doesn’t necessarily benefit from being turned over year after year.
Repeated cultivation can disrupt soil structure and disturb the organisms living within it. In many established beds and borders, applying organic matter to the surface and allowing natural processes to take over is both easier and gentler.
This doesn’t mean adopting rigid rules. Gardens rarely respond well to those.
A border on compacted ground after building work has very different needs from an established bed that has been mulched for twenty years. The important thing is to cultivate because the soil needs it, rather than simply because digging has always appeared on the gardening calendar.
Work with clay rather than fighting it
Clay soil has a rather unfair reputation.
Yes, it can be heavy, sticky and difficult to work when wet. In dry weather it can become remarkably hard. But clay soils are often naturally fertile and can hold water and nutrients extremely well.
Regular additions of organic matter can make clay easier to work and improve its structure over time.
What clay doesn’t appreciate is being cultivated when saturated. Working very wet clay can smear and compact it, making matters worse. Wait until it is moist but workable.
And resist the temptation to believe it must somehow be transformed into sandy loam. A good clay soil is valuable in its own right – and many wonderful garden plants thrive in it.
Help sandy soils hold on to moisture
Sandy soils present almost the opposite challenge.
They tend to warm quickly and drain freely, which is an advantage for many plants. But water and nutrients can pass through them rapidly.
Organic matter again provides much of the answer. Compost, leaf mould and suitable mulches improve the soil’s ability to retain moisture while continuing to preserve its naturally free-draining character.
Mulching becomes particularly valuable during dry summers because it reduces evaporation from the soil surface.
Plant choice matters here too. Rather than continually trying to persuade moisture-loving plants to tolerate dry ground, include species that naturally appreciate free-draining conditions.
Sometimes improving soil means changing the soil. Sometimes it means changing what we ask of it.
Soil improvement takes time
There is no bagged product that will transform garden soil overnight.
Real improvement is slower and, ultimately, much more interesting.
Add organic matter each year. Keep bare ground covered where appropriate. Avoid unnecessary compaction. Allow fallen leaves to become useful again. Grow plants that suit the conditions and pay attention to how the soil responds.
After a few seasons, the changes can be surprisingly noticeable. Ground that once felt heavy may become easier to work. Sandy beds may hold moisture for longer. Earthworms appear in greater numbers and plants establish with less intervention.
You begin to recognise healthy soil not just by looking at it, but by the way it feels in your hands.
The garden beneath the garden
There is something wonderfully grounding about caring for soil.
Unlike a flower border, its transformation isn’t always immediately visible. Much of the work happens quietly, beneath our feet and out of sight.
But the results eventually appear everywhere.
They are in the roots of a newly planted tree, the resilience of a border during a dry spell, the earthworms beneath the mulch and the plants that return each spring a little stronger.
Perhaps that is why improving soil is one of the most worthwhile things a gardener can do.
We may design and tend everything that grows above ground, but a flourishing garden begins with the world beneath it.
Every good garden begins below the surface.
Further Reading: My Garden Journal & Notes
