Applied Soil Ecology, Volume 169, January 2022, 104225
Arbuscular mycorrhizal fungi are like a “life-insurance policy for plants”. They improve soil structure and make nutrients and water available. This increases the stress tolerance of crops and leads to better yields.
Arbuscular mycorrhizal fungi live in symbiosis with plants and are an important element of natural ecosystems. They colonise both the roots and the surrounding soil, thus constituting a sort of extension of the plant root system. Mycorrhizae help to make nutrients available to plants and improve soil structure, thereby increasing the stress tolerance of plants. Studies show that up to 80% of plant phosphorus is absorbed via mycorrhizal symbiosis rather than directly via the roots. These characteristics also make mycorrhizae attractive for farmers. There are several ways to encourage mycorrhizal fungi, for example through continuous soil coverage with living plants and minimal tillage. In addition, there are a range of commercial products containing propagules from mycorrhizal fungi. These preparations are meant to improve the soil and promote plant growth. But how effective are such products? Experts from Agroscope and the Universities of Zurich (Switzerland), Adelaide (Australia), South Dakota and North Carolina (USA) have addressed this question in a global study, in which a total of 28 commercial products were examined in the greenhouse and under field conditions.
The conclusions reached by this study are sobering. In the controlled greenhouse trials, 84% of the products failed to lead to the desired colonisation with mycorrhizae. The study results from Switzerland also showed that the majority of products contained no active fungal spores. This might be the result of inadequate packaging methods and storage conditions. In some products, however, too few spores were found, or germinability was decreased by additives. Despite this, there were also a few positive exceptions and the products clearly enhanced mycorrhizal root colonization. In the case of two products, this increased colonisation of the plant roots also led to significantly higher yields.
Suggestions for improvement
The results of this study show that improved quality control of commercial products containing mycorrhizal fungi is essential. This implies standardised test procedures, including proof of origin and production methods. Manufacturing companies should provide evidence that their product works under controlled conditions and leads to a colonisation of the roots. In addition, quality standards are needed (see Conclusions).
All in all, however, further studies are necessary to ensure the successful use of commercial mycorrhizal preparations in agriculture.
- The majority of the tested products contained no, or too few, germinable spores. The reasons for this were inadequate packaging and storage conditions, or unsuitable additives that inhibited spore germination.
- Mandatory quality control with minimum standards is essential for ensuring reliable mycorrhizal preparations. This implies standardised test conditions, proof of origin and production methods, detailed instructions for use and expiry date, a high concentration of germinable spores and adapted mycorrhizal isolates, and suitable carrier substances for promoting spore germination.