Orange Trees - advice needed

I'd say too much water, or infrequent watering, are probably the most common cause.

Too much water transported to the fruit, and they can't grow quick enough, so the swelling flesh causes the rind to split. Let the roots get dry then water it heavily and you get the same effect.

Similarly, over-feeding also makes the fruit try to grow quicker than they can.

The opposite to over-watering can have similar results, as water in the fruit is either evaporated by heat or is drawn back into the tree, resulting in shrinkage which again may cause cracking.

If you've got a pool and the soakaway when backwashing is near the trees can also result in them getting too much water. Mind you, you're also likely to kill them off because of the chemicals in the pool water.

Thank you, Jim, for your advice.  I don't have a pool so the water they get is either rain or from the tap.  They don't just crack, they split in half.  So it is either too much or too little water, how do I know?

I've never known of citrus fruit splitting in half like yours, so can't help I'm afraid.

Time to ask at a garden centre I think. 

i am afraid i cannot help,but out of interest  ihave noticed a close neighbour ( french and part timer ) has the same with his orange tree this year.

It's both too little and too much. Too little in that you are not watering frequently enough and too much in that the tree absorbs a lot of water all at once and the fruit swells and splits.. If your soil drains well it is almost impossible to overwater. The trees will take on as much as they need. You should let the soil dry out a little between watering, but it should still be damp down a few inches. Make sure you soak the ground when you do water. Make a little well aournd each tree so the water does not run away, but soaks in. Good luck