Semi-Hydro Fertilizer Dosing: EC Targets, Nutrients & Salt Flushing

Why Semi-Hydro Nutrition Is Different

In soil, microbes break down organic matter and release nutrients slowly, and the soil itself stores ions on clay and humus. In Pon, LECA and other inorganic media, almost none of that happens. The medium is a stable physical support, and the roots feed from the solution in the reservoir and the film of water on the grains. That makes the fertilizer you choose and the way you dose it far more important than in soil. The medium itself matters too: Pon holds some ions on its zeolite fraction, while LECA holds almost none, as described in LECA vs Pon. For the physical system, read the semi-hydroponics with Pon guide.

Why Organic and Urea-Based Fertilizers Fail

Standard houseplant fertilizers and organic feeds are built for living soil. In an inorganic medium they cause problems.

• Fish emulsion, compost tea and similar feeds. They contain proteins, fats, sugars and suspended solids that feed bacteria and algae in the reservoir. The result is slime, a sour or fishy smell, clogged slots and wicks, and low oxygen around the roots, which leads to root rot.
• Urea-based NPK. Urea is converted to ammonium by the enzyme urease, which is produced by soil microbes. In a clean inorganic medium that conversion is unreliable, and any ammonium that does form can build up. Together with the extra bacterial growth, this is a common cause of foul odors and rotting roots. Treat the details as hobbyist experience rather than a controlled result, but the practical advice is consistent: avoid urea-heavy feeds.
• Slow-release organic granules. They need microbes and soil moisture cycles to release nutrients. In a reservoir system they either do nothing or release unevenly.

What a Suitable Fertilizer Contains

Choose a complete, fully water-soluble mineral fertilizer designed for hydroponics and check the label for these features.

• Nitrate-nitrogen as the main nitrogen form. Nitrate is taken up easily and is less likely to burn roots or acidify the root zone than high levels of ammonium. A formula may still include some ammonium, but avoid products dominated by urea or ammoniacal nitrogen.
• Chelated micronutrients. Iron should be chelated as EDTA, or as EDDHA for higher-pH water. Look for manganese, zinc, boron, copper and molybdenum as well. Chelated iron stays available in the reservoir, while unchelated iron precipitates.
• Calcium and magnesium. Some one-part foliage formulas include them, and some multi-part systems provide them through a separate component. If the label does not list calcium and magnesium, plan to add a Cal-Mag supplement.
• A balanced N-P-K ratio for foliage. Leafy houseplants use more nitrogen than flowering plants do, and flowering orchids or Hoya in bloom may benefit from a bloom-oriented formula.

Follow the manufacturer's directions as a starting point, then adjust with an EC meter. Label doses are often written for high-light commercial growing, so houseplants indoors usually need less.

EC and PPM Targets

Electrical conductivity (EC) measures how much dissolved salt is in the water. It is expressed in millisiemens per centimetre (mS/cm). Parts per million (ppm) is a calculated figure, and it depends on the conversion factor your meter uses. This guide uses the 500 scale, where ppm = EC (mS/cm) x 500. Some meters use a 700 scale or report TDS directly, so check which scale your meter uses before you compare numbers.

• Sensitive or low feeders (Alocasia, Calathea, Anthurium, orchids, ferns): 0.6-0.8 mS/cm (300-400 ppm).
• Moderate feeders (Philodendron, Epipremnum, Hoya, Peperomia): 0.8-1.2 mS/cm (400-600 ppm).
• Heavy feeders (Monstera deliciosa, Ficus lyrata, Strelitzia): 1.2-1.6 mS/cm (600-800 ppm).
• Baseline water (RO or low-TDS): below 0.1 mS/cm (below 50 ppm).

These are working ranges, not laws. Light level, temperature, plant size and growth rate all change what a plant can use. Start at the low end, watch the new growth for a few weeks, and raise the strength only if growth is pale and slow. The first table above lists the targets by plant group. Remember that tap water adds its own EC, so measure the water before you add fertilizer.

How to Mix to a Target

1. Fill a clean container with your water and measure its EC.
2. Add a small amount of fertilizer, stir, and measure again.
3. Continue in small steps until you reach the target EC, including the EC of the water itself.
4. Check the pH, adjust if needed, and then fill the reservoir.

Keep a note of the amount of fertilizer per litre that gave the right reading, so that you can repeat it. Use the semi-hydro Pon calculator to work out how much solution a pot needs.

The Evaporative Salt Concentration Cycle

A reservoir does not stay at the strength you mixed. Plants lose water through transpiration, and the surface of the medium loses water by evaporation. Both remove nearly pure water, while the dissolved salts stay behind. As a result the solution becomes more concentrated, and salts also wick upward and dry on the surface, pot rim and wick as white crusts. Near the root tips the effective concentration can become high enough to burn them, even if the original mix was correct. The third table above shows how the EC drifts.

The Weak, Frequent Routine

• Use a weaker solution than you think you need and renew it often.
• Between changes, top up with plain RO or low-TDS water, not with more fertilizer solution.
• Replace the whole reservoir every 1-2 weeks.

The Flush and Reset

1. Every 3-4 weeks, pour 2-3 pot volumes of plain, lukewarm water slowly through the medium from above, letting it drain fully.
2. Empty the reservoir and rinse it or the cachepot, removing slime or crust.
3. Refill with fresh solution at the target EC and pH.
4. Flush sooner if you see a white crust, brown leaf tips or if the runoff EC is well above the fresh solution.

The same flush is described for pot systems in the Pon guide. Hard tap water, heavy feeding or high light shortens the interval.

pH and Cal-Mag Management

Target pH

Aim for a solution pH of about 5.8-6.5. Within this range most nutrients, including iron, manganese and phosphorus, stay available. Fresh LECA and some mineral media can shift the pH of the reservoir, so check it a few days after a change. Use a calibrated pH pen where possible. Liquid indicator drops are cheap but coarse, and the colour can be hard to read, so treat them as a rough check. Adjust with a small amount of a pH-down product made for hydroponics, and re-measure after mixing. If your water has high alkalinity, the pH will drift upward again, so check it more often.

Calcium and Magnesium with RO Water

RO and distilled water contain almost no calcium or magnesium. If your fertilizer does not supply them, plants can show deficiency within weeks: curled or distorted new leaves, brown or necrotic leaf tips and edges, weak growth and, for magnesium, yellowing between the veins of older leaves. To prevent this, use a complete fertilizer that contains Ca and Mg, or add a Cal-Mag supplement at the label rate, and then adjust the total EC. Some people blend a little tap water into RO to restore minerals. Whatever you choose, check the final EC and the calcium-to-magnesium balance, because too much of either can block the other.

Foliar Feeding for New Conversions

A plant that has just been converted has few working roots, so it cannot absorb much from the reservoir. During this period you can supply a little nutrition through the leaves, as long as you stay conservative. Mist a quarter-strength solution of a complete mineral formula onto the leaves, preferably in the morning, using a fine spray. Do this no more than once a week, and stop as soon as white roots form. Avoid strong sun right after misting, because droplets can scorch leaves, and keep the mist off flowers and the growing point of delicate plants. Foliar feeding supplements the roots and does not replace them. Details on the conversion period are in the guide to converting houseplants to semi-hydro, which also explains the plain-water start before any fertilizer.

Practical Feeding Schedule

1. Weeks 0-3 after conversion: plain RO or low-TDS water, no fertilizer. Optional quarter-strength foliar mist for cuttings.
2. Weeks 3-6: begin at about a quarter to half of the target EC for the plant group, and increase if growth is healthy.
3. Established plants: use the target EC for their group, change the reservoir every 1-2 weeks and top up with plain water.
4. Every 3-4 weeks: flush and reset.
5. Low light or winter: reduce the strength by a third to a half, because the plant uses fewer nutrients.

If you make your own mineral mix, remember that it holds no nutrients unless you add controlled-release fertilizer, as explained in the DIY Pon recipe. A mix with slow-release pellets already supplies some nutrition, so reduce the liquid dose. To size the substrate for a pot, use the semi-hydro Pon calculator.

Troubleshooting Summary

Related Resources

Learn the full system in the semi-hydroponics with Pon guide, make your own mix with the DIY Pon recipe, move plants over with the conversion guide, compare media in LECA vs Pon, plan quantities with the semi-hydro Pon calculator and see a heavy-feeding aroid on the Monstera deliciosa page.