Standards Development

Scope

This document specifies the test methods for enumeration of culturable microorganisms within samples of biologically active fertilizers, soil conditioners, and beneficial substances, and includes methodological advancements to facilitate accurate enumeration of organisms such as Bacillus which form spreading colonies on plates of standard laboratory culture media.

Purpose

Microbial biostimulants, biofertilizers, soil conditioners, and other plant-beneficial substances have garnered significant attention as enablers of sustainable food production. The global market for microbial biostimulants was valued at $4.59 billion USD in 2023. As growers continue to manage factors such as climate change and fluctuating fertilizer prices, the market is projected to reach $8.16 billion USD by 2031, with a compound annual growth rate of 8.56% from 2024 to 20311,2. These figures underscore the economic potential for this product class; however, this growth will depend in part upon growers’ perceptions regarding the efficacy and quality of these products, as well as those of regulatory authorities. Recent research involving bacterial3 and fungal4 biostimulants has highlighted the need for a science-based composition verification regimen to support the establishment of quality standards for products in this class. Such concerns may be addressed through regulatory audits or through voluntary, industry-led certification programs5,6. To be effective, such efforts must be based upon microbial enumeration methods suitable for industry-relevant sample types and microbial groups. One critical parameter which must be addressed by any composition verification regimen for microbial products is the enumeration of viable microbial cells. The Aerobic Plate Count (APC) assay has been a mainstay of microbiological enumeration testing for over a century7 and is among the most common method used to evaluate the composition of microbial products around the world. However, the standardized APC assays currently in use for enumeration of these products were not developed with agronomically relevant microorganisms in mind; for example, ISO 48338 was developed for food safety testing. While this method is suitable for some agronomically relevant microorganisms, some organisms including members of the genus Bacillus must be enumerated using modified plate counting methods if accurate counts are to be rendered9,10. Bacillus species are common inclusions in biostimulant formulations due to their formation of endospores, which are resistant to a wide range of environmental and chemical stressors and thus are compatible with harsh manufacturing processes and extended supply-chain logistics11. Therefore, a new microbial enumeration method is required for use with products in this class claiming populations of viable microorganisms, including Bacillus spp., on their labels to ensure optimal outcomes for all industry stakeholders.

The main objective of this standard is to define clear and standardized methodologies for enumerating populations of culturable microbial cells within fertilizers, soil conditioners, beneficial substances, and other products intended for agricultural use. By establishing these testing procedures, the standard enables producers, regulators, and industry stakeholders to construct science-based systems for composition verification, ultimately empowering growers to make the best choice possible for their farm.

This standard utilizes ISO 4833 Part 2 as normative reference and has been developed based upon a preponderance of evidence from peer-reviewed publication highlighting the limitations of ISO 4833 for the enumeration of Bacillus-based microbial products