I have a persistant problem with my container bound orange tree leaves yellowing. This season has been the worst. They were productive with fruit. Both are in large containers - 10 gal, and both have had plenty of ironite, maybe too much, if that is possible. I just purchased some Satsumas and it really pointed out to me how bad my leaves on the oranges looked.
My lemon tree's leaves turned yellow and dropped fall-winter 2011-2012, and did not fruit 2012. It turned out to be lack of light - it turns out they are deciduous, not evergreen, if they don't get enough light. So this past fall I sat it next to a south-facing window AND put it under plant lights for 16 hours. I may have given it a little too much light - it is blooming like crazy right now. When ever the temperature is above 50 and still rising, I sit it out on the south-facing back step so the bees can pollinate it. Since this is Colorado, and there isn't much blooming, the bees have learned to come sit on the step and wait for me to bring the tree out. It is a little weird, but quicker than hand-pollinating with a brush.
As for water - I do let it dry out a little between waterings, and I do NOT let it stand & soak when I do water it.
I started using long probe moisture meters when i had the tree nursery, and i do not think i would ever try to grow anything again without one of them. The places i have grown things have had variable soil, different amounts of sun or wind, etc, and those all make so much diffrence on the amount of water required, I just doubt that i could ever guess accurately how much moisture is in the soil one foot below the surface.
Here are pictures of my Lemon tree purchased spring of 2011.
The first sideways (oops) picture is in December of 2011 - the leaves were turning yellow and dropping off - it was almost to the point of more lemons than leaves. No blossoms or lemons in 2012. The second picture is the same tree February 2013. It was inside under lights and in front of a window until it started to bloom. Now that it is blooming, I carry it outside whenever it is warm enough for pollination. It is covered with bees in the picture.