Compost Teas: market survey and microbial analysis

Abstract

Compost teas (CTs) are fermented watery compost extracts that can be made with a wide variety of organic ingredients. These extracts are applied on plants to promote plant growth and increase protection against pathogens. Several studies have shown that the live micro-organisms present in these teas play an important role in plant protection. Still, our knowledge on the identity and functionality of bacterial communities in CTs remains limited. Moreover, CTs are a heterogeneous group, as various combinations of ingredients, fermentation parameters and application guidelines are possible and little is known on the current practices in commercially available products. Consequently, also the microbial communities in these commercially available CTs are unknown. Here, we surveyed 68 CTs on ingredients, claims and guidelines regarding preparation and application. This revealed that most products were sold in dried form which needed to be rehydrated by the customers. Moreover, kelp, humic acid, sugar and molasses, were common ingredients. Additionally, we investigated the bacterial communities of eight products, covering different ingredients and preparations, using sequencing and culture-based techniques. Bacterial communities were remarkably similar at genus level, with Pseudomonas, Massilia, Comamonas, Chryseobacterium, Brevundimonas and Acinetobacter as most prevalent genera. The genus Pseudomonas was more abundant in CTs that were prepared without aeration. Moreover, differences between liquid and dry products revealed the importance of formulation to secure the viability of the desired microbes. This study has given insight in common practices in CTs on the market, their microbial composition, and highlighted several discrepancies with current literature. ImportanceCompost teas are fermented watery compost extracts which are used in professional and amateur horticulture to promote plant growth and suppress plant diseases. Beneficial microbes present in these extracts are suggested to play an important role in the mode of action. However, little is known on the identity and functionality of the bacteria in compost teas. Moreover, compost teas are a heterogeneous group of products that can be made with numerous combinations of ingredient and fermentation parameters and little is known on current practices in products available on the market. Therefore, we surveyed these practices in 68 commercial products and for a subset of these, we explored their bacterial communities using both culture dependent and independent techniques. This study has unveiled the common practices on the market, their microbial communities, and highlighted several discrepancies between commercial practices and scientific research.