On 22/05/19 11:00, Nick Maclaren wrote:
In article ,
Roger Tonkin wrote:

Bit confused about Nicks comment on "soilless" compost. I used
a standard garden centre product Levington Potting compost with
added John Innes, what ever that may mean,

Er, did it SERIOUSLY say that? If so, God alone knows what it means,
because the marketdroids that wrote that text assuredly didn't.

This is from https://www.rhs.org.uk/advice/profile?pid=952:

"Peat-based or peat-free media with ‘added John Innes’

Adding John Innes potting media to soil-less media is sometimes done to
improve buffering, trace element content and weight of these potting
media. As John Innes potting media contain many fine particles gardeners
should be wary of adding John Innes materials to other media as the
‘fines’ may fill or block the air spaces within the media that are vital
for drainage and aeration of the root zone. This is no doubt taken into
account by manufacturers of media that is labelled as having 'added John
Innes' and formulations adjusted for adequate drainage and aeration."

I don't have an issue with that BUT what "John Innes" are they talking
about? JI1, 2, or 3 (or "Ericaceous")? And how much is being added?

John Innes compost is a class of composts, made mostly from 'soil'
(including sand and some clay). Soilless composts are made from
(traditionally) peat and (mostly nowadays) coir etc. The difference
I was referring to is that clay holds mineral nutrients far better
than the soilless material does, which is why soils without it need
so much humus (which also holds them).

Heh. I'm pleased to see you put 'soil' in quote marks. All the recipes
for JI composts call for a good percentage of "loam" - whatever that is.
And who sells it? I see "topsoil" is available from many sources, but
that is hardly a standard product.

The executive summary is that plants in pots run out of nutrients
more thoroughly in soilless than John Innes composts, so need them
replacing.

Indeed, but the main problem is not nutrients but water - turn your back
on a pot recently soaked and you'll find it dry. Water-retaining gel
helps (any idea if it helps retain nutrients, too?), but once those
soilless composts are dry, they are very difficult to wet again, even
with a drop or two of washing -up liquid added to a watering can.

--

Jeff