Growing with Parts Per Million (PPM)

If you’re a new grower, you might be confused about the unit parts per million (PPM).  Even if you’re an experienced grower, you may only understand this unit in an intuitive sense.  Perhaps you know from experience that your plants do well with 1500 PPM at 4 weeks into vegetative growth, but what does that really mean?  In this article you’ll learn what a PPM is, some common misconceptions about it, and how to use the unit properly.

What is a Part Per Million (PPM)?

Parts per million (PPM) is a flexible descriptive unit useful in describing proportional relationships.  A PPM is typically measured and described gravimetrically (by weight) or volumetrically (by volume) but has many other uses.  If you have a million marbles and one of them is blue, then you have one PPM of blue marbles in your marble collection.  When the concentration of something in PPM is described gravimetrically, it’s usually described as how many milligrams of the substance in question (blue marbles) are in a kilogram of the total substance (the marble collection).  So, if I have a nutrient solution with 100 PPM of Phosphorus, I have 100 mG of Phosphorus in every kG of the nutrient solution.  When growers describe concentrations of fertilizer as “#### PPM” without specifying the substance they’re describing, they’re likely referring to the estimated total weight of dissolved fertilizer ions in solution. 

What is Electrical Conductivity (EC)?

In order to understand how growers use PPM, we first need to have a basic understanding of EC and how it is measured.  Electrical conductivity can be difficult to explain in depth, so that will be the subject of another article, but we’ll start you off here. 

Electrical conductivity is a measurement of how well a substance (in this case our irrigation water) conducts electrical currents.  To measure this, growers use an electrical conductivity probe and paired temperature probe for accuracy.  The probe is inserted into the liquid fertilizer solution, and a small current is passed through the solution between two metal contacts.  The electrical conductivity is determined by how well the solution conducts/ resists this current.  Since it would be very difficult and expensive for growers to precisely determine PPM in the field, EC is used to estimate the PPM of the solution.  Ions of different elements influence the electrical conductivity of a fertilizer solution differently.  Some may hardly conduct an electrical current and won’t be easily “seen” by the EC probe.  This doesn’t mean these elements won’t affect the plants, and it doesn’t necessarily mean their real PPM is low.  Other molecules may conduct electricity very well and highly influence an EC reading, leading growers to think there’s more in solution than there is.

How is PPM estimated and empirically derived?

There are multiple PPM standards (500,640,700) and all are derived from EC.  To derive these standards, scientists put a specific substance into a solution until the solution reaches an EC of 1.0, the amount of the substance is recorded, and this becomes the standard for estimation for all ions dissolved in a fertilizer solution.  To convert from EC to one of these scales, you take the EC and multiply it by the standard (ex: EC * 700).  The 700 PPM standard scale is often used in the US and is derived by adding Potassium Chloride to pure water until the water reaches an EC of 1.0.  When this was done, it was found that about 700 mG of KCl per kG of solution was needed for the solution to have an EC of 1.  A grower using the 700 PPM scale is assuming that there’s about 700 mG of ALL dissolved ions per kG of their solution.  Without knowing the scale, a grower or fertilizer manufacturer used to describe a concentration of fertilizer, growers could make drastic mistakes by not using the corresponding scale. 

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