EC (electrical conductivity) values for Cattleyas?
 

Hello all, greetings from Brazil. I started an orchid collection about a year ago, and am planning to expand it (currently some 15 Cattleyas and 1 Phalaenopsis).

My Cattleyas are grown in home-made plastic slatted boxes. I am using a substrate containig ca. 80% wood chips (from either Tabebuya spp. or Mimosa caesalpiniifolia) and 20% terracota pieces (from broken tiles or bricks), on a volume/volume basis. The medium is thus well aerated and will not stay moist very long.

These plants are watered every day, with either tap water (EC=ca. 0.2 mS/dm) or nutrient solution. Nutrients are furnished 2-3 times a week via nutrient solution, which I make up myself based on the MSU formula. The EC values of the solution for N=130ppm stay around 1.3 mS/dm. Thus, for fertigation purposes, I dilute this solution to reach about 0.4 mS/dm, which is the only recommendation for Cattleyas I have been able to find in the available literature.

I wonder whether there is a more up-to-date recommendation for Cattleyas, since the above mentioned value is described as "tentative".

Thanks in advance for your help!

Francisco S. Wechsler

Welcome to the Orchid Board!

Fertilizer needs are highly dependent on growing conditions. Fertilizer is far less important than all the other growing conditions. Watching your plants, and varying things a little to see what happens, will give you more good information than looking up recommendations.

If you are diluting from 130ppm N to about 45ppm N, your nutrient concentrations are in the range many people recommend.

Cattleyas seem to be much more tolerant of dissolved solids than some other orchids. I don't know of research demonstrating optimal EC for Cattleyas, but your EC levels are in the range commonly used. I have water with extremely high total dissolved solids. I can water Cattleyas with it occasionally without causing problems. Long-term use of it is not good. Other orchids don't tolerate my water.

Most water quality reports in the US (and the ones I have read from Europe and Canada) don't mention EC. EC cannot be extrapolated to osmolality, nor concentrations of substances, unless the substances present in the water are know, and their relative proportions. I don't think most US growers measure EC, unless they are large commercial growers. These people know exactly what they have in their water to start, and exactly what compounds they are putting into it. In that setting they can do a titration with their chemicals, measure EC before and after, then use EC measurements to decide how much to add in the future.

I would say the best would be to watch your plants.

The degree of dilution will depend on the frequency of feeding.

If they are fed daily, with every watering, I'd recommend about 25 ppm N. 45-50 is fine for application every other day, and I'd limit it to 150 ppm N for weekly, or less-frequent feeding.

Thank you, Estación Seca and Ray, for your replies. The plants seem to like the environment and several have already bloomed.