Total Dissolved Solids (TDS)

Definition

Total dissolved solids (TDS) is the sum of all dissolved inorganic salts, organic compounds, and other substances in a given volume of water, typically reported in parts per million (ppm) or milligrams per liter (mg/L). Strong evidence^[1]

The official reference method is gravimetric: filter a water sample, evaporate it at 180 °C, and weigh the residue. Strong evidence^[1] In practice, growers and water-quality testers almost never do this. Instead, they measure electrical conductivity (EC)—how easily electric current flows through the water—and apply a conversion factor to convert EC to an estimated ppm value. Strong evidence^[2]

How TDS meters work

A handheld TDS pen measures EC (in millisiemens per centimeter, mS/cm) between two electrodes, then multiplies that value by a conversion factor to display a ppm reading on the screen. The three most common conversion factors are:

• 0.5 (NaCl scale): used by Hanna and most US hydroponic equipment manufacturers
• 0.7 (442 scale): used by Hach and many older or scientific instruments
• 0.64 (KCl scale): used by some laboratory and research meters

The same water sample will show different ppm readings depending on the meter's internal conversion factor. An EC of 2.0 mS/cm, for example, reads as 1000 ppm on a 0.5-scale meter but 1400 ppm on a 0.7-scale meter. The water composition hasn't changed—only the math. Strong evidence^[2] This is why comparing raw ppm values between growers using different meters is unreliable; sharing EC values is more precise. No data

What TDS tells you

In a nutrient reservoir, TDS roughly tracks the total concentration of dissolved fertilizer salts. Monitoring TDS over time gives clues about plant uptake (TDS falls as the plant absorbs nutrients) and water loss (TDS rises as water evaporates, leaving salts behind). Anecdote

In source water—tap water, well water, or incoming water to a system—TDS indicates baseline mineral content. This matters because starting with 300 ppm tap water leaves less "headroom" to add nutrients before reaching target TDS than starting with near-zero reverse-osmosis (RO) water. Anecdote

What TDS doesn't tell you

TDS is a bulk total. It does not distinguish between types of salts: nitrate from chloride, calcium from sodium, or beneficial nutrients from accumulated waste salts. Two reservoirs with identical TDS of 1500 ppm can have vastly different nutrient profiles and therefore different effects on plant growth. Weak / limited

TDS also does not measure dissolved gases (oxygen, CO₂), suspended particulates, or pH. For meaningful plant-nutrition decisions, TDS should always be paired with pH monitoring and, ideally, periodic lab analysis of the actual ionic composition—particularly if you suspect salt imbalance or deficiency. Weak / limited

Use in cannabis cultivation

Growers use TDS meters to track overall nutrient load in hydroponic and coir-based systems. Most schedules target 1000–2000 ppm during vegetative and flowering stages, though this varies by crop strain, substrate, and feeding philosophy. Weak / limited^[4]

Because TDS does not resolve individual nutrient ions, it works best as a trend indicator: watch whether TDS is rising, stable, or falling relative to your watering and feeding regime. If TDS climbs too quickly, salt may be accumulating; if it falls too slowly, water uptake may exceed nutrient uptake. Neither is captured by TDS alone, but TDS changes can prompt closer investigation—particularly with pH and, if available, electrical conductivity or lab analysis. Anecdote

Sources

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