Sterilised Soil – What is it for?

I’m a Mum, a granny, a gardener, a beekeeper, a dog owner and I keep pigs too.  I’m personally not keen on trying to create sterile environments because I think a bit of dirt is good for us – unless it’s in a hospital, doctors, dentists or a veterinary surgery. The concept of using sterilised soil in a garden is, to my mind, a bit odd.  However, when it comes to growing specialised crops or to scientific research, I can see that sterilised soil has a purpose.

What is Sterilised Soil?

Sterilised soil is or growing medium that has been treated with heat or with chemicals.  The process aims to kill off any soil microbes and plant material within it.

Why does soil need to be sterilised?

Some soil microbes can cause disease in plants.  Small seedlings are especially vulnerable to disease, especially if they’re being raised in the warm moist conditions of a greenhouse.  There is one train of thought that suggests that sterilised soil produces stronger plants.

On the other hand, there is an intricate balance between soil microbes and plants.  Some of the miniscule life-forms in soil actually help plants grow.  Some fight off the “bad” microbes and keep diseases under control, others capture nitrogen from the air and turn it into plant food.

When it comes to seeds however, I can see why anyone would want to neutralise them.  When you’re raising plants from seed – especially if you’re doing it commercially – you want to know that what you sow in the soil is going to mature as planned.  You don’t want to spend weeks cultivating plants that actually turn out to be weeds.

Farmers in the UK need to use herbicides to kill weeds in their field crops.  In glass houses and poly-tunnels, the use of weedkillers can theoretically be reduced to zero by growing in sterilised soil.

What does sterilised soil cost?

Because it’s been carefully treated, commercially available sterilised soil is more expensive that ordinary topsoil.  Which begs the question – do you really need it?

I grow flowers, fruit and veggies in my garden.  Some of them in raised beds, some in planters, some in the poly-tunnel and some directly into the garden soil.

If I need to buy soil, I’m perfectly happy to use a good quality topsoil in my garden and poly-tunnel.

What soil should I buy?

For turfing, I would use the best quality topsoil I could afford but don’t see the need for it to have been sterilised.  My lawncare regime will dispense with any weeds that dare to germinate in my new lawn.  I know that professional greenkeepers will disagree, but what they’re doing is so specialised that it warrants having the degree of control that is afforded by using a sterile growing medium.

For the veg patch, I value the ecosystem between plants and soil microbes and actually think that the more creatures are living in the soil, the more life it can potentially support.  Of course that includes weeds; but I find that using weed suppressant fabric or mulching with bark or compost helps to control them.

For wild flowers, a low nutrient soil is essential but again, it needn’t be sterile.

For starting off seeds, yes, a sterile growing medium is probably a good idea and there are plenty to choose from in the garden centre.  Try to choose a peat-free one if you can, it’s ultimately much better for the environment.

Recommended general purpose soil

This is my favourite – it’s teaming with nutrients, stone-free and because it is so dark-coloured it warms up quickly.  An uncle long-since-passed once described this soil as “kind” and I think that sums it up beautifully.