Need help... orange trees yellowing badly
This year the leaves on my orange trees in the backyard have started yellowing, and most of the new growth has been turning yellow, and a lot of new leaves are dropping off. The older leaves remain on the tree, but the new growth is looking twiggy and threadbare, with no fruit set this year.
I have had the trees planted for 3 years now, they are 7-8 feet tall and they have always looked green and healthy and borne fruit in the past. I inspected them for pests, and didn't find any signs of aphids, scale, mites or other pests, some leaves had leaf miner damage, but that seems to be pretty normal in this region. A soil ph test revealed that my soil ph was in the 7.2 - 7.4 range, so I began fertilising this spring with a fertiliser for acid loving plants, and applied Ironite in the hopes that this would help the yellowing. So far, it didn't. The trees recieve water fom the sprinklers 3 times a week for about 15 mins, and there is a 6-7 ft area free of grass or mulch which extends out to the drip line around each tree, so that the top layer of soil dries out well between waterings. They recieve full sun. I have a grapefruit and tangerine tree in the front yard, and they receive the same care, water and sunlight as the orange trees in the backyard, soil conditions are similar, only the trees in the front yard are showing no signs of yellowing at all. Their new growth is light green, the older leaves are dark green and glossy, and the fruit is developing well.
Does anyone have any advice on what I'm doing wrong or what this might be caused by? I've heard that citrus don't need that much water, but unfortunately the same irrigation system supplies other plants that do not like to be dry at all, the soil is basically what they call sugar sand, very well draining, dosen't hold water well, and for the past 3 years it seems to have had no ill effects. The water is usually cut back to once or twice a week during the cooler periods, as less is needed then. Any tips would be appreciated.