Persephone wrote:
I have an azalea in a large pot on the E. side of the house
(So. Calif coastal), that has turned chlorotic. I gave it
iron twice, but has had no effect; leaves still have that
tell-tale appearance.
Chlorisis (yellowing of the leaves between green veins) is a symptom of
poor nutrition, but does not reveal the source. It 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 or magnesium deficiency. 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. DO NOT USE ALUMINUM SULFATE; IT KILLS
RHODODENDRONS AND AZALEAS. Foliar sprays of iron sulfate or chelated
iron can reduce symptoms. 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. There is a tonic that remedies some cases of chlorosis.
Diane Pertson, Otter Point, Vancouver Island, 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. 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.
--
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Cheers, Steve Henning in Reading, PA USA Zone 6