Plant growth-promoting inoculants typically have the greatest effect when applied early in plant development. When applied with the seed, there is a period of time before the seed germinates that the inoculant must be able to survive before root exudates become available. Microbial inoculants have the best chance to survive and compete in the soil environment if they are applied at high concentrations, in a highly viable form, close to the plant roots or seed at planting.
Keeping living microorganisms highly viable in a commercial product and supply chain is technically very challenging. Typically, greater than 99% of viable microbes die or become compromised during processing, shipping, and storage in the supply chain. Consequently, by the time the product is used, many commercial products probably contain far fewer numbers of the claimed microbe than product labels suggest. Delivery of a high concentration of microbes is important to overcome inoculant losses during application and in the soil environment so that there is sufficient survival of cells for long enough periods to establish and colonize the emerging plant root system. Inoculants are best introduced when there is adequate moisture and temperature conditions for expedient seed germination. Further, applying inoculants along with a protective, nutritive resource (e.g., complex nutrient medium) can aid inoculant survival until seed germination. Read the full article here.