fertilizing plants

Fertilizing plants: measure fertilization with an EC-tester for proper plant care

Here you will find a list of frequently asked questions about fertilizing plants. These can be house plants, container plants or other potted plants. But also information about the care of your lawn and plants in the yard or vegetable garden.

EC meters and fertilizing plants: frequently asked questions about fertilization of plants

EC meter measuring water

EC meter liquids


fertometer in potted plant



Rapitest 3 way digital analyzer in use

Soil analyzer (fertility, pH, temp.)


What does an EC meter measure?

All EC-meters measure the concentration of the total soluble salts in a liquid. For potted plants, these are the fertilizers plus the salinity altogether. The meter gives an indication of the total salt concentration.
Measurements are expressed in EC (electrical conductivity) or TDS (total dissolved salts). EC has the unit mS/cm and TDS in g/l (gram per liter) or ppm (parts per million). To convert EC into  TDS factor 640 is used. Hence  1,00 mS/cm = 640 ppm = 0,64 g/l.

Here you will find more information about our EC/TDS meter.

What do insertion EC-meters measure then?

Nowadays there are numerous EC-meters with a probe that is directly inserted into the soil. You will then have an instant measurement result, the only condition is that the soil has to be very moist. This is in stark contrast to the above mentioned EC liquid meters, where you have to get soil out of the pot first, mix that soil with distilled water and have to wait for 30 minutes so you can filter it to finally measure the fluids.
Professional digital insertion EC-meters cost at least 300 euros and are not really interesting for an individual. The fertometer is a simple EC-meter that measures the total salt concentration in the soil, and then immediately shows you if this is too low or too high, or if the salt content and thus the amount of nutrients is appropriate. Now you know if you need fertilizing plants.

How high should the salt concentration (EC) be in a pot?

During the growing season, an EC 0.35 mS / cm and under is really low. Therefore the fertometer alerts below this value by means of a yellow light. Is the value above 1.00 mS / cm then that’s harmful to the plant in the long term and you should stop adding fertilizer, a red light will turn on. Even if you can fertilize more with some plant species there is still a sufficient buffer of nutrients in the soil present. So no need fertilizing plants any further.

How high is the EC of fertilizer?

This depends on whether or not the plant is a major consumer, how often water is given and how often you want to fertilize. Also, the growing temperature plays a role here. Generally most fertilizers on the market have an EC of 1.2 mS / cm and a feeding of once a week is advised. But there are also ECs of 2.4 mS / cm and even higher. The frequency to fertilize is then often lower (eg fertilizing once every two weeks). Please note that the water itself also has an EC and this value should be added to the feeding water. You can measure the EC value of fertilizers with our EC/TDS meter for liquids.
The EC value of liquid fertilizer is about 2.5 times higher than the EC value measured by a fertometer directly in the soil. The concentrated fertilizer is slowly (takes at least 30 minutes) absorbed by the soil, divided and buffered.

How high is the nutritional value in sowing soil?

Sowing soil always has a low nutritional value in order to prevent that the first little roots get a full load. With the fertometer, the yellow light comes on.

I bought soil with slow-acting fertilizer. The packaging states that the EC value is 0.4 mS / cm, but it is higher than 1.00 after opening.

Slow-acting fertilizer starts to work in moist soil after 2 weeks, which means as soon as the fertilizer is in a humid environment at a certain temperature. Sometimes that process starts at 3 °C, but usually only from 10 °C. The higher the temperature, the faster it will go. If the soil has been in the store for a year or so, then the nutritional value is already relatively high, too high sometimes.
So it is better to buy basic soil and stir the fertilizer granules in it yourself, if you are going to use soil. The pgmix the soil provides the feeding for the first 2 to 6 weeks and then the slow-acting fertilizer will take it over afterward.
Slow-acting fertilizers usually have an EC of 0.4-0.6 mS / cm in the soil and this value is in the green range of the fertometer. In this case, this is a good starting point. But fertilizing plants would be fine though.

Can the Fertometer also be used for organic soil?

In organic soil you also measure all present salts and ions (absorbable). You measure the exact current salt concentration. So, all salts, also unwanted and unrecognizable salts. This should be taken into account. Is the organic soil pure, then you measure the actual nutrient salts, just as with chemical fertilizer.
Because the nutrients disappear much slower compared to potted plants in actual full ground (regarding flushing, drying out etc) it is better to fertilize in full ground, like a vegetable garden for example, between yellow and green.

Our tap water has no less than an EC of 0.8 mS / cm?

Tap water is tricky because there are many minerals in it and you do not know exactly what kind of salts these are. Often it involves salinity which the plant cannot use. It is also a good idea to know the pH value because it is often too high (for example. 8.0). And what’s even more important is the concentration of the bicarbonates that are responsible for the slow increase of the pH-value in the soil, which makes the plant absorb less and less nutrients. Especially older plants have this problem and brackish. If the plant is dry, (which is common in houseplants) these salts will crystalize and the next time you give it water these crystals will be driven to the top due to capillary effect. A white crust forms slowly around the edge of the pot.
Try to dilute your tap water with rainwater or neutralize it with Nitric acid for example.

With the EC/TDS meter you can measure the electrical conductivity of your water.

With the pH meter you can measure the acidity level of the water.

Sometimes you read that you have to flush older plants thoroughly and then have to fertilize them again in order to boost nutritional levels.

The problem with older plants is that there is indeed an accumulation of salinity in the potting soil (see above). You can remove it by flushing, the problem however is that you also flush out little soil particles and after thoroughly flushing a few times the buffer capacity of the potting soil reduces significantly. You have created an anorexic-plant! The plant dries out quickly, can no longer buffer any food and the soil solely consists out of roots. An infusion is then the solution or just provide a fresh growing environment every spring in the form of new potting soil. Hence normal fertilizing plants can restart after 6 weeks.

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