Measuring the conductivity and pH of growing media

The nutrient level of the plant is monitored by measuring the electrical conductivity (EC) of the liquid fertiliser in the growing media. There are many ways to measure the conductivity and nutrient level, and the results of different methods and instruments may vary greatly.

Growers usually use press water from the substrate to measure conductivity. In practice, this means waiting for about an hour after fertigating and then squeezing the growing media to obtain a sample (one sample from  multiple pots). The probe of the conductivity meter is placed in the sample and the meter displays the results. Most conductivity meters display the results in milli Siemens per centimetre (mS/cm).

Usually, conductivity measured in this way varies between 0.8 and 4 mS/cm, depending on the irrigation method, plant, stage of growth and substrate composition. Some meters use the measuring unit of micro Siemens per centimetre (µS/cm), whereupon the results are thousandfold compared to the mS/cm results (in other words, about 800–4,000 µS/cm).

Some meters also use or offer the unit of Deci siemens per centimetre (dS/cm), whereupon the results are a hundredth compared to the mS/cm results (in other words, about 0.008–0.04 dS/cm).

In addition to measuring conductivity from press water, there are various other methods for measuring the nutrient levels and conductivity of the growing media. Usually the meter is the same as the one used in the press water method, but the sample is processed differently before the actual measuring. This means the results are different.


Official press water measurement

These types of measurements conducted by growers are usually called the squeeze method or the press water method. In these methods, the substrates are irrigated normally with a liquid fertiliser. After irrigation, the grower waits about an hour and then squeezes the substrate for a press water sample which is measured using a conductivity meter.

The official press water measurement (EN 27888) is conducted by adding distilled water to the substrate sample until the litre weight of the sample is 550 g. After a waiting period of  24 hours, the sample is squeezed and the squeeze out moisture  is measured. The values of the official press water measurement are slightly higher than the results of the usual measurements conducted by growers, but the difference is not significant.

Press water measurements, both official and on-site, use the measuring unit mS/cm.

Statutory standardised 1+5 measurement (EN 13038)

Growing media suppliers must report the nutrient levels and acidity of the growing media on the packaging and in documentation in accordance with the official EN standard.

The sample is prepared by taking 1 volume part growing media and adding 5 volume parts of distilled water. The sample is mixed thoroughly and lleft for 24 hours, after which the conductivity is measured from the suspension.

The result is a 14-fold figure compared to the official press water measurement. In other words, the results of the 1+5 method corresponding to the usual mS/cm values ranging from 0.8 to 4 in the press water measurement fall between 11 and 56 and the measuring unit is mS/m.

 Dutch method/Sonneveld method 1+1.5

Cultivators in the Netherlands, a country known for its flowers, have used the so-called 1+1.5 method for measuring conductivity and acidity for a long time. Relevant literature and publications of Dutch scientists and research institutes often refer to this method, and conductivity and pH values are nearly always reported using this method.

The sample for the 1+1.5 method is prepared similarly to the sample in the 1+5 method, but only 1.5 parts of distilled water are added. In other words, the sample is prepared by taking 1 volume part growing media and adding 1.5 volume parts distilled water. The sample is mixed thoroughly. After a 24 hour waiting period it is filtered through filter paper and the conductivity is measured from the filtrate.

The results are lower than the values obtained using the press water method. In the 1+1.5 method, the values corresponding to the 0.8–4 mS/cm press water values fall between 0.3 and 1.45 mS/cm. In other words, the figures of the traditional press water divided by 2.75 or the figures from the 1+5 method divided by 38.1.

Conductivity values measured in a laboratory

A so-called conductivity value can be measured with a substrate nutrient analysis using a growing media sample.. Due to the processing of the growing media sample (drying and crushing) and the extractant used in the process, the results of the conductivity value method are integers and the measuring unit is 10xmS/m. The resulting conductivity value of a limed and regularly fertilised substrate usually varies between 3 and 10 10xmS/m.

As the processing of the sample reveals all the soluble nutrients in the growing media, including soluble nutrients retained in a convertible form, comparing the figures directly with the press water results is difficult.

Peat-based growing media from Kekkilä Professional have been measured to discover how to compare the conductivity value with conductivity. According to the measuring results, the conductivity value from a growing media analysis using limed and fertilised peat-based media must be divided by 2.6, yielding a numerical conductivity value corresponding to press water measurement in mS/cm.

Therefore, the figures of the growing media analysis corresponding to the 0.8–4 mS/cm press water values fall between 2 and 10 10xmS/m.

Acidity of the growing media

Different measuring methods also affect the reported pH value, but the differences are smaller.

Legislations require growing media suppliers to report the pH value, measured with the EN 1+5 method, on the packaging. The values of the EN 1+5 method are about 0.4 units higher than those of the press water method. The packaging of growing media prepared for bedding plants, for example, shows the pH value as 5.9, which sounds slightly high for cultivating bedding plants, but when measured from the press water, the pH value is the safe and familiar 5.5. Similarly, the packaging of growing media designed for blue hydrangea varieties may show the pH value as 5.2, whereupon the press water value reflects a  pH value of 4.8.

pH measured with the 1+1.5 method follows the pH value obtained with a press water measurement.

Conversion table

This conversion table shows the nutrient levels of the growing media using different methods.

Conversion table conductivity measurement methods