How to adjust a mixture in 5 steps

28 april 2017

The growing situation at a grower changes or a new raw material is introduced. There could be several reasons to change a (substrate) mixture. The smallest change can already have big consequences. As knowledge centre for substrates, RHP is more and more approached for advice on problems in cultures because of mixture adjustments. Avoid problems with these 5 steps.

One change in the mixture often has multiple consequences. These consequences need to be clear, before the grower can safely use the new mixture. This can include new user advice. In many cases it is wise to test the new mixture first to determine whether your estimation is correct. Follow especially the nutrition situation and the water content during the growth.

Predictable peat 

RHP sees an increase in problems caused by mixture changes. These often have multiple causes. Up until now most mixtures exist for a large part of peat. A lot is known about peat concerning the expected characteristics in a mixture. The outcome is less predictable when the non-peat part is increased or when there are multiple non-peat raw materials in one mixture. We call this “stacking”.


The effects of stacking are not yet well known. Many non-peat raw materials bring in different characteristics that can differ per raw material. It is often possible to anticipate on a deviating characteristic. But this is more difficult when more characteristics change. This is still apart from the interactions that can occur. To develop more knowledge on the topic, the Steering Committee Research will soon discuss it to give it a place in the collective RHP research. With the knowledge that follows from the collective research, RHP certified companies will be able to predict mixture changes in the future even more accurately.

Limits in RHP-norms

As long as not all effects are known, stacking is limited within RHP-norms (see module 330 of the Product Certification Scheme in My RHP). This needs to avoid the development of practical problems. Of course earlier research and experience has already shown us something. 3 examples:

3 examples

1.      Example chemical properties
The pH research of a couple of years ago has shown that the pH-buffer can change strongly when a mixture is adjusted. Peat has the characteristic to buffer changes in the pH. Other raw materials do this less or not at all. Then the effects of, for example, extra fertilization on the pH are much bigger, causing it to be impossible for the optimum pH for the culture to be reached. Nowadays the pH-buffer of a mixture is well measurable.

2.      Example physical properties
Sometimes a raw material that increases the air content is sought. For example to replace a peat fraction in a mixture. It is important to realize that apart from chemical effects, this can have consequences for the water uptake speed and the water distribution in the substrate. These last ones can be determined with a WOK-analysis/water uptake characteristic analysis.

3.      Example biological properties
For several reasons, the last couple of years the sector is confronted more with saprotrophic fungi in cultures. Apart from the presence of the fungus, for example in the form of spores, the circumstances need to be optimum to allow the fungus to grow. Think of climate and the susceptibility of the substrate to let the fungus thrive. Replacing one raw material with another can (strongly) increase this susceptibility, causing more fungus to grow. While the replacing raw material doesn’t introduce the fungus and doesn’t have to be susceptible itself. The optimum circumstances for the fungus are formed, purely by the combination with other raw materials.

Want to know more?

Check My RHP for several research reports about mixtures. Or ask your technical advisor at RHP about mixture changes.

More news? 

//Enable form button reCAPTCHA