Two things leave the reservoir and they leave at different speeds
Water goes out through the leaves and off the surface. Minerals go out only through the roots, and only in the proportions the plant is actually taking up. When water leaves faster than minerals, whatever is left behind is more concentrated than what started, and the meter reading climbs. When minerals leave faster, the solution thins out. Neither is a fault. It is just what a reservoir does.
The ratio between those two rates is the one number that describes the whole behaviour, and it is what this calculator calls the uptake ratio. A ratio of 1.0 means minerals and water leave in exactly the same proportion they are present in, and EC holds perfectly steady. Below 1.0 the solution concentrates. Above 1.0 it dilutes. Real values move around constantly with the crop, the stage, the light and the temperature, which is why the honest way to use this page is to measure your own ratio over a week and then project forward.
Measuring it is straightforward. Fill the reservoir, record the volume and the EC. Do not top up for a few days. Then record the volume left and the EC again. The salt mass at the start is the starting EC times the starting volume, and the salt mass at the end is the ending EC times the ending volume. The fraction of salt that disappeared, divided by the fraction of water that disappeared, is your ratio.
Why the meter lies to you after a top-up
If you top up to full and then measure, you are measuring diluted solution. The reading looks stable week after week while the underlying trend does whatever it is doing. The high point in a top-up cycle is immediately before the top-up, when the water is at its lowest and the minerals are at their most concentrated, and that is the moment a root system experiences the strongest solution it will see.
Measure before topping up. It takes the same ten seconds and it is the only reading that shows the trend. If you want both, take one before and one after and record the pair, because the gap between them is a direct readout of how much drift is happening in a day.
Plain water or solution
The arithmetic for holding the feed portion of EC level is unexpectedly clean. To keep it constant, the top-up should be at the uptake ratio times the current feed strength. If the crop takes minerals at seventy percent of the rate it takes water, top up at seventy percent strength and the feed portion stays where it is. Topping up at full strength when the ratio is below 1.0 is how EC climbs; topping up with plain water is how it falls away.
| Uptake ratio | What is happening | Plain water top-up | Full strength top-up |
|---|---|---|---|
| Well below 1.0 | Water leaving much faster than minerals | Feed portion drops slowly | Feed portion climbs steadily |
| Around 1.0 | Roughly balanced | Feed portion drops | Feed portion roughly holds |
| Above 1.0 | Minerals leaving faster than water | Feed portion drops fast | Feed portion still drops |
Notice what is missing from that table: the source water minerals. Every top-up with plain water brings in another dose of whatever the supply carries, and the crop takes very little of it. Those minerals accumulate no matter what strategy you use, which is why they are modelled separately here and why the final reading is split into a feed portion and a source portion. On a high-mineral supply that accumulation can become a substantial fraction of the reading within a week or two.
When to change rather than top up
There is no fixed interval and this page does not suggest one. What the model shows is the shape of the problem: as the run goes on, the reading becomes a less and less reliable description of what the solution actually contains, because the accumulated fraction grows and the balance between elements shifts in ways conductivity cannot see. At some point the number on the meter stops meaning what it meant on day one.
Practical triggers people use are a drift beyond what they are comfortable with, an accumulated source-mineral fraction that has become a large share of the total, pH that will no longer sit still, or simply a schedule. What all of them have in common is that they are decisions about a specific crop in a specific setup, informed by readings rather than by a number from the internet.
Reservoir temperature belongs in the same conversation and is not modelled here. Warm solution holds less dissolved oxygen and the root zone changes character quickly, so a reservoir that sits in the warm exhaust of a lighting rig behaves very differently from one on a cool floor. If temperature is what is moving, the fix is upstream of any dosing decision.
Related pages
To mix the solution in the first place, and to stop the ppm scale confusion from wrecking the target, use the nutrient solution EC and ppm calculator. The water loss rate that drives everything here is transpiration, and whether the air conditions permit it is the subject of the VPD calculator. For a reservoir that is really an irrigation supply feeding zones on a timer, the irrigation zone calculator covers the delivery side, and for the actual water volume of an odd-shaped tank the pipe volume calculator and aquarium volume calculator both handle geometry this page assumes you already know.
One safety line that belongs on every page in this group: nutrient concentrates and pH adjusters are corrosive. They go into water rather than water into them, they never meet each other undiluted, and they stay off skin and out of reach of children.
Questions people ask
Should I top up with plain water or with nutrient solution?
It depends on your uptake ratio, and the arithmetic gives a clean answer once you have measured it: the top-up should be at roughly the uptake ratio times the current feed strength to hold that portion level. If minerals leave at seventy percent of the rate water leaves, a seventy percent strength top-up holds steady. Plain water lets the feed portion fall; full strength lets it climb. What neither choice fixes is the source water minerals arriving with every top-up, which accumulate regardless. That accumulation is what eventually forces a change rather than another top-up.
How do I work out my uptake ratio?
Stop topping up for a few days and take two measurements. Record the volume and EC at the start, and the volume and EC when you come back. Multiply volume by EC at each point to get a figure proportional to the salt present. The fraction of that salt which disappeared, divided by the fraction of the water which disappeared, is the ratio. If you lost a quarter of the water and a sixth of the salt, the ratio is roughly 0.67. Do it again a fortnight later, because the number moves with the crop stage, the light and the temperature, and a value measured on a small plant will not describe the same plant at full size.
Why does my EC keep climbing even though I feed the same recipe?
Almost always because the crop is taking water faster than minerals and the top-ups are at or near full strength. Every top-up replaces the water that left and adds back minerals that did not, so the concentration ratchets up a little each day. The second contributor is the source water, whose minerals are barely taken up at all and accumulate with every top-up regardless of strength. The third possibility is that the reading is misleading rather than the solution changing, which happens when readings are taken at inconsistent times relative to top-ups or at inconsistent solution temperatures. Measure before topping up, at a consistent time, with a freshly calibrated meter.
How often should I change the reservoir completely?
There is no universal interval and this page does not offer one. What it can show you is when the reading has stopped describing the solution: when the accumulated source-water fraction has become a large share of the total, when the drift has gone further than you are comfortable with, or when pH will no longer hold. Those are the signals people actually act on. The interval that follows from them depends on the crop, the reservoir size relative to the plants, the source water quality and the season, and a schedule copied from someone with different water will not transfer.
Does this model pH drift as well?
No, and pH behaves quite differently from EC. It moves with which ions are being taken up rather than with how much water has left, it is buffered by the alkalinity of the source water, and it can swing within a single day rather than over a week. A reservoir with low alkalinity moves fast and one with high alkalinity resists adjustment. Because the mechanisms are unrelated, a model built for concentration would give you a false picture of acidity. Measure pH directly and often, and handle any adjusters as the corrosive chemicals they are: into water, never the reverse, and out of reach.