pH and EC: what the numbers actually mean
Most feeding problems are not feeding problems. The nutrient is in the tank, the plant just cannot get at it. Two numbers tell you whether it can.
They measure completely different things
pH is acidity. It decides which nutrients are chemically available for a root to take up. It says nothing about how much nutrient is present.
EC is electrical conductivity. Dissolved salts conduct electricity, so the more dissolved nutrient in your water, the higher the reading. It tells you how strong the solution is. It says nothing about what is in it, or whether the plant can use it.
You need both. A perfectly mixed feed at the wrong pH is useless, and a perfectly pH-adjusted feed at half strength is a hungry plant.
pH and lockout
Nutrient availability shifts with pH, and different elements peak at different points. Iron, manganese and zinc become hard to take up as things go alkaline. Calcium, magnesium and phosphorus drop off as things go acidic. There is no single pH where everything is perfectly available, which is why you aim for a window rather than a number.
- Coco and hydro: 5.5 to 6.2, and letting it drift across that range through the week actually helps -- different elements get their turn.
- Soil and living soil: 6.0 to 6.8. Soil buffers itself far more, so chasing decimal points is wasted effort.
When a plant shows a deficiency and the feed chart says the element is there, check pH before you add anything. Adding more of a locked-out nutrient just raises your EC and makes things worse.
EC and how strong is strong enough
EC is usually read in millisiemens per centimetre. Rough starting ranges, and these vary enormously by species, media and light intensity:
- Seedlings and fresh cuttings: 0.4 to 0.8
- Early growth: 1.0 to 1.4
- Full vegetative growth: 1.4 to 1.8
- Heavy fruiting or flowering: 1.8 to 2.4
Two things to know before you use those numbers. First, measure your source water first and subtract it. If your tap water reads 0.3 and your mixed feed reads 1.5, your nutrient contribution is 1.2, not 1.5. Second, ppm is a trap. Meters convert EC to ppm using either a 500 or a 700 scale, and the same solution reads 700ppm on one and 980ppm on the other. If you are following someone else's numbers off the internet, find out which scale they used, or ignore ppm entirely and work in EC.
Input versus runoff
Reading what goes in tells you what you fed. Reading what comes out tells you what is happening in the root zone, which is the more interesting number.
If runoff EC is climbing well above input, salts are accumulating and the plant is not taking up what you are giving it -- back the strength off or water to more runoff. If runoff EC is well below input, the plant is stripping the solution and you have room to feed harder. A gap of a few tenths either way is normal.
The bit everyone skips
A pH probe is a piece of glass that drifts. It drifts a little every week, and it drifts faster in warm rooms and strong solutions. An uncalibrated meter is not a meter, it is a random number generator with a screen.
Two point calibration, once a month, takes four minutes. You need two buffer solutions -- pH 7 to set the zero point, and pH 4 to set the slope. Rinse the probe in clean water between each, never wipe the glass, and do not pour used solution back into the bottle.
EC meters need calibrating too, usually against a 1.413 mS/cm standard, though they drift far less than pH probes do.
Storing the probe
This is the one that quietly kills meters. A glass pH probe must never dry out. The membrane needs to stay hydrated or it stops responding, and once it has properly dried it does not come back.
Store it in KCl storage solution. Not tap water -- that leaches the reference electrolyte out and shortens the probe life. Not calibration fluid -- that is not what it is for. Not the cap it came in, dry. Keep the sponge or the cap wet with proper 3 mol/L KCl and a decent probe will last years instead of months.
A sensible routine
- Calibrate pH monthly with pH 4 and pH 7. Diarise it.
- Check EC calibration every couple of months.
- Top up the storage solution whenever the sponge looks dry.
- Read your source water at the start of every mix.
- Adjust pH last, after all nutrients and additives are in -- they all move it.
- Read runoff weekly, not daily. You are watching a trend, not a moment.
What we stock
We keep pH-4, pH-7 and KCl 3 mol/L storage solutions on the shelf, because they are the thing people run out of and cannot be bothered ordering. Calibration solution is genuinely our most repeatedly purchased product, which tells you something about how many growers are doing this properly.
Meters and pens are here, including the Aquamaster P110 Pro-2 combo, which reads pH, EC and temperature in one unit.
If your meter is reading strangely and you are not sure whether it is the meter or the mix, bring it into the shop and we will put it in fresh buffer for you.
