pH and EC are the two figures you come across most often on a substrate specification sheet. They say a lot about how a plant will develop in the first few weeks, and they remain a good indicator throughout the crop. Yet they are regularly measured or interpreted incorrectly. A brief explanation.
pH: acidity
The pH indicates how acidic or alkaline the substrate is, on a scale of 0 to 14. A pH of 7 is neutral, lower is acidic, higher is alkaline. The scale is logarithmic: a pH of 5 is ten times as acidic as a pH of 6.
The pH mainly determines how available nutrients are. If the pH is too high, iron, manganese and phosphate are less readily taken up and you may see, for example, yellow young leaves. If the pH is too low, some trace elements can actually be released in excessive concentrations. For most crops in organic substrates, the right range lies between pH 5.5 and 6.5. That is lower than in a mineral garden soil. Acid-loving crops such as rhododendron, heather and blueberry prefer it even more acidic, between pH 4 and 5.
EC: the amount of dissolved salts
EC (electrical conductivity) measures how many dissolved salts are present in the substrate solution, expressed in mS/cm. Nutrients are salts, so a higher EC generally means more nutrition. But unwanted salts such as sodium and chloride also count.
Too low an EC means too little nutrition: the plant grows slowly and looks pale. Too high an EC makes it harder for roots to take up water and can cause root damage in young plants and cuttings. That is why propagation substrates have a lower EC than substrates for vigorous pot plants.
The measuring method determines the figure
An EC value only means something if you know how it was measured. In the Netherlands, potting mixes and substrates are usually tested using the 1:1.5 volume extraction method: one part substrate is mixed with one and a half parts demineralised water, after which pH and EC are measured in the extract. The indicative values in our specifications are based on this method.
If you measure in a different way yourself, for example in drain water or with a sensor placed directly in the substrate, you will get different figures. They are not wrong, but they are not directly comparable either. So always compare measurements that were taken in the same way.
Measuring it yourself: how to go about it
- Take samples from several pots and from the root zone, not just from the top layer.
- Combine the samples into one representative sample.
- Use demineralised water and a calibrated meter.
- Let the extract stand for a while before measuring, so the salts dissolve properly.
- Record the date, batch and measuring method, so you can compare measurements over time.
Want a complete picture, including the individual nutrients? Then have a substrate analysis carried out by a laboratory.
What affects the pH during the crop?
- Irrigation water: water with a high bicarbonate content gradually raises the pH. Rainwater and reverse osmosis water hardly do so.
- Nitrogen form: feed with a lot of ammonium lowers the pH in the root zone; feed with mainly nitrate raises it.
- Lime: lime that has been mixed in continues to react during the first few weeks; the pH may then rise a little further.
- Raw materials: peat and bark are naturally acidic, coir is roughly neutral.
Adjusting
If the EC is too high, one or more generous irrigations with clean water will help to leach out excess salts, followed by an adjusted nutrient concentration. If the EC is too low, increase supplementary feeding gradually and measure again after a week.
A pH that rises too far can be corrected with a feeding programme containing more ammonium or by acidifying the irrigation water. A pH that is too low can be corrected with a feeding programme containing more nitrate, or with more lime in the next batch of substrate. Spreading lime on the pot afterwards works slowly and unevenly.
Common mistakes
- Sampling only the top layer.
- Using tap water for the extract.
- Using a meter that has not been calibrated for months.
- Comparing values from different measuring methods.
- Only measuring once symptoms appear, instead of at fixed times.
