In article , Rod rodcraddoc
writes

Do you know what JI contains? I can't remember the exact proportions -
it's a long time since I made any, but I can tell you that it contains
a significant amount of peat. That's if it's made to the original JI
specification - if not it shouldn't really be called JI compost,
though trade usage seems to be to call any soil based compost JI
compost. Again the JI spec calls for loam produced in a specified
manner but most so called JI composts I've seen seem to contain any
old soil they could get hold of.

A.G.L.Hellyer gives the following John Innes mixtures in the 1981
edition of Sanders Encyclopedia:

Basic formula for potting:
Medium loam 7 parts by bulk
Peat 3 parts by bulk
Coarse sand 2 parts by bulk

A basic fertiliser is prepared separately consisting of:
Hoof and Horn meal 2 parts by weight
Superphosphate of lime 2 parts by weight
Sulphate of potash 1 part by weight

For most greenhouse plants add 4ozs of basic fertiliser plus 3/4oz
ground chalk or limestone to a bushel of basic potting mixture,
increasing the amount of fertiliser for strong growing plants.
For lime hating plants the chalk or limestone can be left out.

Basic mixture for seed germination:
Mdium loam 2parts
Peat 1part
Coarse sand 1part

To each bushel of this mixture add:
1.5ozs superphosphate of lime and
3/4oz ground chalk or limestone.

John Innes mixtures were first demonstrated and standardised at the John
Innes Horticultural Institution by W.J.C.Lawrence in the 1930s, which
explains the antiquated nature of the measures given.
--
Alan & Joan Gould - North Lincs.