My fruit trees will be planted into sandy soil that I am planning to add old horse manure to. I am thinking of using liquid seaweed extract and liquid manure made with horse manure as fertilizer should I be adding anything else?
For planting in October? Run a green manure over them too. That will create mulch and insert OM into the ground.
Yes, green manure would be good if you have time before they go into the ground. After they're in, mulch heartily with anything you can get... (although not up to the tree trunks - you probably know that already). I have found great improvements in my sandy soil just from keeping it mulched, pretty much nothing else. I am sure if I added more compost or was more active with green manure they improvements might be faster, but in the meantime I just keep it mulched with a variety of materials (weeds, grass clippings, hay, straw, manure, leaves... etc) and the soil improves itself.