VX wrote in
s.com:

I still don't really understand the differences between the peat-type
and the loam-type. I've read that the latter holds water better/longer
and so plants don't need so much water, but I don't get the other
differences, such as which type holds nutrients longer, or what other
advantages/disadvantages they have.

Peat-type potting composts are entirely composed of organic matter - ie,
mostly-rotted or broken down bark, coir, or other sorts of vegetation.
(I once had some brilliant potting compost made from brewery waste -
excellent stuff!)

The ingredients can vary quite a lot. For example, kitchen waste is
mostly leftovers of stuff you fancied eating with your soft human
digestive system, so compost made from that tends to be very rich and
gooey like chocolate cake.

Potting compost made from fibrous things like straw or coir or hedge
trimmings is usually less rich and gooey, but tends to be longer
lasting. I mix compost from my wormery with a coir block or 2 to make
potting compost that is really good for hanging baskets - not so good
for long-term plantings like box in pots.

Organic matter is good at absorbing moisture and holding air too, so
roots can easily grow through it and get plenty to drink and breath.

This stuff is sort of living in a way - if you let it dry out
completely, it loses its structure, and also any minute organisms living
in it die off, so it goes all hard and it's very difficult to get it
back to how it should be.

A lot of the things used to make compost, such as peat or coir, don't
contain all the trace elements plants need to grow, so these are often
added by the manufacturers, and after a bit they get washed away or used
up.

Organic matter, being just rotted-down leaves/bark/veg/straw/whatever,
goes on breaking down gently all the time. (It is mostly just carbon and
oxygen mixed up until it becomes a sort of All Bran, so it can easily
rot down to make carbon dioxide and just drift away as a gas).


So a potful of peat-type compost will slowly get smaller and smaller as
the organic matter in it decays, and even if you feed a liquid feed,
you'll also need to top up with more compost.

Loam type compost contains organic matter (it's often made by mixing a
peat-type or garden compost with other things), but it also contains
tiny ground-up bits of rock (sand or even clay), like normal garden soil
does.

Because these bits are hard and more or less inert, they don't lose
their volume when they are dehydrated like purely organic (peaty)
compost does, they can help with drainage as the compost doesn't become
so much of a soggy lump, and they can also release trace minerals over
a much longer timescale than the fertilisers added to peat-like compost
tend to.

However, because it has these tiny bits of rock in it, loam-based
compost can be heavier than peat-only compost, so I find it can be a
pain in situations where you want lightness - hanging baskets for
example, or big pots that need to be light enough to move.

Although loamy compost does reduce in size as the organic matter breaks
down, it doesn't do so as much or as fast as the stuff that is all
organic material with no sand/loam/clay.

Thus, loam-based composts are often good for things like trees that will
be in their pots a long time with just a top dressing or a liquid feed
from time to time.

I have found that most commercial loam-type / John Innes potting
composts contain peat - if you want to avoid using peat in your garden,
you may be stuck with the 'peat-type' alternatives such as coir or bark-
based composts. These have improved a lot recently, but I think could
still be better. They often aren't rotted down enough, or they are made
of very fibrous things like bark that tend to be dry and lacking in
nutrition even when well-rotted.

This is one good reason I have found to have a wormery for kichen waste
- I can then use the worm debris to make really good peat-free potting
compost with no weed seeds in it, and the worm wee to water and feed
pots that might otherwise be struggling because the bought potting
compost in them is a bit light on nutrients.

Victoria
--
gardening on a north-facing hill
in South-East Cornwall
--