What the two readings mean
pH describes the acidity of a solution and affects nutrient availability. EC measures electrical conductivity, which relates to the dissolved ions present. In a reservoir, those ions come from fertilizer, the source water and any adjusters you add.
An EC meter cannot distinguish nitrogen, potassium or sodium. Two solutions with the same EC can have different compositions, so a normal reading does not prove that nutrition is balanced. Use EC to follow concentration changes; use the recipe to know which nutrients you supplied. Record both if you want to understand a problem later.
EC units and the ppm problem
Guides commonly express EC in mS/cm. A reading of 1.2 mS/cm is equivalent to 1,200 µS/cm. Check the display units before comparing your meter with a table. An apparent difference of a thousand times may just mean the two readings use different units.
Many TDS meters estimate ppm by multiplying conductivity by a conversion factor. At a factor of 500, an EC of 1.2 gives 600 ppm; at a factor of 700, it gives 840 ppm. These are examples of conversion, not two different solutions. Until you know the factor, ppm readings from different scales cannot be compared directly. Recording EC avoids that ambiguity.
Find a range for your crop and stage
Use the feeding program for your crop, stage and growing method. A mature tomato's target should not become the target for young lettuce. For basil, an example reference range is EC of 1.0–1.6 mS/cm and pH of 5.5–6.0. Compare reference values with the recipe you are using.
Also check where a recommendation applies: the reservoir, irrigation water or an extract from the growing medium. Those measurements are not interchangeable. Seedlings, mature plants and fruiting crops can have different feeding requirements even within the same system.
A small pH change within the working range does not automatically need correction. Repeatedly pushing the reading back to one exact point can add unnecessary salts while distracting you from the cause of the drift. Follow the range specified by your feeding program rather than trying to hold a display perfectly still.
Prepare and calibrate your meters
- Read the instructions for your model. Check the electrode, battery and temperature requirements for the calibration standard.
- For pH, use fresh buffers that cover the working range, commonly pH 4 and 7. Pour a small amount into a clean container.
- For EC, use a conductivity standard with a known value. Tap water is not a calibration standard.
- Rinse between solutions, wait for a stable reading and do not return used standard to the bottle.
- Store the sensor as its manufacturer directs. Many glass pH electrodes need a suitable storage solution and must remain moist.
If a reading suddenly changes without any top-up or other intervention, check the meter against a standard before treating the reservoir. Cleaning an electrode or recalibrating can sometimes explain an apparent emergency without changing the feed. Temperature compensation does not remove the need to wait for the reading to settle.
Measure and adjust in sequence
- Inspect roots and equipment. Note the water level and temperature.
- In a system with conventional top-ups, restore the known working volume using appropriate water. Preserve the required air gap in a Kratky system.
- Mix and measure EC. Decide whether concentration needs adjustment or the solution should be replaced.
- If feeding is needed, add small amounts of the complete mix according to the recipe. Mix and remeasure after each addition.
- Check pH after setting EC. Add adjuster gradually according to its instructions, mixing and remeasuring between additions.
- Record the final readings and the quantities added. Compare the next measurements at the same working volume.
Do not combine fertilizer concentrates and pH adjuster directly. Each product should enter water as its instructions specify. If you overshoot pH and start alternating acid and base to undo each correction, stop and reassess. Repeated neutralization also changes what is dissolved in the reservoir.
A worked example: less water, higher EC
Imagine a tank containing 10 litres at EC 1.5 mS/cm. Later it contains 8 litres and reads 1.8. It is too early to conclude that you added too much fertilizer: the water volume has changed. In a system with ordinary top-ups, restore the volume to 10 litres, mix and measure again.
If EC returns to the working range, no feed is needed. If it stays high, check the source water and the record of additions. If it falls below the range, consider adding the complete mix. The numbers here are illustrative. They explain the decision order and are not a feeding recipe.
A concentration balance can roughly estimate dilution for a fresh, uniform solution. It is less informative in an old tank whose salt composition is unknown. You still need to measure after mixing because EC does not analyse individual nutrients.
Why pH keeps moving
Check the source water, tank volume, correction history and meter. High water alkalinity can drive pH upwards again. The pH reading does not tell you the bicarbonate reserve, so two water sources with the same initial pH can behave differently after adjustment.
Plants also alter solution composition as they take up nutrients. A small reservoir needs closer attention because the same addition of water or adjuster represents a larger fraction of its volume. A sharp change accompanied by smell or deteriorating roots calls for an inspection, not just another dose of adjuster.
When replacement is more useful than another adjustment
Replace solution when the addition history is lost, repeated top-ups have left the composition uncertain, or the solution is contaminated. Use your growing program to set the planned replacement interval. Prepare fresh solution first, clean accessible surfaces and avoid leaving roots to dry during the change.
After replacement, record the source water, recipe and readings. If the problem returns immediately, look for a shared cause such as water quality, the meter, temperature or equipment. Replacing solution repeatedly without checking those factors may repeat the same mistake.