I planted 6 citrus trees this summer. Before I planted them I dug in old horse manure and bio char and clay into my very sandy soil. I have been feeding them Mg and iron as both of these are low in my soil tests. They are being attacked by scale. As they are baby trees I am concerned that the scale could really knock them. I have sprayed with white oil. This seems to kill the scale but only for a very short time and then they are back. I'm thinking that it is probably a soil problem. In that the trees are not growing well enough to beat the scale. Has anyone got any ideas, either to eradicate the scale or improve the health of the trees so that the scale don't effect the trees as much as they are.