Potting compost

[MereDintofPandiculation]

Advice from people who know about gardening wanted! It’s usually said that you can use as potting compost a mix of two parts garden soil to one of garden compost. Why is this is? Is it

a) to make the garden compost go further
b) to get nutrients from the soil that are lacking in garden compost
c) some other reason?

And how can they be so prescriptive given the vast range of garden soil and garden compost?

John Innes potting compost is a mixture of loam (soil), compost, sand and some fertiliser. Mixing garden soil with your own compost is a pretty good subsitute, unless you have a heavy clay soil when some extra sand will help.

I have an RHS book (Essential Gardening Techniques)which says "the advantages of loam-based composts is that they retain moisture and nutrients for longer than soilless compost".

And how can they be so prescriptive given the vast range of garden soil and garden compost?
The above quotation goes on to say "the quality of the compost depends upon the quality of the loam". Note that they talk specifically about loam and not just any old soil.

Is it prescriptive or more a rough rule of thumb?

The thing I don't understand is the composts being sold nowadays are the ones described as 'with John Innes', as the various JI formulations are themselves mixes.

The thing I don't understand is the composts being sold nowadays are the ones described as 'with John Innes', as the various JI formulations are themselves mixes.
Isn't that just the marketing department doing their thing?
It's like when they describe a T-shirt as being 'made with fairtrade, organic cotton". It implies 100% of the bestest cotton but it isn't (that would be "made of"). "Made with" can mean something much less, like a mere 5%.

I realise I wasn’t very clear, I’m basically asking why I need to add garden soil to my garden compost. What specific thing is the soil adding for me?

JI is loam, peat, grit and fertiliser. I don’t use it because of the peat content. To my mind, garden compost is an adequate substitute for loam + peat, I’ll add grit or sand if I need it free-er draining, and fertiliser as needed. I’m not understanding why diluting my garden compost with garden soil makes it a better medium

If your garden compost works for you as potting compost then I'd just use it. Maybe some peoples isn't as good/mature enough?

Otoh during a period in lockdown when I had no compost (DH got rid of our compost bins when neighbours caused a rat problem, I've only got leaf mould nowadays) I used some soil to pot up some seedlings and they were ok too.

When plants grow in the ground they have a complex and symbiotic relationship with fungi in the soil. Bagged compost is sterilised and so won't have any good soil fungi in it. So adding good quality loam will help the plants by giving them access to this fungi. I would also guess that it could help with moisture retention?

Whoops. Didnt spot the original posters name. I assume you already know about plant fungi relationships @MereDintofPandiculation ?

I have a forty year old compost heap with very well rotted, lovely crumbly compost. I pot up plants just in neat compost- no added soil - and they do fine.

By “garden compost” do you you mean home composted organic material, and not bought multipurpose stuff? (We really need to invent some new names for stuff, it’s so confusing).

I can’t say I’ve heard anyone say you can pot things up in a ⅔ ⅓ mix - pretty much everything seems to assume you’re using bought bagged potting mixes. And Monty uses a range of recipes and ingredients for his home made mixes to suit the plant.

But I would imagine the original mix would be to add the texture and microbiology of home made compost to the heft and structure of soil.

I wasn’t talking about bagged compost! I was talking about my garden compost which certainly isn’t lacking in fungi and microorganisms. And it’s garden compost, not bagged compost, that the advice relates to

Yes, home composted.

I can’t say I’ve heard anyone say you can pot things up in a ⅔ ⅓ mix Most recently in a recent issue of the RHS magazine, but other places too. I’ve since found
. this I hadn’t thought it worth looking on the website - if they didn’t explain in the magazine I didn’t expect them to explain on the website - but it appears mainly to increase water retention.

You’d be lucky to find peat in any John Innés Compound now, they use ‘peat substitutes ‘ instead.

I say lucky, because I haven’t yet found one which is any good. The Horticultural Trades Association say the same.

Mine’s only 30 years. I used to use it to half fill big containers, topping up with bought compost, but when Covid hit and we were self isolating, and I didn’t think it was fair to delivery drivers to order online, I started using my compost for everything. Haven’t seen the need to go back.

Using soil would be a problem for me, there’s not often any visible!

Peat bog is a better carbon sink than forest. I certainly can’t justify it to myself to use peat for recreational gardening.

I think an awful lot of bought multi-purpose compost comes from council-run waste recycling centres. I always add a handful or two of garden soil when using it, otherwise it's just organic matter and nothing 'solid' (plus a healthy dose of living organisms )

The general advice with using garden soil in pots is to not do it. There's the introduction of undesirables present in the soil (which you wouldn't get with loam-based compost as it has been sterilised), and that the soil structure will break down in a pot and cause problems with drainage and aeration. The beneficial minerals present in the soil to start with would also tend to leach out of the drainage holes fairly quickly and not be replenished.

I haven't read that RHS article yet @MereDintofPandiculation

By the way, as a pp says, a lot of the peat-free compost you get now is produced from garden waste recycling centres. All very well, until you consider that there will also be chemicals present from treated weeds and lawn clippings, and goodness knows what else. I don't know about you lot, but I don't want to use potting compost with unknown weedkiller and other toxins in it.

But equally they'd leach out of bagged compost. That's why you fertilise plants in pots when you wouldn't fertilise them in the open ground.

I wouldn't either. But I am very clear that we need to stop destroying the world's diminished supply of peat bog for recreational gardening.

Maybe we need to work harder at changing perceptions and reducing the recreational gardener's use of weedkillers and pesticides.

And work on perception. Seen in my Facebook feed last night in a discussion on where to plant trees "Why not plant the field above the rugby pitch ... it’s a derelict piece of land to my mind" - the field in question is a wildflower meadow with ox-eye daisy, yarrow, red and white campion, black knapweed, yellow rattle ....

I agree, although what I meant is that ordinary soil contains trace mineral elements and salts such as selenium and boron that you wouldn't find in fertiliser. Everyone's soil is different though, isn't it? We have free-draining fine sandy loam here.

I have water-retentive clay much improved by years of mulching. I don't need to water in the summer, but I do have to be aware that one area of the garden is repeatedly under 6 inches of water in the winter, such is the slow drainage. I have a swamp cypress.

If I wanted to add soil to my compost mix, the only place I could get it from at this time of year is the "woodland", and that's basically leafmould, which is not what they're thinking of.
..

Everyone's soil is different though, isn't it? We have free-draining fine sandy loam here. That's one reason why I was pondering the prescriptive advice of 2 part to 1 of garden soil to garden compost (or 70:30 in the article I quoted). How can they say that when there's so much difference in people's garden soil and garden compost?

As you say, it would depend entirely on the sort of soil you have in your garden. Chances are that many RHS members reading that article will have been improving the soil in their garden for many years, so they are more likely to be able to benefit from that article, and their soil would be in good condition and relatively pest & weed free already.

I grow a lot of trees in pots, and they are usually in the same substrate for a number of years. Okay, they are bonsai trees, but they are trees all the same, and need the same sort of nutrients as other plants. Using garden soil in those pots would not be beneficial, as they need a moisture-retentive but free-draining mix. Soil would fairly quickly fill all the gaps in the substrate, thus preventing the free movement of air, and the roots would not grow through it. The tree would then up sitting in a pot with a dead airless (and either bone dry or soggy) mass in the middle, and all the roots fighting to escape round the edges. That then causes problems with watering. Ordinary soil is also the ideal medium for growing a crop of vine weevils.

I'm with you on the international need to protect peat bogs from further damage, and that we do need to find alternatives, but I'm not all that sure that garden soil is it.

but I'm not all that sure that garden soil is it. Is anybody suggesting it is? The articles on making your own compost aren’t usually coming from a “avoid peat” perspective.