In years past, we’ve all heard about feeding orchids weakly weekly or diluting the fertilizer to 1/4 strength. These were probably good generalizations at the time. Several months ago, however, the Atlanta Orchid Society had a wonderful speaker, Jan Szyren, who gave the society much more specific information about feeding orchids. Not only did we learn that we should use a basic fertilizer (that is, a fertilizer with nitrate as the nitrogen source), we also learned that plants, including orchids, will grow best if fed at a nitrogen concentration of 125 ppm. Jan introduced us to the Michigan State University Reverse Osmosis fertilizer (MSU RO fertilizer), which was designed to be used with low alkalinity water, the type of water that Atlanta has. Because low alkalinity water has low levels of calcium and magnesium, the MSU RO fertilizer is supplemented with calcium and magnesium as well as with the micronutrients needed by orchids.
Many AtOS society members are now using the MSU RO fertilizer, and Jan suggested that ½ teaspoon of fertilizer be mixed with each gallon of Atlanta water. Jan also mentioned that she is experimenting with higher concentrations and her initial results appear good. After reading the Bill Argo articles about pH and plant nutrition, I’d like to give you some additional information that might be useful.
Using the MSU RO fertilizer and diluting it at ½ teaspoon per gallon, the nitrogen concentration will be 105 ppm. If you use 1 teaspoon of MSU RO fertilizer and a gallon of water, the nitrogen concentration will be 210 ppm; so, you should probably add between ½ and 1 teaspoon of the MSU RO fertilizer to each gallon of water so that you get somewhere between 100 and 200 ppm nitrogen. Little information exists to guide us in deciding what is the correct nitrogen concentration for orchids. Jan suggested 125 ppm nitrogen because this seems to give the best result for plants in general. Jan suggested that orchids be fed with every watering and that every 4th or 5th watering be without fertilizer so that salts (like sodium) do not build up in the orchid mix.
You might look at the plants in your collection to decide if you should feed at 125 ppm or try something a little higher (200 ppm, for instance). Large orchids in active growth or orchids that we typically think of as heavy feeders, such as Cymbidiums, might benefit from using nitrogen concentrations close to 200 ppm. More delicate orchids, such as many Pleurothallids or seedlings, probably will do better at 125 ppm. My impression is that feeding at a nitrogen concentration of 300 or 400 ppm or higher with every watering is probably too high. Remember, no one knows for sure what is best so you will have to use some judgment along with personal observation of your orchids to decide what is appropriate. For now, I plan to feed at every watering with a nitrogen concentration of 150 ppm and omit fertilizer every 5th watering or so.
Determining the fertilizer concentration
Since other fertilizers are also used, I would like to give you an example of how to calculate the amount of fertilizer to dilute in water to give you the desired ppm nitrogen. First, you need to know the nitrogen concentration of the fertilizer. The nitrogen concentration of the fertilizer is the first of the three numbers. In the case of a fertilizer that is 20-10-10, the nitrogen concentration is 20%. Here are the steps to determining the amount (in volume) of fertilizer to add to water:
Step 1 Multiply the desired nitrogen concentration by the gallons of fertilizer you want
Step 2 Multiply the percent nitrogen in the formula by 75
Step 3 Divide the value from Step 1 by the value from Step 2.
Here’s the formula:
Amount of fertilizer to add = (Desired nitrogen concentration) x (# gallons of water)
(% nitrogen in the fertilizer) x (75)
Here’s an example if you want 125 ppm nitrogen as the final concentration in 1 gallon of water and your fertilizer is 20-10-10:
Amount of fertilizer to add to 1 gallon of water = (125) x (1) = 0.083 ounces
(20) x (75)
If you want a higher or lower nitrogen concentration, replace the 125 in the preceding formula with the desired nitrogen concentration.
Here’s how to convert 0.083 ounces to a volume measured in teaspoons.
Volume in teaspoon = 0.083 ounces = 0.4 tsp
Therefore, if you are using a fertilizer that is 20-10-10, just a little less than ½ teaspoon in a gallon of water will give a fertilizer concentration close to 125 ppm nitrogen. You could easily round this off to ½ teaspoon and the concentration will be just a little above 125 ppm. If you want a nitrogen concentration of 200 ppm, add about 3/4 teaspoon of the 20-10-10 fertilizer. If you added 1 teaspoon, then the nitrogen concentration will be about 320 ppm so you can now gauge how to dilute the 20-10-10 fertilizer to give the amount of nitrogen you think you should feed your orchids.
If all of this confuses you to know end, call the manufacturer and talk to one of their technical representatives to find out how to dilute the fertilizer to get the desired ppm nitrogen. And still another option is to look at Part 5 in the Bill Argo articles on pH and plant nutrition. Part 5 will be in September 2004 AtOS newsletter and on the AtOS website. The bottom of Table 1 gives the amount (in teaspoon) of different fertilizers that can be added to 1 gallon of water along with the corresponding concentration in ppm nitrogen. In this table, the MSU RO fertilizer is the 13-3-15-8 Ca-2 Mg. The table also lists other fertilizers, too.
Practical Advice About the Concentration of Fertilizer
Here’s the practical advice I promised:
# Feed your orchids at 125 ppm nitrogen (or a little higher) with every watering, skipping the fertilizer with every 4th or 5th watering in order to flush salts from the orchid mix.
# If you are using the MSU RO fertilizer, use between ½ and 1 teaspoon of fertilizer for each gallon of water,
#Use ½ teaspoon of MSU RO fertilizer for orchids that are “light” feeders,
#Use 1 teaspoon of MSU RO fertilizer for orchids that are “heavy” feeders,
#If you are using another fertilizer, use the formulas in this article to calculate how much fertilizer to add to a gallon of water to get the desired nitrogen concentration,
#Call the manufacturer and have them tell you how much fertilizer to add to a gallon of water to get the desired nitrogen concentration,
# Look in Part 5, Table 1 of the Bill Argo articles (on the AtOS website and in September 2004 AtOS newsletter) to find the amount of fertilizer to add to 1 gallon of water to get the desired nitrogen fertilizer.
The next article in this series will describe using pH and EC meters to measure the pH of the orchid mix and the fertilizer concentration.