Hydroponics is a method of growing plants by suspending their roots directly in a water-based nutrient solution instead of using soil. Since the plants cannot draw minerals from a growing medium, the success of the system depends entirely on the precise management of that nutrient solution. Monitoring the amount of dissolved plant food is paramount to ensuring healthy growth and maximizing yields. Hobbyists and professional growers alike rely on a specific measurement to quantify this concentration: parts per million, or PPM.
Defining Parts Per Million (PPM) and Total Dissolved Solids (TDS)
Parts Per Million (PPM) is a unit of concentration, representing the number of parts of a solute dissolved in one million parts of a solvent. In hydroponics, a reading of 500 PPM indicates 500 units of dissolved solids exist for every million units of water and solution, gauging the strength of the nutrient mix.
The dissolved solids being measured are primarily the mineral salts that make up the hydroponic fertilizer, such as nitrates, phosphates, and potassium. Because of this, PPM is used interchangeably with Total Dissolved Solids (TDS), which is the total weight of all substances dissolved in the water. For practical purposes, 1 PPM is roughly equivalent to having one milligram of dissolved substance in one liter of water.
By monitoring the TDS/PPM, a grower ensures the plants have an adequate supply of nutrition. If the concentration is too low, plants will exhibit signs of deficiency; if it is too high, it can lead to nutrient toxicity. Maintaining the solution within an ideal range is a continuous process because plants constantly absorb water and nutrients, changing the concentration over time.
Measuring Nutrient Strength: PPM and Electrical Conductivity (EC)
Although growers refer to PPM, the measurement device does not count individual particles. Nearly all hydroponic meters primarily measure Electrical Conductivity (EC), which is the water’s ability to transmit an electrical current. The mineral salts dissolve into charged ions, which facilitate the flow of electricity, meaning a higher concentration of dissolved salts results in a higher EC reading.
PPM is an inferred measurement, calculated by the meter using a conversion factor applied to the raw EC reading. Technical confusion arises because there is no single, universal standard for this conversion. Different manufacturers or regional standards use different conversion factors, resulting in different PPM readings for the exact same solution.
The two most common conversion scales are the 500 scale (used in North America) and the 700 scale (used in Europe and Australia). For example, an EC reading of 1.0 mS/cm will display as 500 PPM on a 500 scale meter, but 700 PPM on a 700 scale meter. This discrepancy is why EC is often considered the more universal measurement for communicating nutrient strength.
Optimal PPM Levels for Plant Growth Stages
PPM requirements change significantly throughout a plant’s lifecycle and vary depending on the species. Seedlings are delicate and require a very low nutrient concentration, typically 100 to 400 PPM. They rely on stored seed energy and can easily suffer from toxicity if the solution is too strong.
As the plant enters the vegetative stage, nutrient demand increases substantially to support rapid growth, typically requiring 500 to 1,000 PPM. Fruiting and flowering plants, such as tomatoes or peppers, require the highest concentration during their reproductive phase, often needing 800 to 1,600 PPM. Leafy greens like lettuce, however, prefer a consistently lower concentration, thriving near the lower end of these ranges.
Practical PPM management involves making precise adjustments to the reservoir solution. If the measured PPM is too low, concentrated nutrients are added until the target level is reached. If the PPM is too high, plain, pH-adjusted water is added to dilute the mix. Brown or burnt leaf tips are a classic sign of excessively high PPM (nutrient burn), requiring immediate dilution.