Thoughts on fertilizer. - Page 4 - Orchid Board

Team Ferret · #1 09-05-2007, 07:32 PM

This question comes up often, “how much should I fertilize”, or something close. When the answers come it is everything from “weakly weekly” to a rather high dose at 125 ppm N. This has added to my confusion until I took a long hard think about it. So I am going to throw this out for discussion, but I don’t claim that this is correct.

What I am thinking is the difference is the basic culture. The volume of water, relative to what will evaporate and how it will evaporate, is key to how much fertilizer that can be added. I will throw out some examples with the understanding that this is in general and some orchids just don’t like high amounts of fert.

Bare root culture such as with Vandas. A very low volume of water will be absorbed by and cling to the roots. What isn’t absorbed evaporates completely in a short period of time relative to humidity, temperature and airflow. Any nutrients not absorbed will precipitate out of the evaporating water on the roots and be deposited on the root itself. When this residual salt reaches a level that is damaging to the roots, and I don’t know what level this would be at, the roots are damaged. So to prevent root damage on a Vanda, you would fertilize with a very weak solution.

Bark or mix that a lot of orchids grow in would allow for a bit more fertilizer because the evaporation rate is slower and the volume of solution present is greater than the example above. As moisture evaporates or is absorbed by the roots, more moisture/solution is supplied by what remains in the medium. To this the “weakly weekly” feeding makes sense because as the water evaporates the residual fertilizer remains in the medium which would be rehydrated at the following watering.

Non S/H rock culture (we could include sphag moss here), depending on the type of rock used, even more fertilizer could be used because there is more volume of solution available. Many of the “rocks” available are porous and wick solution to and away from the roots. Ill skip to S/H and this will make much more sense.

Semi Hydro culture at 125 ppm N or greater. Because there is a relative large volume of solution which is wicking up as the top evaporates would prevent the fertilizer from precipitating onto the roots until the concentration of the fertilizer is too great to continue to be suspended in the water. If we start with a volume of 1 pint of solution at 125ppm and over two weeks that evaporates to ½ pint, we should be at 250ppm, which should still easily remain suspended in the solution and not burn the roots.

I think where this gets tricky with new growers like myself is when we have a culture that can handle moderate or high doses of fertilizer but the orchid has new roots that the ends are exposed and not buried in the medium. Fert solution that remains on these roots when watering/feeding will evaporate and the salts will precipitate on the roots and burn them. I learned this one the hard way but cured it with a clean water misting after watering with fert solution.

Ok, so these are my thoughts as to why there are many answers to the question, “how much fertilizer”. If I am correct here, it will serve to explain that all of the answers are correct given that they are properly matched to the culture.

Dave