Research update: combining substrate components

28 february 2019

At the end of 2017 RHP started research in combining multiple components into one substrate in a conscious way and the possible stacking effect of properties. What is the progress in this multiannual, complex research? An update!

The goal of this RHP research is to achieve a way to combine components safely in substrate mixtures, even if the current maximum dosage is exceeded. Under the RHP quality mark a number of products have a maximum allowed dosage in substrates. However, in mixtures, more and more components with individual allowed limits are combined. In such cases it is not taken into account whether these components, when combined, have negative effects on crops, whereas when applied separately in a mixture no harm is done.

The first experiences in this research are as follows:

Nutritional aspects
In the first phase the five components peat, green waste compost, rice hulls, bark and wood fibre were analysed with various extracting agents (water, CAT and hydrochloric acid/HCl) to describe the availability of nutrition in the components. It is not yet clear how to interpret these analysis results. But the test results do give initial insight in how elements are buffered at organic matter complexes (CEC).

Together with the guidance committee the next steps for this part are defined as follows:

• From now on a standard analytical approach will be used to characterise materials; extraction with water and hydrochloric acid (HCl).
• Separate mixing trials will be carried out in order to get insight in the availability of elements and the adsorption of elements to organic matter complexes.

Nitrogen fixation
Bark, wood fibre and rice hulls were tested on N-fixation. In this test wood fibre showed a low N-fixation, whereas bark and rice hulls showed higher values. In a first run of bio assays however, the N-fixation showed to be much less severe for these last two products. There was especially no significant difference in N-fixation between the mixture with rice hulls and its reference mixture. Based on this, the current N-fixation test for these products should be evaluated.

For this part the next step is defined as follows:

• Nitrogen immobilisation will be separately tested by incubation of mixtures in comparison with the current standard RHP test.

High dosages in bio assays
In a bio assay with dosages up to 50% of coarse raw materials in a mixture with fine peat, it was observed that growth of the test crops declines strongly at dosages of 50%. However, it was also observed that water uptake and capillary action of the substrates were severely limited. This probably also did lead to growth inhibition. The question is now whether the standard bio assay is suitable for mixtures with high dosages of coarse components or not. To be able to test coarse components, we are now looking at the use of so-called extraction tests.

The next steps for this part are defined as follows:

• Bio assays will be conducted at 100% dosage of materials by means of an extraction test.
• Combination of materials in bio assays will take place in combination with adapted references or adapted fertilization.

The first exploratory phase in this research has now been completed. RHP continues this research to get a better grip on the aforementioned aspects of nutrition, nitrogen fixation and bio assays. New analytical methods are necessary to eventually establish a safe way of combining multiple substrate components into one mixture. These methods should lead to a correct interpretation and assessment of the properties of the substrate components and their interactions.

Also read the previous news item:
COMBINING CONSTITUENTS RESPONSIBLY? (30 NOVEMBER 2017)