On Nov 12, 6:21 pm, "mark" wrote:

A.N.Other volunteered to get me some compost for my cuttings. They couldn't
find any specifically for cuttings and came back with John Innes number 1
and a warning for me not to moan if it was the wrong one. As if! I haven't
used this before and it was quite nice to do so. Carrying it about, not so
nice.

A decent quality JI No 1 is ideal for rooted cuttings and seedlings.
It doesn't 'run out of steam' as quickly as soil-less mixes, which is
important when you are potting at this time of year, since you don't
want composts running out so soon as to force a too-early repot.

So my question is what are the relative merits of loam based and peat/peat
substitute composts?

It's raining cats 'n dogs and I've not looked in for a while so here
goes for a long one I think:

There are two main types of compost: soil-based (predominantly John
Innes) and soil-less, which encompass both peat-free and peat-based
composts.

Purely from the plant's perspective, good quality loam (soil) based
composts are much better because they can hold on to their nutrients
and traces for much longer and release them to the plant as the plant
needs them. Loam also acts as a buffer so that over or under-feeding
is less damaging to the plant and helps prevent 'salts build-
up' (excessive concentration of released nutrients in solution),
which can damage or kill susceptible plants. It releases moisture to
plants more slowly, but does not dry out as easily as soil-less
compost.

Plants that are expected to remain in containers for more than a
season or so-called 'heavy feeders', really do best in these types of
composts because they have the durability and nutrient absorbing and
retaining qualities that soil-less compost do not possess. If you see
any of your 'gardening heroes' using anything other than loam based
compost for potting up long-term plants, demote them in your
admiration immediately! They demonstrate a singular failure to
understand the basic principles.

The disadvantages of JI composts are that they relatively expensive
nowadays, heavy to move about and less pleasant to handle if you're
fussy about your finger nails. They are less free-draining than soil-
less composts so over-watering can be an issue if you're not skilled
with the watering can. Some consider them to be 'cold' composts, but
that is also very true of any wet soil-less compost. Good ones take
some tracking down since the quality of the loam is all-important and
really good loam is now almost in the 'hen's teeth' category. The
cheapest types seem to use what is little more than sterilised 'dirt',
so beware if you're the sort with cobwebs on your purse strings.

The compost should be friable (nice and crumbly) and open, without
resembling black or very dark 'dust'. In use, it shouldn't form a
solid mass (pan down) and it ought to retain its open structure for a
very long time. If you buy a very cheap 'John Innes compost', it may
have been made with substandard ingredients and perform poorly. You
can't see or feel what's in the bag, so it’s pot-luck until you get it
home. If you've bought a good one you've hit the jackpot.

Pure peat-based composts are light, easy to use, drain quickly and
fairly consistent, hence their dramatic rise in popularity in the past
30 years. The best are always made from sphagnum moss peat since it
is more porous, longer lasting and does not 'pan down' as quickly as
those made with the cheaper sedge (black) peats. It is completely
devoid in any plant nutrients so these have to be added as well as the
traces.

The earliest peat-based composts were pretty poor because early slow
release fertilisers behaved erratically depending upon temperature and
the amount of watering. Times have moved on and the latest slow
release fertilisers are infinitely better; providing a progressive
release of nutrients as temperatures rise, thus reducing the risk of
salts-build up in the compost. Really good peat-based composts are
amongst the best you can use, but they still do not have the
durability of loam based and are therefore only suitable for short
term (one season) plants.

Peat-free composts are the answer to the eco-friendly gardener's
dreams. They derive their bulk and organic matter from composted
organic waste or non-peat (bark, coir etc.) alternatives and go some
way to satisfying anyone's conscience about gardening activities
affecting the environment. There are no performance advantages over
soil-based or peat-based composts, but their use helps you ‘do your
bit’ for the environment. All well and monumentally good, but they
are heavier to use than peat-based and can 'waterlog' with even
greater ease than loam-based. Nutrient balance seems to have been a
‘bete-noir’ with peat-free, although many modern peat-free composts
appear to have this sorted.

Of all of the compost types, they are shorter lived, decomposing
rapidly in containers and plants growing in them require just a bit
more careful management if they are to remain vigorous and in good
health. There are some very good peat-free composts that almost match
their peat-based counterparts, but dire ones are still around so you
really need to know what you’re about. If you buy this type cheaply,
you really do buy at your (plants') peril.