Yellowing of a leaf between dark green veins is called chlorosis and is usually caused by an iron deficiency. Many conditions can be responsible for an iron deficiency. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency, magnesium deficiency or too much phosphorus in the soil. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts, but is in an unusable form, due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways. Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. For a quick but only temporary improvement in the appearance of the foliage, ferrous sulfate can be dissolved in water (1 ounce in 2 gallons of water) and sprinkled on the foliage. Some garden centers sell chelated iron, which provides the same results. Follow the label recommendations for mixing and applying chelated iron. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Chlorosis caused by magnesium deficiency is initially the same as iron, but progresses to form reddish purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer. The "rhododendron tonic" is, allegedly, a good formulation for rhododendron and azalea problems indicated by chlorosis. Diane Pertson, Otter Point, Vancouver Island , otterpt@macn.bc.ca wrote: I have found the following foolproof formula for chlorotic leaves or a rhododendron that isn't looking healthy: Purchase a bag of Epsom Salts crystals (magnesium sulfate) (available here in bulk at farm-and-feed outlets), about $4.00 for a 5 lb. bag - and a bottle of FULLY Chelated Iron & Zinc (this is a very concentrated liquid - the chelation means it is in a form that can be readily absorbed by the plant), about $7.00 for 1 quart; In a one gallon watering can, put in 2 Tbsp. of Epsom Salts crystals and 2 Tbsp. of Iron and Zinc liquid - fill with warm water and stir to dissolve; Sprinkle this over the rhododendron - by that I mean drench the leaves with the solution and pour the remainder around the drip line of the root ball. In 1-2 weeks, the leaves should be nice and green. You could repeat the process at this time if the leaves aren't fully green. This works even better if, a month before, you have sweetened the soil by sprinkling a little Dolomite Lime on the roots. [Note, she lives in the Pacific Northwest where very acidic soil is not unusual. Most other areas don't have that problem. Soil under conifer trees tends to be acidic or very acidic.] Very acidic soil can prevent the roots from taking up nutrients. As many of my rhododendrons are planted in very acidic soil under a canopy of giant cedar trees, I find an application of Dolomite and a light topdressing of mushroom manure in late spring is all they need. If soil is too acid, the symptoms can be the same. Very acidic soil can prevent the roots from taking up nutrients. In the western USA where many rhododendrons are planted in very acidic forest soil, an application of Dolomite and a light topdressing of mushroom manure in late spring is all they need. Sprinkle the lime on in late winter, very early spring. Don't overdo it - just a light sprinkle. If it is mid-spring, get the lime on right away so the rhododendron roots will be able to take up the soil nutrients in time for new growth. If you don't have rain, water it in well.

Some of them were planted in October, and some were planted in March. I watered them everyday for the first month or so. If I didn't, they would begin to droop. Now, they get watered 3 times per week with the water sprinkler, and I use a soaker hose on them for about 2 hours once a week.

I am not aware of any restrictions that the product advertises. See my comments on your other thread. Luis

I think it's normal. Mine (and other evergreens) do this every year about this time, but it's only a small percentage of the leaves. Then they seem to drop off quickly and you don't see any yellowing leaves again until this time next year. Eddie, that seems strange to me about adding lime. I'm not disputing what you're saying, but would like to know more....I thought you were supposed to keep lime away from evergreens to keep the ph on the low side. Could it be that they're not taking up enough calcium without the ph being higher; or couldn't you just add calcium to stop the yellowing?