| I must first mention that the soil
base which we use is a fine sand, quite infertile in nature, with a fairly
neutral pH. Since the sand does not have an appreciable calcareous component,
the pH can be adjusted fairly readily through the use of ferrous sulphate
and fine sulphur to the proper range of 4.5-5.5 pH along with our native
acid peat. Soils with a clay component will be much more difficult to alter
practically, and growers would be well advised to remove clayish soils
from proposed planting areas to a depth of 10-15 cm (4-6") and apply a
more suitable well-drained, sandy loam raised to a level of at least 15cm
(8") above normal grade. Sub-surface water from the surrounding calcareous
clay soil should not pass through the raised bed or the pH will rise quickly,
nullifying the advantage of incorporating the acidic media.
Dig up the bed to break up any "hard
pan" which may restrict proper drainage, and ensure the bed does not have
the ground water impeded in any way. If using a fork, remove any pernicious
weeds such as perennial thistle or quack grass which will be difficult
to control after planting. Soil from the pathways around the bed is thrown
into the bed to raise it about 10cm (3-4") and crowned slightly to ensure
quick drainage to the outside of the bed and to prevent water from puddling
around the stems. Amendments are added as follows:
Per 100 square feet (3 x 3 m or 10'
x 10')
1 6 cu. ft. bale coarse grade sphagnum
peal
5-7 bushels pine bark compost
3 bushels sedge peat (black humus)
5 1 lb. Magamp (Magnesium Ammonium
Phosphate 7-40-6) slow release fertilizer
15 tb. fine sulphur (to lower and
maintain acidity, from 7.0 pH to 5.0 pH)
1 lb. ferrous sulphate FeSO4 to lower
pH quickly and provide Iron.
These materials are added uniformly to
the bed in layers, resulting in a bed of soil and media components about
15 cm (6") in height. The bed is then rototilled or forked well, bringing
up about 5 cm (2") of the sandy loam and working the organic matter well
down so that a homogeneous mixture results, leaving a bed of 15-20cm (6-8"),
which is about the depth of the root ball of a young rhodo or azalea. Ideally,
for a bed where the plant will become a permanent part of the landscape,
the bed should be even deeper, to allow better volume for the roots to
explore over time. For our purposes the plants are in the bed for a maximum
of 3-4 years so 20 cm is adequate.
A vital fact which bears constant repetition
is the need to work the coarse organic materials well down into the bed,
to allow large pore spaces where the root ball will be well aerated. I
feel that some people confuse "aeration" with drainage. While drainage
is crucial and does affect aeration, many soils can be well drained and
very poorly aerated. Sandy soils are good for drainage, but in our location
and in many other parts of southern Ontario these sands are overlying clay
deposits which can reach saturation and not allow easy drainage. This causes
the water table to rise, restricting aeration to the roots as water displaces
the air. The type of sand predominant in our area is of a very fine texture
which, when compacted by years of settling and water penetration have become
very deficient in aeration porosity.
The root systems of Ericaceae seem
to be very sensitive to high carbon dioxide concentrations and low oxygen
levels that occur in poorly-aerated soils. The lack of large pores within
the soil due to a tight soil matrix, or through pore displacement due to
poor drainage will restrict the soil volume that the fine roots can explore.
We have found that the root systems
will go down as deeply as there is good aeration. Where the organic component
of coarse material stops in depth, so does the root system. A deep bed
of suitable texture is important to give the plants the cool, moist root
run that they desire, and also improves the plants' ability to survive
our severe winter conditions. A deep root system can delay and shorten
the midwinter period when the entire root ball and the essential water
source are completely frozen. This is especially true for the elepidote
rhodos which, having a large evergreen leaf surface, are prone to dessication
and tissue damage if the roots are frozen and the water is tied up in ice
crystals and unavailable to the plant.
Mulching applied after planting, and
maintained through the season helps to keep the roots cool and moist and
slows the freezing process as well.
The organic materials we incorporate
are very coarse in nature. The peat has fistsized chunks which do not break
down rapidly and offer a good media for the soil matrix. Plants dug up
after 2 years are found to have a myriad of roots through these chunks.
The typical peat, normally available, is milled by rototilling to a fine
powder and vacuumed from the bog for ease of packing. The fine material
is acidic and does offer exchange sites for nutrients but loses some moisture
retaining characteristics and almost all aeration potential. We pay a premium
for the coarse peat which is hand cut from the peat bogs, but find it irreplaceable
in providing proper soil conditions.
pH
The breakdown of organic matter through
microbial decomposition called "subsidence" is a problem when a lot of
organic matter is used in the bed. This is especially rapid when the pH
of the media is above 6.5, since the microbial systems responsible favour
alkaline conditions. With more rapid decomposition there is danger of severe
depletion of Nitrogen in the soil matrix. These micro-organism.' require
large amounts of Nitrogen for their life processes and are much more competitive
than the plants for this vital nutrient.
The amount of Nitrogen provided in
our bed preparation through the incorporation of the Magamp (7-40-6) is
sufficient, and we therefore use soluble fertilizers intermittently through
the year to keep the nutrients at their proper levels to encourage growth,
and also to develop flower buds on our larger sale-sized plants.
Growth phases of the rhodos should
be observed to time feedings when the plants need them most. Deficiency
symptoms are expressed often through leaf colour and lack of proper development.
Interveinal chlorosis is indicative of improper pH, which causes important
elements such as Iron (Fe) and Manganese (Mn) to become unavailable to
these acid loving plants. Pale, uniformly yellow leaves on rhododendrons
and azaleas usually denote lack of available nitrogen. Azalea leaves showing
reddish tinges also indicate nitrogen deficiency.
In general, feeding programmes mentioned
in other articles should be used in conjunction with the bed preparation
I have outlined. However, bed preparation is the single most important
aspect of growing Ericaceae well, along with site selection. If enough
forethought and work is done prior to planting to provide your acid-loving
plants with the soil characteristics they need, a lot of headaches will
be alleviated and your garden can be enjoyed as it ought to be.
Rhododendron Society of Canada, Niagara Region.
© 1999
All rights reserved (http://www.rhodoniagara.org/)
Nick Yarmoshuk
- 905 684 4703
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