It’s easy to think of soil as one thing, or to not think of it at all. But the ‘dirt’ as Americans like to call it is a vibrant ecosystem, made of many different things that come together to give it – and our plants – life. Humans tend to suffer from ‘plant blindness’, concentrating on the animal life in our world and relegating the plants to the backdrop. Whilst gardeners redress that balance, I think that most suffer from ‘soil blindness’, not realising that the key to good gardening is in the soil. Certainly as a species we are content to poison our soil, cover it over and generally forget that it’s there, but it’s something we should be paying a lot more attention to.
With the exception of peat soils that are found in natural bogs and wetlands, and which can be up to 50% organic matter, soil is mineral based. It is made, over long periods of time, as mineral particles are worn away from the underlying rock by various natural processes. For gardeners, trying to change your soil on a large scale would be like trying to move a mountain.
The texture of your soil is determined by the size of the mineral particles. Sand particles are the largest, and feel gritty. They’re non-porous, and their ability to hold water in the soil depends on their size. Coarse sand particles make for very free-draining soil.
Silt particles are smaller, and so hold more water, but they still act very much like sand particles. They don’t have what it takes to hold clumps of soil particles together – that’s the job of clay particles, which are the smallest. They’re formed from different minerals, many of which have electrical charges. Negatively-charged clay particles will attract positively-charged ions in the soil. There’s no need to get bogged down in the chemistry, but these include many of the important plant nutrients, which are less likely to wash out of a clay soil than a sandy one. Clay soils are also much better at holding onto water, but they can hang on so tightly that not all of it is available to plant roots!
Pros and cons
Although there is such a thing as a ‘perfect’ soil (loam) for gardening, you’re unlikely to find it in your garden. We all have to live with soils which have advantages and disadvantages. Sandy soils don’t get waterlogged in winter, and heat up quickly in the spring, but they are likely to lack nutrients and to require a lot more watering in summer.
Silty soils can be hardy to manage, and clay soils can waterlog in the winter and bake hard in the summer. They take longer to warm up, but hold more food for your plants.
To get a feel for your soil, you can dig up a small sample to see what it contains. A gritty texture means a high proportion of sand; silty soils feel silky when wet. A lot of clay makes it easy to form your wet sample into a ball or a sausage.
You can also test the pH of your soil, which tells you how acid or alkaline it is. Most vegetables and fruits enjoy a fairly neutral soil (pH 6-7), but if yours is more acid or alkaline then there are plants that will enjoy those conditions – and it is possible to change them in small areas if there’s a fussy plant you simply must have. (Although sometimes it’s easier to grow them in containers.)
Given that you can’t change the underlying nature of your soil, gardening is about managing your soil to get the best results, and learning to live with what you have.
Most plant roots grow in the uppermost layer of soil, the topsoil, which is where the organic matter is found, along with mini-beasts and microorganisms that help to break it down and make nutrients available to plants. It’s here that there’s water, but also the air that plant roots need. The depth of your topsoil depends on the area’s history; you may have more than a foot, or have to make do with a couple of inches.
Below the topsoil is the subsoil. It’s less inviting, wetter and colder. Fewer creatures live there, but some plants do send down deep roots to mine the subsoil for its mineral content. Again, the depth is determined by the local geology and the history of land use. Underneath the subsoil is the parent rock.
A lot of the things that gardeners and farmers do to their soil aim to improve one of the major aspects that it under our control – the soil structure. Compaction is a common issue, whereby the topsoil is crushed so that all of the spaces for air and water are squeezed out. Roots have trouble penetrating, and when they do there’s nothing there for them to find. Compaction can be caused by heavy machinery, but it can also be caused by feet. The traditional way to relieve compact is to dig (or plough, if you’re a farmer), but it can also be improved – more slowly – with No Dig methods.
Soil that is left bare over winter can really suffer. Heavy rain can easily wash plant nutrients out of sandy soils, and dry topsoil of any kind is prone to being whipped away by the wind, or washed away by sudden rains. Damaged soils can form a cap, a hard surface layer that seedlings find impossible to break through. Keeping your soil covered over winter, with plants or mulches, avoids these problems and reduces weed issues at the same time.
The number one thing you can do to improve your soil is the same, whatever type of soil you have – add organic matter. Organic matter works miracles in the soil. It can simultaneously help to prevent waterlogging (by improving the soil structure) and hold on to water to make it available to plant roots. It feeds all the bugs and beasties in the soil, which in turn will make nutrients available to plants. Organic matter in clay soils stops them baking so hard in dry weather, and makes them much easier to dig.
The ideal solution is to make your own garden compost, and apply it to the soil when you’re digging, or as a mulch that the earthworms will incorporate for you. It’s very hard to overdo adding organic matter, and compost never goes far enough in the garden! If you’re lucky then you might be able to find local sources of animal manure, which need to be properly composted before they’re added to soil, but you do need to know they haven’t been contaminated with weedkillers that may affect your crop. Well-rotted organic manure is also available to buy in bags, and is a great addition to areas of the vegetable patch where you’re planning to grow hungry crops, or to the rose garden.
For areas where the soil doesn’t need such heavy feeding, leaf mould makes an ideal low fertility soil improver, and is easily made from fallen leaves in the autumn, although it does take a couple of years for them to break down.
Ultimately, soil management is a balance between improving your soil to make the best possible growing environment for your plants, and choosing the plants that will be happy in your soil. Large-scale alteration of the soil composition or pH is time consuming, expensive and ultimately temporary, and is best confined to small areas such as the vegetable garden.
Emma Cooper has been gardening, and blogging, since the dawn of the new millennium. She’s utterly smitten with edible and useful plants, and is never happier than when she’s in the garden, up to her elbows in compost. She’s in the process of building a new garden, and you can follow her progress on her gardening blog, The Unconventional Gardener.