What do you use to feed fruit trees in pots?

[out in the cold]

Is horse poo any good for fruit trees?

[boundtothesoil]

Is horse poo any good for fruit trees?

I'd say 'definitely', although I'm not a 'user' myself simply because I don't have a ready source. As a newbie to this forum I'd guess that the subject has been discussed before, or at least 'the use of fresh manure verses composted manure'. However, horse manure usually includes dung, urine and bedding, whilst you're talking about straight/pure dung, and there's the additional question relevant to this thread of using it for 'pot-grown trees' rather than traditionally applying in a circle around trees grown in open soil.

One of the best sites for straightforward nutritional info on manures is:-
NPK Values Of Manures
However, I don't know of any that list the nutrient contents of pure dung as opposed to manure. Clearly there's a fairly wide range of contents, but, for arguments sake, taking a typical fresh horse manure value of :

0.7% N: 0.3% P2O5 : 0.6% K2O , expressed on a fresh dung weight basis. This means that 1 kg of fresh dung would contain 7g of N . According to Defra's fertiliser recommendations for fertigation, that's not an unreasonable annual application of N to apply to a young tree in an orchard. Similarly, it's not unreasonable for a tree growing in a 20+ litre pot, if it was the only source of N available. However, I'd be careful in the way I actually applied this to a pot, avoiding any contact with the trunk and, if possible, incorporating the dung below the top inch or so of soil/compost - to lessen the loss of N from the dung by volatilisation as ammonia. Those of a more prudent nature would, I'm sure, recommend composting the dung (6- 12 months) before applying it, thereby lessening any negative weed seed / E.coli, worming chemical, scorching effects etc. Safer definitely, but in doing this you would inevitably reduce the absolute (N,P,K)nutrient content per kg of the material you apply to the pot. That said, it should be a great compost.