Guide to Measuring Tools for Substrates in Ornamental Horticulture

Peat based substrates have been used for decades. Peat is relatively sterile, has good buffering capacity and its properties are easy to predict. New growing media though, such as those containing wood fiber, coir or bark, are less easy to predict.

The so-called new substrates have lower buffering capacity for both water and nutrients, require higher N fixation and are more susceptible to pH fluctuations. Therefore, it can be necessary to adjust the nutrient supply and irrigation throughout the growing season to ensure good quality plants. The right measurements and measurement technology can help make decisions on how to adjust.

Here below a zoom-in in the most common tools to measure EC, salt, pH and moisture in growing media, as well as the reasons to prefer full substrate laboratory analysis.

EC and Salt Measuring Devices for Ornamental Plants’ Substrates

If interested in the different behavior of EC and salinity in peat substrates and non-peat-based substrates, here below a table showcasing the main 2. For a deeper dive, click on the link to go to the article.

Looking at the classic measuring instruments for these parameters, the EC measuring devices are available as compact, hand-held devices or as a set consisting of a basic device with probe to test a nutrient solution up to an EC of 200 mS/cm. These devices are easy to use and guide you through the calibration program.

In recent years, the direct measurement of salinity in the substrate with the PET 2000 activity meter has also become established. All salts dissolved in the substrate are measured.

To take measurements, the substrate needs to be moist. Insert the substrate and read the salt content in g/l by pressing the measuring button. These devices are accompanied by reference values in addition to the instructions for use. The optimal salt content of the crop can be also self-assessed, by halving the EC optimum. EC optimum divided by 2 corresponds approximately to the salt optimum.

pH Measuring Devices for Ornamental Plants’ Substrates

In the below table we highlight the 2 main differences between peat-based growing media and its alternatives in relation to pH. If you are looking for more detailed information, at this link a full article about the different behavior of pH in peat substrates vs non-peat-based ones.

To check the pH value of substrates in pots, pH meters with probes for measures directly in the substrate are often used. The pH meters have a glass electrode that sometimes requires pre-piercing of the substrate. Regular calibration of the pH meters is required to ensure accurate measurements.

In addition to the glass electrodes, there are also simpler ways to determine the pH value. An example are indicator tests, which are easy and quick to use. These tests consist of a measuring plate with a color scale and a small hole into which a substrate sample is poured. This is filled with the indicator solution after which it runs along the color scale. The measurement accuracy is far below that of glass electrodes. However, it does provide an indication that is sufficient in most cases.

Moisture Measuring Devices for Ornamental Plants’ Substrates

Moisture management varies significantly between traditional, peat-based substrates and peat-reduced or peat-free ones. Here below we highlight the main two while, to know more about the specificity of moisture management when growing plant in peat-free or peat reduced substrates, click here.

In general, available water in the substrate can be measured with tensiometers which are available in different lengths and designs and are easy to use. They are available for use in nursery goods from 12 up to 100 cm. Tensiometers measure the suction tension, i.e. the force that plants need to extract the necessary water from the substrate in mbar. This can give valuable insight into the moisture levels of a substrate and the need for irrigation.

Other methods of measuring soil moisture are also available. Sensors that measure the percentage volumetric soil moisture (vol.%) or soil moisture in m³/m³ can be used to determine the amount of water in a substrate. This can be measured with handheld sensors or wireless sensors that can be placed in pots all over the nursery to get a real-time insight into soil moisture.

Substrate Laboratory Analysis: the Most Accurate Set of Checks

Although handheld or wireless sensors are becoming more accurate and can give valuable insights into EC and pH, lab analyses for sure give an even more elaborate view of the substrate. Providing a lab with a substrate sample (either from the starting growing media or from pots throughout the growing season) can give insights into not only pH and EC, but all nutrient element levels. Choosing the right analysis method is important to get the desired results.

An often-used substrate analysis method is with water. In this method, the substrate sample is mixed with demineralized water to dissolve all water-soluble elements present in the substrate. For plants to take up nutrients, these elements need to be water-soluble. Therefore, a substrate analysis with demineralized water can give good insights into readily available nutrients present in the substrate.

However, in growing substrates there is a Cation Exchange Capacity (CEC), which -in a nutshell- is an indication of how well a substrate can hold and exchange cations.

Just to clarify the concept, let’s take coir as an example: it has a relatively high CEC and for this reason it has to be extensively flushed before being used in growing media. In fact coir naturally has high levels of K+ and Na+ on its exchange sites and have to be washed out otherwise in the substrate these are often replaced by Ca+ and MgO2+. This means that over time, more K and Na will be released by the substrate while Ca and Mg become bound to the coir. The nutrients bound to the substrate will slowly be released by the substrate making them available to be absorbed by plants. By taking a CAT analysis rather than a water analysis, the elements bound to the CEC are also measured.

CEC analysis gives a more detailed insight into all elements present in the substrate, even the elements which are not readily available for plants and may need to be ‘pushed’ of the complex through addition of cations.

Conclusion

Crop safety is granted also through regular measurements, which help to detect inconsistencies at an early stage, even before the plant is visible or damage occurs.

For personalized guidance, contact your regional technical support by visiting your country’s website or fill in the form you can reach from the banner at the bottom of this page.