Note:  These figures refer to the contribution of the major components only, so any minors or trace elements in the fertilizer will add to the TDS, as will any solids dissolved in your water.  For example, Dyna Gro "Grow" formula is labeled as 7-9-5, which yields an N-P-K contribution of 199 ppm at one teaspoon per gallon, but if you add the other ingredients in that complete formula, the total contribution jumps up to 235 ppm.

Nitrogen Management

As I learn more about orchids and their nutritional needs, it becomes apparent that there are two primary factors to be considered: availability of a range of nutrients, from the macro- to the micro- and trace elements (see this for more info), and the amount of nitrogen that is supplied.  If you use a decent, complete fertilizer, the first criterion is satisfied, so you only have to focus on the nitrogen loading.

Studies at Texas A&M have shown that for phalaenopsis growing under their greenhouse conditions in bark-based media, nutrient solutions providing about 225 to 250 ppm N is optimal for growth and flowering.  Considering the lower light flux and average temperatures in my greenhouse in Pennsylvania, plus the varied collection I have - from "heavy feeding" vandas to phrags that don't like a lot of dissolved solids in their irrigation water - I shoot for a considerably lower target, more in the range of 100- to 150 ppm nitrogen.

This simple calculator below allows you to plug in the nitrogen content of your fertilizer and your target nitrogen loading to determine the amount to be used when mixing your solution.  Note that this was designed for liquid fertilizers, but don't let that concern you.  I did a bit of research and calculation that suggests that using this estimator is reasonable for both powders and liquids.  Click here if you'd like to read about it.

Please enter the label formula Nitrogen number for your fertilizer (the first one) and the target nitrogen loading, in ppm, you desire.