In an extract from her book City Veg, Cinead McTernan shares her tips for healthy soil – from choosing the right compost to creating a wormery
It’s staggering to think that in one tiny teaspoon of soil there are, quite literally, billions of microorganisms that play a part in the soil’s life cycle. These microorganisms are hard at work, breaking down plant matter and animal tissue, creating airways and introducing nutrients, which ensure plants can access air, water and nutrients. Once you understand this, it makes sense that replenishing the organic matter upon which these vital microorganisms feed and thrive is an essential responsibility for us growers. Think of it like a drink of squash – when you have a bit left in the bottom of the glass, you’ll need to add more concentrate as well as water, otherwise the mixture will be very weak, with only the merest hint of flavour.
If you’re waiting for your homemade compost to be ready, or if you don’t have the room for a compost heap or bin, there is a great range of peat-free, organic composts available – they are more expensive than the alternatives, but I believe it’s worth the outlay. For some years now, I have also used a biochar-based compost, which helps establish roots quickly and encourages nutrients and microbial populations in the soil that noticeably improve plant health and productivity.
The go-to option for sustainable gardeners as harvesting peat from lowland bogs affects the wildlife that depends on these habitats as well as local ecosystems. With peat compost sales to gardeners to be banned from 2024, there are plenty of alternatives on offer. In the early years of peat-free, it really wasn’t a viable alternative as it dried out so quickly, but these days, recipes and ingredients have improved and it’s much more effective. Read the label and go for brands that provide plenty of information about how to use and care for the product. Look out for composts that have beneficial additives like biochar, seaweed and mycorrhizal fungi, because these all help container-grown crops.
Made from any combination of materials, such as peat, coir fibre, bark or vermiculite, these composts are specially created for containers and particular types of plant. They offer good drainage, aeration and a neutral pH. They are also light and, generally, you don’t need to apply any feed for the first six weeks.
This provides the right acidity for crops such as blueberries and cranberries that need soil with a pH level between 4 and 5 (a neutral soil has a pH of 7; alkaline soils have a pH above 7). It is readily available to buy from garden centres, DIY stores or online.
Some crops, especially Mediterranean herbs, need a very open, free-draining growing medium. Adding horticultural grit or perlite to general composts helps to provide the right conditions by improving aeration and preventing the compost from becoming waterlogged and compacted. A good ratio to use is 3-parts compost to 1-part horticultural grit.
• Fill containers with compost to about 2.5cm from the top to prevent the soil from washing away over the sides when you water.
• As a rule of thumb, one 20-litre bag of compost will fill a 30cm-diameter pot.
• It is best to use fresh compost each year if you are completely replanting a pot with new plants. But, with large containers, you could remove only the top two-thirds of compost and replace it with new compost for each planting season. Top-dressing (scraping off the top layer of old compost and refreshing with new compost) each year is sufficient for perennials that are staying in the same container until they need repotting (after three or four years of growth).
The humble earthworm is the unsung hero of the soil. Most of us are probably aware that it’s a good thing if you dig a hole in your garden and see lots of worms. However, the benefits of them taking up residence in your soil are broader than just churning it up or forming a tasty treat for our feathered friends.
The key thing to know is that earthworms chomp their way through all manner of organic matter. Once processed through their guts, this transforms into worm casts that are rich with microbial activity, have a high nutrient content and help to improve the soil’s structure and improve water retention.
If you don’t have soil in your backyard, don’t despair. You can still enjoy the benefits worms bring by setting up a wormery. Not only will they create a perfect, friable compost that’s ideal for seed sowing or potting up your plants, and produce a liquid that’s a fantastic fertiliser, but they’ll eat your kitchen waste too – cooked food scraps, veg peelings, coffee grounds and tea leaves, not to mention hair, wool and vacuum cleaner dust.
And other wonders
Nitrogen is essential in plant growth, and nitrogen-fixing plants are hugely beneficial in a vegetable garden. While this might sound a little heavy on the science, it’s actually very straightforward. Legumes – such as peas, beans and clover – are also known as nitrogen fixers, which work together with bacteria to collect nitrogen from the atmosphere and store it as they grow (they require very little nitrogen themselves). When the plant dies, it releases the stored nitrogen, which in turn, raises the nitrogen levels in the soil, from which other plants will benefit.
It is a great idea to plant nitrogen fixers next to any leafy vegetables (lettuce, silverbeet, cabbage, broccoli, bok choy, etc), which are known to require a lot of nitrogen. Deep-rooted herbs like comfrey, borage and dandelion collect nutrients from deep in the soil and hold this huge variety of nutrients in their leaves. The leaves of these herbs can be chopped off and lightly dug into your soil to act as a fertiliser for nearby plants. Be careful with comfrey in the veg-growing area, as whenever you cut a root a new comfrey plant springs up, so it can become invasive.
An extract from City Veg: Inspiration from an Urban Garden by Cinead McTernan (Bloomsbury Wildlife), which is out now in hardback and ebook.
This story originally appeared on House & Garden UK