The practice of adding organic amendments to crop soils is undergoing resurgence as an efficient way to restore soil organic matter content and to improve soil quality. The quantity and quality of the organic matter inputs affect soil physicochemical properties and soil microbiota, influencing different parameters such as microbial biomass and diversity, community structure and microbial activities or functions. The influence of organic amendments on soil quality has also effects on crop production and plant health. The enhancement of soil suppressiveness using organic amendments has been widely described, especially for soil-borne diseases. However, there is great variability in the effectiveness of suppression depending on the nature of the amendment, the crop, the pathogen, and the environmental conditions. Although the effects of organic amendments on soil properties have been widely studied, relationships between these properties and soil suppressiveness are not still well understood. Changes in soil physicochemical parameters may modulate the efficacy of suppression. However, the parameters more frequently associated to disease suppression appear to be related to soil microbiota, such as microbial biomass and activity, the abundance of specific microbial groups and some hydrolytic activities. This review focuses on the effect of organic amendments on soil microbial populations, diversity and activities; their ability to enhance plant health through disease suppression; and which of the parameters affected by the organic amendments are potentially involved in soil suppressiveness

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One of the main avocado diseases in southern Spain is white root rot caused by the fungus Rosellinia necatrix Prill. The use of organic soil amendments to enhance the suppressiveness of natural soil is an inviting approach that has successfully controlled other soilborne pathogens. This study tested the suppressive capacity of different organic amendments against R. necatrix and analyzed their effects on soil microbial communities and enzymatic activities. Two-year-old avocado trees were grown in soil treated with composted organic amendments and then used for inoculation assays. All of the organic treatments reduced disease development in comparison to unamended control soil, especially yard waste (YW) and almond shells (AS). The YW had a strong effect on microbial communities in bulk soil and produced larger population levels and diversity, higher hydrolytic activity and strong changes in the bacterial community composition of bulk soil, suggesting a mechanism of general suppression. Amendment with AS induced more subtle changes in bacterial community composition and specific enzymatic activities, with the strongest effects observed in the rhizosphere. Even if the effect was not strong, the changes caused by AS in bulk soil microbiota were related to the direct inhibition of R. necatrix by this amendment, most likely being connected to specific populations able to recolonize conducive soil after pasteurization. All of the organic amendments assayed in this study were able to suppress white root rot, although their suppressiveness appears to be mediated differentially.