John Innes compost is comprised of 7 parts sterilised loam, 3 parts peat and 2 parts sharp sand (except for the seed and ericaceous ones) with some nutrients added. J. Arthur Bowyers ordinary general purpose contains bark fines, composted green waste, limestone and nutrients, with some peat unless you've bought peat free.

Essentially, modern, standard potting composts are made principally out of rubbish - hot composted green waste and bits of wood, with either very little or no peat, which is why most of us complain that its all lumpy and needs sieving before use. Adding 20% John Innes helps to give it a more 'soil like' and friable feel.

The all purpose is more fibrous & randomy. Thats where my pottery,glass, rusty nails & shredded plastic & lumps of wood come from!!