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 are watered 3 times a week, 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 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, sandy, well drained soil, moderate water, 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, and we do have very sandy, well draining soil that never gets soggy, 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.
Dave

Watering 3 times a week is probably the problem--it can cause root rot.

Where are you?

sw florida, on the border of zone 10-11. I doubt seriously you'd get root rot in our soil. It's pretty much like beach sand, extremely well draining. Even after tropical storms that dump several days of hard rain on the area, there are no puddles, and the soil is dry to the touch again after a few days of no rain. The reason we water often during the dry season is that we also have other plants growing, such as bananas, brugmansia, roses, and papaya that are not drought resistant. We get almost no natural precipitation here from november til june, and 16 mins of sprinkler irrigation every few days certainly does not saturate the soil. It dries out quickly in this heat, especially around the citrus trees where there is no mulch. Once the rains begin in june we do not irrigate. The other reason I was hesitant to suspect the irrigation is that the tangerine and grapefruit trees also get the same amount of irrigation and same care as the minneola and cara cara, but they are as green as green can be, and loaded with rapidly developing fruit. Can you think of any other things that might also cause this? Dave

We're pretty sandy here in NW FL, but I don't water more than once a week and then only the younger trees during the fruit setting period for the most part-- my lemon never gets added water. The grass needs once a day, but I keep it to every other day. I keep the sprinklers from hitting the citrus trees. Do you have leaves dropping without the petiole attached?

Citrus trees in the tropics use the dry season to supply the stress needed to initiate blooming in place of cold stress. In Italy they withold water during the summer to force a fall bloom on lemons. Mature citrus really do not need much watering.

No, the sprinklers don't hit the trees directly, but they still do get watered. They are basically collateral damage. We bought a new home already built with the irrigation system already installed, and the sprinklers cover a broad area. The flower gardens, roses, and other fruit trees such as bananas need regular water in this heat, and I have the sprinklers adjusted so that most of the water is concentrated on these areas.
You guys are a lot cooler than us. Winters here are still 77-81, with nights in the upper 50's lower 60's, and spring and summer from mid 80's to lower 90's with nights in the mid to upper 70's.
The leaves dropping are mostly the tiny young ones that just grew, the older ones are remaining on the trees, they just have turned mostly yellow except for the veins, which are blotchy green. I have read citrus like a lower soil ph, around 5.5-5.7 perhaps? So with our soil ph in the low 7's, I used fertilizer for acid loving plants hoping this would help lower the ph. Is there something else I should perhaps use to lower soil ph? I also read a couple places that irrigating frequently with well water in sw florida may raise soil ph, if this is true , then perhaps I do need to take more drastic steps to lower the soil ph around the citrus.
Dave

Citrus need more N than anything-- You can use Ammonium Sulfate for that and it will acidify the soil. Do what you can to adjust the sprinkers off the citrus.

Clay soils tend to have higher than normal pH while sandy soils have pH on the low side.

The sandy soils tend to lose their bigger cations via rapid draining and leaching and to achieve balance, the only readily cations that remain to keep the total charge balanced are the H+ ions, so the soil tends to be acidic. Remember that pH is based on concentration of H+ ions (the negative log of the molar concentration of these ions).

Generally, soils in the tropics with abundant rainfall are acidic due to the effect of leaching, while those in the desert, they tend to be alkaline. So it surprised me that you have above normal pH. There are specific cases when sandy soils have high pH, such as sources of underground water or presence of limestones, lime or calcium carbonate deposits. Which could be the case in some Florida soils.

Perhaps, your soil is too acidic, and should be checked. Application of dolomite or small amount of lime just in case it is. But have your soil pH checked by your agricultural extension or other reputable persons, just to be sure. If your soil pH is above 7, follow advice of Skeeter.

After correcting your soil pH, amend big areas of your soil by increasing organic matter content. Mulching with manure (keep clear from trunk), compost, coir dust, etc. This will help in the buffering of your pH to keep it in the ideal range much longer, besides, it will help reduce watering too. Organic matter helps in water and nutrient retention.

Also, nematodes are a problem in sandy soils:
http://edis.ifas.ufl.edu/CH008

The soil is pure limestone sand. Lime is not needed and would raise the pH even higher. Local advice is best. Ask your local county ag agent!

Perhaps the ironite I applied a couple of times dosen't work as quickly as they say. The problem seems to be slowly working itself out, and the latest flush of growth on the affected trees was normal in color and seems to be holding up just fine. The red navel is also blooming in addition to the new growth. The minneola dosent seem to be flushing with much new growth, but it's color has deepened and no more leaves are dropping.