"JRB" wrote:

Sorry, I should have wrote 'quite alkaline' soil instead of 'quite
acidic'.

This is a problem but it will show up in the color of the leaves, not
the lack of flowers. Yellowing of a leaf between dark green veins is
called chlorosis. Chlorosis can be caused by malnutrition caused by
alkalinity of the soil, potassium deficiency, calcium deficiency, iron
deficiency or magnesium deficiency. A combination of acidification with
sulfur and iron supplements such as chelated iron or iron sulfate will
usually treat this problem. Chlorosis can also be caused by nitrogen
toxicity (usually caused by nitrate fertilizers).

The most important factor in achieving vigorous growth is an acid soil
mixture high in organic content. Rhododendrons and azaleas need an acid
soil with a pH of 5.0 to 6.0, well mulched with organic material.
Rhododendrons thrive in a moist, well-drained, humus-filled soil,
enriched with peat moss or leaf mold. Prepare the soil by thoroughly
mixing equal parts of loam, coarse sand and ground oak leaves or redwood
before planting. Many commercial growers root rhododendrons and azaleas
in pure sphagnum peat, or in a 50-50 mixture of sphagnum peat and coarse
sand or perlite. A favorite mixture on the West Coast is 1/2 sphagnum
peat and 1/2 ground bark dust, but in such mixtures, plants must be fed
regularly. My favorite soil mix is a 50-50 mix of peat humus and the
natural soil. Soil around the rhododendron's shallow roots must be kept
cool and moist but well drained.

If the soil is too alkaline, acidity may be increased by adding flowers
of sulfur (powdered sulfur) or iron sulfate. I add 1 tablespoon of
sulfur powder around the base of any plant showing signs of chlorosis.
Do not use aluminum sulfate. Aluminum can build up in the soil to toxic
levels eventually. One very common source of chlorosis is when lime
leaches out of concrete, such as from a foundation or walkway, making
the nearby soil more alkaline. This problem decreases each year as the
concrete ages. An annual application of sulfur can compensate for this
problem.

Sphagnum peat or peat moss is a super soil amendment. Researchers claim
that plants planted in mixes containing sphagnum peat will resist
disease better. The sphagnum peat in the soil does regulate the
availability of water so the roots are not too wet, but also the
sphagnum is said to provide protection against disease.
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