Soil isn’t one thing, it’s a collection of different things that come together to make the life-giving, plant-growing ‘dirt’ that we love. We have a tendency to poison it, cover it over and generally forget that it’s there, but good soil is the heart of a good garden and something we should pay a lot more attention to.
Most soils are mineral based, made from mineral particles that are weathered away from the underlying rock. (Weathering is the collective term for various natural processes that gradually break down rock – it’s not the impervious, eternal substance we generally believe it to be). Soil is therefore a product of the local geology, and you have to live with what you get.
The texture of mineral soils depends on the levels of three classes of mineral particles. Sand particles, which are mainly quartz, are the largest – they range from 0.06 mm to 2.0 mm in diameter. Even the smallest sand particles feel gritty, and they are non-porous. Their ability to hold water depends very much on their size – coarser sand particles make soil very free-draining.
Silt particles are smaller, between 0.002 mm and 0.06 mm across, and behave very much like sand. They can hold more water, but they can’t form stable crumbs (clumps of soil particles) and can be hard to manage. Some silt particles are less inert than sand and act more like clay.
Clay particles are the smallest, with a diameter of less than 0.002 mm. They are made from various minerals, many of which have surface electrical charges. Most of these charges are negative, and so clay particles tend to attract positively-charged ions (cations). Positively-charged ions in the soil include many of the plant nutrients we have already encountered, and they are far less likely to leach out of a clay soil than from something more sandy. Clay soils are also very good at holding onto water – but some of it is held too tightly to be available to plants.
Each type of soil has advantages and disadvantages. Sandy soils heat up quickly in spring and resist waterlogging, but they are more likely to lack nutrients. Silty soils can be hard to manage, but retain more water. Clay soils can be waterlogged in winter rains and take a long time to warm up in spring. They are harder to work, but retain more plant nutrients.
You can get a feel for what kind of soil you have by digging up a small sample and seeing how it feels. Sandy soils are always gritty. If you wet a silty soil it will feel silky in your hand, and the more clay your soil contains then the easier it will be to roll it into a ball or a sausage.
Loam is an ideal mineral soil, consisting of a mixture of each kind of particle in proportions that make it perfect for gardening. But the idea of soil improvement is not to try and turn your soil into loam, it’s to try and create the best soil environment possible given what you have.
Topsoil is the uppermost layer of soil, consisting of mineral particles, organic matter and the mini-beasts and microorganisms that make their living there. It’s also home to most plant roots, and is a nice nutrient-rich environment with water and air and spaces for roots to creep into. The depth of the topsoil layer depends very much on the local geology and the history of the garden. You could have no more than a scattering of topsoil, or more than a foot.
Underneath the topsoil is the subsoil layer. It’s a less inviting zone, with more water and less air. Less things live there, but some plants do send down roots to mine for water and nutrients. Again, the subsoil depth depends on location. Underneath the subsoil is the parent rock.
There are various structural problems that can affect your topsoil and the plants that grow in them, which mostly occur as a result of the way we abuse our soils. Compaction is a big issue. Healthy soils are not solid, they contain spaces filled with air and water. Compaction (either through the use of heavy machinery or the pitter patter of football-playing feet) fills in those spaces and makes the soil far less hospitable. Relieving the compaction improves plant growth, an effect you may well be familiar with if you have a lawn.
Soils left bare can suffer from the weather – sandy soils will have all of the nutrients leached out by heavy rain. Dry topsoil is very susceptible to erosion, literally being washed off or blown away on the wind. Clay soils sometimes benefit from weathering – traditional advice is to leave clay soils bare over the winter to allow frost to break down the heavy clumps for you. Damaged soils can also form a cap, a solid layer that makes it hard for seedlings to grow or plant roots to penetrate.
Traditional advice also recommends digging or cultivating your soil, which helps you to create a fine tilth (texture) suitable for seed sowing and mix in bulky soil improvers like well-rotted animal manure. You may prefer the No Dig approach, which keeps soil covered and protected from the weather. No Diggers avoid treading on their soil, to avoid compaction, and leave soil improvers on the surface for earthworms to distribute.
There are advantages and disadvantages to both approaches, and the one you adopt is a personal choice – the point here is that your soil is a living thing and needs to be treated as such. Chemical fertilizers and pesticides damage the microorganisms that help build soil; constant cultivation can damage soil structure. Keeping your soil healthy is the key to growing healthy plants.