Total Dissolved Solids (TDS)
A measurement of the combined dissolved inorganic and organic substances in a liquid, used in cannabis cultivation to gauge nutrient solution strength.
TDS is a useful but indirect measurement. Most 'TDS meters' sold to growers are actually EC (electrical conductivity) meters that convert their reading using a conversion factor — and different brands use different factors, which is why a '700 ppm' reading on one meter can be '500 ppm' on another. Learn what your meter actually measures and report EC when sharing numbers with other growers. It's more standardized.
Definition
Total dissolved solids (TDS) is the sum of all dissolved inorganic salts, organic matter, and other substances in a given volume of water, typically reported in parts per million (ppm) or milligrams per liter (mg/L). The reference method is gravimetric: evaporate a filtered water sample at 180 °C and weigh the residue [1]. In practice, almost nobody does this; growers and water-quality testers estimate TDS by measuring electrical conductivity (EC) and applying a conversion factor [2].
How meters actually work
A handheld 'TDS pen' measures EC — how easily current flows between two electrodes — then multiplies that reading by a conversion factor to display a ppm value. Three factors are common [3]:
- 0.5 (NaCl scale): used by Hanna and most US hydroponic meters
- 0.7 (442 scale): used by Hach and many older meters
- 0.64 (KCl scale): used by some scientific meters
An EC of 2.0 mS/cm reads as 1000 ppm on a 0.5 meter and 1400 ppm on a 0.7 meter. The water is identical; only the math changed. This is why sharing EC values is more reliable than sharing ppm across grower forums Strong evidence.
What TDS tells you
In a nutrient reservoir, TDS roughly tracks the total concentration of fertilizer salts. Watching it rise or fall over time gives clues about plant uptake versus water loss. In source water, TDS indicates baseline mineral content — important because starting with 300 ppm tap water leaves less headroom for added nutrients than starting with near-zero RO water [4].
What TDS doesn't tell you
TDS is a bulk number. It doesn't distinguish nitrate from chloride, calcium from sodium, or beneficial nutrients from accumulated waste salts. Two reservoirs with identical TDS can have wildly different nutrient profiles. TDS also doesn't measure dissolved gases, undissolved particulates, or pH. For meaningful plant nutrition decisions, TDS should be paired with pH monitoring and, ideally, periodic lab analysis of the actual ionic composition [5].
Sources
- Government U.S. Environmental Protection Agency. Method 160.1: Residue, Filterable (Gravimetric, Dried at 180 °C).
- Peer-reviewed Walton, N.R.G. (1989). Electrical Conductivity and Total Dissolved Solids—What is Their Precise Relationship? Desalination, 72(3), 275-292.
- Government U.S. Geological Survey. Specific conductance and total dissolved solids: Water Science School.
- Peer-reviewed Caplan, D., Dixon, M., & Zheng, Y. (2017). Optimal Rate of Organic Fertilizer during the Vegetative-stage for Cannabis Grown in Two Coir-based Substrates. HortScience, 52(9), 1307-1312.
- Book Resh, H.M. (2013). Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower (7th ed.). CRC Press.
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