Diarmuid Gavin: The secret to a good garden is in the soil
Before you plant, get to grips with your soil type, which holds the key to your garden's success
In the early months of the year, our gardens get bashed about quite a bit with wind, snow, rain and temperatures which can vary within a single day from delightfully mild to decidedly frosty.
Our garden environment, however, is very forgiving; soon things will settle and before we know it a new season - the most hopeful one - will be upon us.
But at the moment, if we do venture out, the texture underfoot is likely to be a mite squelchy or even frozen.
And that's because our garden's foundation, the soil, is absorbing what the elements throw at it. So, what is soil and why is it so important that we know something about the type we have? And how can we improve our gardening methods by understanding and maybe even conditioning it?
The base of every garden is its soil. Before you plant a single species in your plot, it's best to know exactly what type of soil you have: acid or alkaline, chalky or boggy, free-draining or clay. Once you analyse your soil, you can find out how to improve it and what species will do best in your soil type. So, before we launch into the growing season and planting, let's get down to earth and examine what's in the ground.
Soil is a mixture of organic matter, minerals and living organisms, all of which contribute to the fertility of the soil. Plants need good supplies of oxygen, nutrients and water from the soil, and its general structure has a direct effect on how well these elements are retained. Plant roots are designed to take in water but also need air pockets to be available in order for them to breathe. If soil is too free-draining, the water and nutrients will leach away too quickly. The balance is quite a delicate one; a good soil is hard to come by and, once you have it, care must be taken to look after it, as compaction and digging can destroy a soil.
In general, a good soil has a loose structure, is water-retentive and has a good amount of organic matter - the blacker the soil, the higher the organic content. The main inorganic content of your soil will be made up of the bedrock that has been eroded and deposited over time - clay, sand and silt, as well as larger rock and stone. It is the relative proportions of these different-sized particles that affect the overall structure of the soil; the different-size particles stick together and determine how much air will be trapped in the resulting spaces. Clay particles are small, flat and sticky, whereas sand is larger and rounder; clay will therefore retain more water but will squash air out to produce the wet, heavy soil that we associate with a very clay soil. It is, however, great at holding onto nutrients, being less free-draining than a sandy soil. The perfect soil is an approximately equal mixture of the three - clay to hold onto nutrients, sand and silt to build good structure for air and water. Soil can be improved over the years by adding amounts of whatever element that is deficient in your mix.
So, what type do you have? If it is crumbly and dark in colour and you can see some earthworms, lucky you - your soil is good. If it is light in colour and very dry, it is more likely to be a sandy soil and will need some treatment. Mediterranean and drought-tolerant plants are best suited to a sandy soil. Clay soil will stick together if you squeeze it and retains water in the winter, making it unsuitable for those Med plants like lavender and cistus.
Organic matter is the absolute key to achieving a good soil. Organic matter, as it rots down, helps to stick the soil particles together in sandy soil. As earthworms digest matter, they move through a heavy clay soil, improving the structure. So dig in plenty of well-rotted farmyard manure, garden compost and leaf mould. Slow-release additions such as bark and shredded twigs that can be applied as a mulch and left to rot down every year will also help. Sand can be added to lighten up heavy clay soils.
One of the most important factors of your soil is its measure of acidity of alkalinity; a very simple pH kit can be found in any DIY store so that you can establish this before investing in any plants that won't suit your soil. The ideal soil on which most plants will grow is pH 6.5 to 7. If you have acidic soil, there are many beautiful plants that you can grow well, such as rhododendrons, camellias, pieris, heathers and kalmia. However, if you want to grow veg on acid soil, it's well worth adding some lime to sweeten it up a month before you start planting. If you don't have acidic soil and want to grow rhodos and azaleas, do so in pots with ericaceous compost - don't try to make your soil acidic. Matching the right plant to the right soil will produce the best results: good gardening is keeping things simple and staying realistic.
Your soil is the most important factor in the success of your garden, and investing a little time and thought into its care will reap huge rewards later.