Not all soils are the same! Therefore, the correct soil care is essential for your garden to thrive.
Garden soils are traditionally divided into three camps though nearly all of them contain various quantities of all three.
The divide is along the grounds (;D) of soil particle size with clay being the smallest, silt is medium and sand the largest. This is the key to understanding how they function in your garden.
Sand particles being very large have many gaps between them even when compressed. Sand lets water and air move through them very well which is perfect for plants and why we call them ‘light soils’. However they dry out very fast and then becomes loose and easy to move around.
Clay particles are extremely small. When they are compressed in the ground there are very few gaps left between them. This can be a problem if there is no space for air and water. Clay holds water very well, permanently if it isn’t allowed to dry out*, and it takes a long time to do so. They are ‘heavy soils’ to move around when wet and hard to dig when dry.
Silt soils are between clay and sand in size and effect in the environment.
They are all made from particles of rock eroded by water, ice and other means, and transported thousands of miles by glaciers, rivers and the wind. They take thousands of years to break down, getting smaller and smaller over time. For this reason clay soils are often rich in mineral plant food.
How To Know What Soil Care You Need
You can test what kind of soil care you need in several ways. The easiest thing to do is see if you can roll a small amount into a sausage. Officially known as the Sausage Test; high clay soils will easily soften and mold in the hand. Much easier to do when damp. Sand soils do not hold their shape well even if wet. Another way of testing is to place a teaspoon of your soil in a jar of water, add a drop of dish soap and wait. After a small amount of time light sand soils will create a layer at the bottom of the jar and relatively clear water above. Heavy clay soils take a long time to form a layer on the bottom with many particles remaining suspended in the now murky water indefinitely.
Most plants prefer a balanced mix of all three soil types, giving their roots oxygen to breathe (as we would say, it has ‘good drainage’) and water when they need it. A mix of sand, silt and clay with a good amount of organic-rich compost thrown in should create a soil that acts like a giant brown sponge for your plants; soaking up water when it is available and releasing it slowly when needed.
What is crucial and often overlooked when caring for your soil, is the need for oxygen. Clay soils are particularly vulnerable to being compressed when they are wet, squashing out both water and air from spaces that have been created very slowly over time by plant roots, growing, dying and rotting, and by animal activity at all scales.
It takes a very long time for clay soils to regain their original textures once compressed. It creates badly drained conditions suitable only for plants that like bog lands and swamps. Moss is a key indicator of problems with soil drainage and clay compression.
London happens to be famous for its clay – it sits on a vast bed named after the city which was created by the Thames over millions of years. Most of us have a foot or so of top soil before we reach the clay bed in our gardens though it varies locally.
Even with a deep top soil London gardens will likely remain high in clay content and it is highly recommended you minimise walking or digging these wet soils especially in winter.
It is a good idea to make use of stepping stones of any description when gardening beyond the reach of a path.
Over time you can decompress garden soils by adding to the surface organic, well composted matter, mulching with dead leaves, bark and sticks and doing everything else you can to encourage worms. Earthworms are, along side the many other species that live in the soil, natural tunnelers helping to move air and water around. Many species actively pull dead leaves and plants underground into their burrows, consuming and fertilising the soil as they dig and turn it.
It is also possible to add sharp sand and grit particles over time as you plant in your clay-heavy garden. Spring bulbs are often very happy sitting on something to help their drainage underground, and it wouldn’t hurt to add sharp sand or grit to the compost added to the planting holes of most species.
It is possible also in difficult cases to buy soil amendments such as lime or gypsum that may change larger areas more rapidly if needed.
It is always a good idea to take care of your soil and garden it as you would your plants. Treat it to your compost, dig carefully, if not at all**, and it will reward you with strong, healthy and resilient garden plants.
*Clay ‘puddled’ by animal feet, including intentionally by humans, creates ponds naturally if the squashed clay surface never dries and ‘cracks’.