Fertilizing Orchids, TDS and PPM (All You Should Know)

water and fertilizing orchids

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The sensitive air roots of orchids can get fertilizer burn or even rot if the water or liquid fertilizer is too “hard”, in other words, containing too many minerals or soluble salts.  The easiest way to understand how hard your water is is in terms of Total Dissolved Solids (TDS).

What is the best TDS level for orchids?

Epiphytic orchids have a low tolerance for soluble salts in water at a maximum concentration of 200 ppm.  The ideal level also depends on the tolerance level of the orchid species, water hardness, watering method, and growing medium.  Tap water that is too hard should be replaced by rainwater or filtered water.

In this article, I’ll explain why you should measure TDS, what it is, the best level for orchids, and how to use a TDS meter.

1. What is TDS?

Total Dissolved Solids (TDS) measure the total amount of mostly inorganic soluble salts in liquids. 

In the context of watering or fertilizing plants, a TDS measurement can tell us how many macro and micronutrients such as potassium, magnesium, calcium, sodium, etc. are in the water. 

TDS is measured in units of parts per million (ppm), or milligrams per liter (mg/L).

For example, distilled water has a low ppm level below 50, but ocean water has an extremely high TDS level, about 35,000ppm, due to a high concentration of salts.

2. Why should you measure TDS?

There are two reasons why you should measure the TDS or the strength of your liquid fertilizer.

First, orchids generally have a low tolerance for soluble salts and minerals in the water because of their sensitive roots.  Different species have different tolerance levels (more about the next part of this article).  When mixing liquid fertilizer for your orchids without measuring their level of salinity, you could easily overdose your orchid and can stunt growth and burn roots, causing root rot. 

Also, it is important to find out whether the water you use to mix and dilute fertilizers is suitable for the sensitive roots of orchids.  Even though you follow the manufacturer’s dosage and dilution instructions, the hardness of your tap water, or concentration of salts in your tap water, can vary. The tap water in your area could contain exceedingly high levels of salts.

You will be surprised that the concentration of soluble salts in most tap water is too high that is not suitable for even watering orchids, let alone fertilizing them.

3. What TDS level is best for my orchid?

The ideal TDS level for all orchids, as recommended by several Orchid Societies, is below 200 ppm. Terrestrial orchids are hardier than epiphytic orchids and can tolerate a higher level at 500 ppm.

The St. Augustine Orchid Society shows a specific breakdown of the different levels for different types of orchids.

SpeciesTDS Levels  
Excellent quality for all kinds of orchids (especially epiphytic orchids)<175 ppm
 Good quality you can grow many orchids175 – 525 ppm
Questionable quality Grow only the toughest orchids 525-875 ppm
Unsatisfactory quality Unsuitable for orchids>875 ppm
Source: Bottom (2019)

In general, epiphytic orchids have a very low tolerance of soluble salts in water. 

This is because in their natural habitat, orchids grow above ground with roots exposed to the air and attached to trees.  Mineral nutrients are hard to come by, and so their velamen-lined roots have evolved to absorb every atom they can find whether it is from the decomposed debris of organic matter on a tree or from the rain.

Although their roots have evolved to be efficient scavengers in their natural habitat, that may not be suitable for the fertilizer strength many home growers are used to.

That is why the best way to apply fertilizer on orchids is to adopt the principle of “feed weakly, weekly” to feed a low dose frequently.

4. Your water source matters

Before you dissolve the powdered fertilizer, it is important to first determine the starting TDS level of your water source as it can vary with different sources. 

4.1 Tap water

In the United States, the TDS in drinking water, including tap water, is set to a maximum of 500ppm according to the Environmental Protection Agency.

But in reality, the level can vary well above 500 or even 1000 pm due to factors such as the erosion of heavy metals from the water pipes, and agricultural runoff into wells, near coastal areas. 

One simple way to know the hardness of your water is to check for any white residue or even white flakes on the bottom of your water bottle, teapot, etc.  If there are, your water is too hard for orchids.  And you should use a water source with a lower TDS level, such as rainwater and Reverse Osmosis water. 

4.2 Rainwater

Rainwater is low in TDS, often below or equal to 50ppm as it comes from the atmosphere and is not filtered through layers of mineral-rich soil. In coastal regions, rainwater tends to have a higher saline concentration from ocean evaporation.

4.3 Reverse Osmosis water

Many orchid growers install Reverse Osmosis (RO) equipment to purify tap water or bring the TDS level close to zero and supplement it by adding Calcium and Magnesium to prevent nutrient deficiency problems.

5. Potting media, and watering habits also matter

Your potting media type and watering method can also affect the concentration of TDS accepted by your orchids. 

If you only soak your orchids or use a water retentive media such as sphagnum moss, the soluble salts can accumulate in the medium as well as the roots in each watering/drying cycle.

Also, if the potting medium is not properly flushed under running water, the salts can accumulate.  When the roots dry up, the high levels can burn roots, stunting growth or even causing root rot.

6. How to Use a TDS Meter?

These are the three steps you can follow in preparing your orchid fertilizer of the right concentration of soluble salts using a TDS meter:

  1. Know the TDS sensitivity or tolerance of your plant. With the epiphytic Phalaenopsis​ species, you’ll ideally want to reach a TDS fertilizer solution level of about 200 ppm or less.
  1. Get the TDS reading of your water source. Ideally, the water to be used should be at room temperature.  Dip the probe of your TDS meter into your water sample. Within a few seconds, you should get your reading.
  1. Mix your liquid fertilizer to the right level.  As an example, let’s say you have the MSU fertilizer formula (13-3-15) packaged by Tezula Plants or rePotme as the major source of NPK. And you would like to add additional Calcium and Magnesium (to avoid die-back or nutrient deficiency for the active growing season).

Since you already have 10 ppm of TDS in your rainwater, you will need to dissolve an additional combined total of 165 mg between the two fertilizers to make up a total of 175 ppm. 

Don’t worry about getting a special scale to measure the quantity.  Simply pinch a little bit (about ¼ teaspoon per gallon of water) of the powder fertilizer to mix into the water, and check the TDS after each time.  For liquid fertilizer, go up drop by drop and usually, it would just be a few drops per gallon. 

 TDS level
Targeted total175 ppm
Rainwater10 ppm
Cal-Mag15 ppm
NPK150 ppm
An example of adding up fertilizers to reach a targeted total of 175 ppm

TDS Meter

7. Are TDS meters accurate?

When used on their own, TDS meters may not be very accurate because they measure the electrical conductivity of charged ions in liquids, and converts that reading to TDS levels measured in parts per million. The reading can be impacted by the type of salts existing in your water sample.

However, they still offer a useful guide for home orchid growers to check the TDS levels comparatively, for example, the comparison between one liquid with another, or the different TDS levels of a liquid before and after mixing in fertilizers.

To be more specific, you can calibrate your TDS meter by first dipping it in rainwater or RO water which should give you a low reading close to or below 50 ppm.  Assuming your starting position is indeed 50 ppm, you will know that you have another 125 ppm of fertilizer to add as you add it drop by drop.

Conclusion

The TDS level accepted by orchids should be below 500 ppm, most ideally below 200ppm.  
Ultimately it depends on your orchid species, water source, and watering habit.

When mixing your own liquid fertilizer for your plants, it is important to understand the different TDS levels from different sources of water, such as rainwater, tap water, groundwater, and water filtered through reverse osmosis.  

Happy gardening!

Related

Top 10 Orchid Fertilizers: A Detailed Review (2022)

Orchid Nutrient Deficiency: Symptoms & Solutions (Pictures)

What Nutrients Do Orchids Need? (A Complete Guide)

Why You Should Foliar Feed Orchids (& How To Do It Right)

References

Bottom, S. (2019). Soluble Salts and Your Orchids. St. Augustine Orchid Society.

Hancock, N. (2018, August 16). TDS and pH. Safe Drinking Water Foundation.

Water Systems Council. (n.d.). Wellcare information for your about Total Dissolved Solids.

Barkalow, R. (2020, July 20). Do Not Trust Your TDS Meter. First Rays.

Barkalow, R. (2021, July 18). Simple TDS Meter Calibration. First Rays.

Carol loves to garden and research to help others grow their green thumb.

She is working towards her dream of living close to nature.

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