CF, EC, PPM 500, PPM 700, TDS… What’s the difference?

The shop has continued to stay busy, and our outdoor growing season is progressing beautifully! After a mellow winter, an early spring has arrived in Oregon, and many folks got an early start on their gardens. While the task of prepping and planting is behind us, it is no time to sit on our laurels! Staying on top of preventative spraying regimens, building support structures, and developing liquid feeding regimens are important routines to establish.

As plants continue to feed on organic nutrients from soil amendments, there is often room for supplemental liquid feeding. Some gardeners feed lightly with organic liquid nutrients, fish emulsion, liquid kelp and compost teas to foster live microbial activity in their soil, while others aggressively feed with synthetic nutrient solutions. Whatever your method may be, an important factor to consider is the parts per million or PPM of your feeding solution. This is a fancy way of referring to the concentration of your feed solution, water, nutrient concentrates, and supplements. In general, feeding solutions for clones or seed starts measure low in PPM (100-250 ppm) and steadily increase with the growth and size of plant through bloom (up to 1600-2000 ppm).

It is always recommended to start light, and steadily increase the ppm of your solution over time. During this process, “reading” your plants nutrient needs along the way is important. Why does PPM matter? As a point of reference, PPM allows a grower a greater awareness of the plant’s nutrient needs as it matures. Measuring PPM also serves as a safety measure, mainly when using strong synthetic fertilizers. An abrupt feed of strong nutrient solution has the potential to burn or even kill a plant! This can be prevented by checking the feed solution with a PPM measurement first.


There are a number of different scales of measurement for PPM, here is the nitty-gritty:

PPM measures parts per million.

PPM is known as dimensionless quantities; that is, they are pure numbers with no associated units of measurement. A mass concentration of 2mg/kg – 2 parts per million – 2ppm – 2 x 10-6.

There are many different scales used for different industries around the world and for many different reasons! Did you know there are more than two scales? The most widely used scales in Hydroponics are the 500 scale, 650 scale and the 700 scale.

CF and EC are measures of electrically-charged nutrient ions in a solution.

Pure water will not conduct electricity. Water usually conducts electricity because it is full of impurities (in our case, electrically charged nutrient ions). The two black dots on the end of Bluelab nutrient probes are called electrodes. When they are placed in a solution, an electrical current passes from one electrode to the other, counting the number of electrically charged ions present. This represents the units measured – CF or EC.

What’s the difference?

The ppm 700 scale is based on measuring the KCl or potassium chloride content of a solution. The ppm 500 is based on measuring the NaCl or sodium chloride content of a solution and is also referred to as TDS – total dissolved solids. Individual nutrient ions have different electrical effects! The true ppm of a solution can only be determined by a chemical analysis. ppm cannot be accurately measured by a CF or EC meter. They are present on Bluelab products as a conversion guide only. The conversion is the following:

2.4EC x 500 = 1200ppm (500 scale) or 1200ppm / 500 = 2.4EC

2.4EC x 700 = 1680ppm (700 scale) or 1680ppm / 700 = 2.4EC

If written instructions say you should grow your crop at 1100 ppm – how do you know which scale the author is referring to? Is the scale on your ppm meter right for the job? If the contents were compiled in the USA, it could be the 650 or 500 scale. If the book is written in the UK, it could be the 700 scale. If it was from Australia,  it could be any of the three!

If you grow using ppm, you will need to know the following:

  • What ppm scale is the book referring to?
  • What ppm scale is your meter using?
  • Which standard or calibration solution should you use for your meter?
  • What ppm scale is the nutrient formula referring to?

We have a conductivity converter that can help determine what type of ppm. If we start with a general idea of where certain crops should be grown using EC or CF as a guide, this can help determine what ppm the instructions are referring to. If you need help determining what conductivity your crop should be grown at we have a nominal value chart that may help.

What PPM scale is your nutrient formula referring to?

Manufacturer                                                         Scale

Advanced Nutrients                                                    700 scale

Botanicare                                                                     700 scale

Cutting Edge Solutions                                               500 scale

Dutch Master                                                                500 scale

Dyna-Gro                                                                       500 scale

FoxFarm                                                                        700 scale using dechlorinated tap water

General Hydroponics                                                  500 scale using reverse osmosis water

General Organics                                                         500 scale using reverse osmosis water

House & Garden                                                          700 scale

Humboldt Nutrients                                                   500 scale

Hydro Organics/ Earth Juice                                    500 scale

Miller Chemical & Fertilizer

Nectar for the Gods                                                     700 scale

Rock Nutrients                                                             700 scale

Roots Organics                                                             500 scale

Soul Synthetics                                                             500 scale

Technaflora                                                                   500 scale


Here is a handy PPM conversion chart:

[gview file=””]