Composition of growing media

31 May 2021

The first three webinars that RHP organised for certified companies were about safety, pH and nutrition and physical aspects. Composition of growing media was the theme of the fourth webinar. How do raw materials interact with each other? And how can certain properties be realised in a growing medium during substrate production?

It starts with determining the wishes of the customer and of the plant. The most important variables to take into account in substrate production are: crop, type of plant material, cultivation method/experience of the grower, potting behaviour, processability of the growing medium, fertilisation options, irrigation system, cultivation time and cultivation system. Let's call this 'the setting' at the grower. When a growing medium is in the pot, the physical and chemical properties can (almost) no longer be changed. RHP-certified growing media must therefore meet predefined specifications.

Grower takes over

When a growing medium is delivered to the grower, he takes over. The grower will irrigate and fertilise the culture. When using a new growing medium, the grower must pay attention to necessary adaptations in culture measures. The less the growing medium fits in his experience, the more adaptations are necessary in the culture. The growing media can be different in terms of water uptake and water buffering, which means that the grower has to adapt his irrigation strategy. The growing media can also be different in terms of nutritional aspects, which means that adapted fertilisation is required during substrate production and later on in the culture.

Checklist composition of growing media 

Below is a checklist for the composition of growing media. A number of aspects are interrelated. It concerns three main areas: physical aspects, nutritional aspects and biological aspects.

Multiple effects of raw materials

Raw materials have multiple effects and interact when combined. A raw material in a growing medium has an effect on the following aspects:

  •   Physical aspects: structure stability, gradation, internal pores, retention curve (water buffer, Easy Available Water) and WOK (water uptake characteristic).
  •   Chemical aspects: initial pH, buffer (pH, cations), nutrient load, nitrogen immobilisation and biological aspects.

Interactions mainly appear for nutrition: buffer size, buffer occupation and nutrients offered, mineralisation, conversion and immobilisation. This interaction continues in the culture. Fertilisation by the grower also has influence.

In short, for each raw material in a mixture, the effect and interaction with other raw materials must be evaluated. When composing growing media, it is important to think in terms of properties. And it is also important to talk to the grower about properties, not recipes. RHP continues to research the physical aspects of raw materials and continues to work on modelling nutrition. Next month we will summarize the them of the final webinar: packaging, storage and application.

For RHP-certified companies: you can download the hand-outs of the presentations of the webinars at My RHP.

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