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When I started growing orchids I bought a few books to learn about their culture.
What I understood about fertilizing was :
- fertilize with high nitrogen plants that are potted in bark,
- fertilize with a balance fertilizer plants that are potted in other media.
- use a high phosphorous fertilizer to promote flowering & root formation.
Recommended dosage was 200 PPM nitrogen (for plants in bark).
Some of the questions that came to mind were :
- what PPM nitrogen for plants in media other than bark ?
- should the fertilizer dosage be the same year round ?
Then as I read more and more about orchids and fertilizing I started being confronted with more issues :
- TDS (total dissolved solids)
- PH.
And I heard that some growers recommended the use of Epsom salts, the occasional use of a fertilizer called Mag Pro, the application in the summer of another fertilizer, Peter’s Cal Nitrate, and yet another fertilizer, Peter’s Plant Starter during potting,...
At times I got thoroughly confused.
All fertilizers contain as principal ingredients Nitrogen, Phosphorous and Potassium, which are commonly referred to as N-P-K.
The percent of these elements are shown on the fertilizer container as 3 numbers separated by dashes, always in the order N-P-K.  For example: 30-10-10, 3-12-6.
In a 30-10-10 formulation, 30 % of the contents are Nitrogen, 10 % are Phosphorous and 10 % are Potassium.
In a 3-12-6 fertilizer, 3% of the contents are Nitrogen, 12% are Phosphorous and 6 % are Potassium.
Nitrogen fertilizer is derived either from urea, ammonia or from nitrates. Nitrogen derived from urea is not readily available to orchid plants, therefore the Nitrogen in the fertilizers we use must be derived from ammonia or nitrates, not from urea.
Fertilizers are basically made of minerals which are an essential ingredient of proteins. All living things need minerals. We, humans,  find many of the minerals we need in fruits and vegetables, in meat, fish, vitamins...
Plants also need minerals and that’s what fertilizers will provide.
Nitrogen
Is an essential element for the chlorophyllian function, that is for the leaves to convert light and nutrients into carbohydrates.
Nitrogen is necessary for the plant to grow.  But excess nitrogen will cause plants to grow excessively in size and may delay flowering ( think of those Vandas ! ).
On the other hand nitrogen deficiency will result in stunted plants.
A recent study by Dr. Yin-Tung Wand of Texas A & M University suggests that “under severe nitrogen deficiency, the proteins in the lower leaves are digested and the nitrogen is transferred out of the older leaves into the younger upper leaves (note : of Phalaenopsis). As a result some of the lower leaves start turning yellow and eventually fall off”.
Phosphorous
Is believed to regulate many activities.
It is necessary for the formation of cells, it promotes root growth, it induces and stimulates flowering.
Deficiency is phosphorous will also result in stunted plants, with dark green leaves.
Potassium
Is necessary for healthy growth. Deficiency may result in dwarfness.
Micronutrients
Besides the essential elements (Nitrogen, Phosphorous and Potassium), plants need other minerals such as calcium, magnesium, iron, manganese,...usually referred to as “trace elements” or “micronutrients” because plants need them in much lower concentrations than Nitrogen, Phosphorous or Potassium.
An ideal fertilizer is one than contains the essential elements and all the micronutrients plants need.
Organic fertilizers such as guano and cow manure must be decomposed by bacteria before the nutrients they contain can be absorbed by plants (the same applies to nitrogen from urea).
Decomposition does not occur readily in the type of potting materials used for growing orchids, therefore organic fertilizers are not suited for orchids.
Also, unprocessed organic fertilizers can host diseases that will affect our plants.
For these reasons, we use, as our basic fertilizers, inorganic fertilizers.
They may be benefits in an occasional application of organic fertilizers such as fish emulsion, but we are not experienced in their use or effects, therefore we have not included them in our regular fertilizer program.
I don’t know how many different formulas of fertilizers there are, but there are a lot.  So which fertilizer(s) should we use ?
Fertilizer chart 1
Here is a chart of the fertilizers we are using in 2002.
As you can see we basically alternate between high nitrogen (10 - 5 -5 ), high phosphorous ( 3 - 12 - 6 ) and balanced ( 7 - 7 - 7 ) fertilizers.
In the beginning of the summer we make 2 applications of 15 - 0 - 0 for additional calcium, and in the summer time we use more frequently the high nitrogen.
But this is only part of the story.
Fertilizer chart 2
Because one of the other questions in our mind was should we give our plants the same amount of fertilizer year round ?  Do they need the same amount when they get 14 hours of strong light as when they get 8 hours of low light ?
I do not remember getting or finding a clear answer to this question, but it seems logical to me that orchids (and all plants) could not possibly use the same amount of nutrients on a short winter day as on a long, sunny, summer day.
We found in the August 1998 issue of the magazine Greenhouse Product News a table of illumination, by month, for the Northeast.  We adjusted this table to a weekly schedule and used it to determine the amounts of fertilizer we should give our plants. This is what Fertilizer chart 2 shows.
We can not talk about fertilizers without mentioning some other useful additives.
Superthrive
Makers of Superthrive claim their product achieves miracles. We do not know from first hand experience but we hear from a lot of people that it is very beneficial.
Robert Fuchs, a commercial grower in Florida, claims that continuous use can cause mutations, so he limits it’s use to once a month application.
Pro-Tekt
Consists of 3.7 % soluble potash and 7.8 % silicon derived from potassium silicate.
Field tests have evidenced silicon improves heat & drought tolerance.
In addition Dyna Grow (manufacturer of Pro-Tekt) claims it increases resistance to environmental stress, enhances healthier, stronger growth, and produces hardier plants.
Considering several field tests confirm the better results achieved when adding silicon, we made it part of our fertilizer program.
KLN
A rooting hormone that was developed by Dyna Grow to promote root formation on cuttings.
Dave Neil of Dyna Grow recommends to use it for 3 waterings after repotting plants.
He also recommends to use it monthly as a maintenance to promote stronger root growth.
But how much fertilizer should we put in a gallon of water.
This depends on what kind of nitrogen PPM we want to achieve.
Here is how to calculate the amount of fertilizer to add to 1 gallon of water in a watering can:










Of course an easy way is just to follow the instructions on the fertilizer container.
If you decide to follow the instructions on the fertilizer and if they give you a range, such as 1/4 teaspoon to 1/2 a teaspon per gallon, then use the higher dosage in the summer and the lower one in the winter.
A-Determine the desired nitrogen PPM. For example:
(PPM = parts per million)
B-Find out the % nitrogen in the fertilizer
As in Dyna Grow 10-5-5
C-Number of oz in a gallon
( we started saying this calculation was for one gallon of water)
D-# of oz of fertilizer required per gallon of water is
=A divided by B : 100 / 0.10 or 200 / 0.10
further divided by 1,000,000 (to get
Parts Per Million)
multiplied by C (the number of oz in a gallon).
100

10 %

128

0.128
200

10 %

128

0.25