Help me rejuvenate an old apple tree

IronSquirrel

So on top of a multitude of reasons to dislike our landlord, it appears her need to make everything look a certain way meant laying huge paving slabs and pebbles around a century-old apple tree.

The slabs are in a circle surrounding the tree, and they stop maybe a meter out from where the branches above them stop. After that, it's 5cm of pebbles to stop any benefit from rotting foliage.

It's got all kinds of signs of age- stripped bark here and there, knobbly bits and so forth. Now we may be leaving in six months, who knows, but I'd like to give it a feed.
I've been told the slabs have compacted the soil, preventing the tree from getting much out of the soil, so I thought I'd start there. I'll take off a few slabs, leave some gaps here and there for the tree roots.
I don't care for the fruits, I just like to give it the right ratio of nutrients for a fighting chance.

What do people recommend? Expensive, or cheap internet hacks welcome 
Is there anything I can make maybe from the mountain of lawn cuttings I've got?


Thanks

fidgetbones

A liquid feed would go through the gravels.

IronSquirrel

What sort, 
I've just checked the cupboard, I've about 250 ml of tomorite and half that of Miracle Gro organic performance plant feed,

and a regular supply of about a bin liners worth of lawn clippings from now til end of summer.