 Hawaii Cooperative Extension Service
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
Department of Horticulture, CTAHR
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