Having just purchased a tree fern ( Dickinsonia) I am at a loss as to understanding the potting on instructions i.e. eracacous feed and John Innes nos 3 compost mixed with leaf mould . Having looked at the J.I. nos 3 instructions it says not suitab

Mix your own J I NO. 3
7 parts good soil, 3 parts peat, 2 parts sharp sand.
To each wheelbarrow full add 12 ounces bone meal, 12 ounces superphosphate, 6 ounces sulphate of potash (no lime) this makes J I NO. 3.

I don't know about the soil requirements of these, but I do know that the trunk is in fact a root and you should water the trunk as well as the soil in dry weather and when you're getting it established.

I have 3 of these and in the past I have had them grow successfully in pot of multi purpose compost. F?or the last 3 years they have been planted in my garden in a semi shaded and wind free position. They thrive in this situation.
During warmer weather, water the whole trunk and the crown. During the winter stuff the crown with some straw and wrap in a horticultrul fleece.

Please look at my photos to see how they look at different seasons.