STANDARDIZED POTTING MIX FOR MANOA
Over the years we have evaluated a number of
components and amendments for use in a soilless mix for potted plant
production in our foliage plant program in the Department of Horticulture
shade house at Manoa This is to satisfy one of the requirements for
exporting potted plants out of Hawaii. These studies have evolved into a
standardized mix for our research protocol, which has resulted in the
production of high-quality plants of the species being grown. A summary of
the accumulation of these studies is being presented here to provide those
wishing to standardize their production with some guidelines. We know that
this works for our growing conditions here on Oahu, but it may need some
modification for other growing conditions.
We are using peat moss and perlite as the basic
components because of availability, in a 1:1 ratio by volume because of
convenience of measuring small quantities of mix. It has been determined
that other ratios (from 1:4 to 4:1) work equally well, and thus the ratio
may be adjusted depending on the perlite grade (coarse, medium or fine),
the peat moss quality, and the crop to be grown. Screened black cinders of
appropriate size can be substituted for perlite. Trials to date with other
organic materials, especially compost, have not given satisfactory
results.
The emphasis for the fertilizer program has been on
controlled-release products, as this seems to be the most logical method
for container production in Hawaii. To date, the only products that have
given consistent high performance in the soilless mixes have been those
with approximately equal proportions of nitrate and ammonium forms of
nitrogen. Osmocote 18-6-12 has become our standard for comparison with the
newer controlled release fertilizers evaluated. Research trials have
consistently shown that the optimum rate seems to be slightly higher than
recommended by the manufacturer, or about 1.5 times more for the
production of plants under saran under Hawaii conditions. Caution:
Production under full-sun conditions will result in higher growth medium
temperatures, faster release rates, and damage to the root system due to
higher levels of soluble salts.
The rates suggested in Table 1 are for long-lasting
formulations for long-term crops.
Table 1. Fertilizer amendments for a 1:1 (volume:volume)
peat:perlite mix for the production of foliage plants under saran shade.
grams/cubic ft
Osmocote 18-6-12
143
or
Osmocote 17-7-13
199
or
Nutricote Type 270 20-7-10 123
and
Dolomite (AG65)
85
Micromax
28
Treble superphosphate
17
Normally, we find that the resin-coated controlled-release fertilizers
last slightly less time under Hawaii conditions than they were designed to
last under Mainland conditions. If short-term crops are being produced,
the release time of the fertilizer should be selected accordingly. These
rates do not take into consideration the use of additional fertilizer
applications, such as applied by an injection system.
Of the various minor-element mixes evaluated, Micromax has
given us good results and is used at the manufacturer's recommended rate.
Liming trials here and elsewhere have demonstrated that pH
is not critical in productions systems without soil. We have found that
good-quality plants can be produced at a pH of 4.0 in a peat-perlite-based
medium. Dolomitic lime is added at reduced rates to provide calcium and
magnesium as nutrients rather than to adjust pH. Excessive lime in the
medium can result in reduced plant growth, even though symptoms are not
expressed.
The use of superphosphate appears to be optional,
because the literature suggests that it is readily leached from artificial
mixes and is apparently supplied in adequate amounts in the
controlled-release formulations. However, we do not have any research data
to support this, so it is often added until its requirement can be
verified.
While this may not be the most economical production
medium, we are confident it provides for the production of consistently
high-quality plants.
These fertilizer materials can be weighed out in
advance and used as needed. Incorporation of the slow release fertilizer
at mixing has given the best plant growth for most species, but if the
potting mix is to be stored for some time after mixing, it would be better
to make a "top-dress" application at potting time to avoid
release of soluble fertilizer salts.
Fred D. Rauch, rauch@hawaii.edu
Paul K. Murakami, pmurakam@hawaii.edu
Department of Horticulture, CTAHR
University of Hawaii at Manoa
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