This study focused on the microbial profile present in an agricultural soil that becomes suppressive after the application of composted almond shells (AS) as organic amendments. The role of microbes in the suppression of Rosellinia necatrix, the causative agent of avocado white root rot, was determined after heat-treatment and complementation experiments with different types of soil. Bacterial and fungal profiles based on the 16S rRNA gene and ITS sequencing, the soil under the influence of composted almond shells revealed an increase in Proteobacteria and Ascomycota groups, as well as a reduction in Acidobacteria and Xylariales (where R. necatrix is allocated). Complementary to these findings, functional analysis by GeoChip 4.6 confirmed the improvement of a group of specific probes included in the “soil benefit” category was present only in AS-amended soils, corresponding to specific microorganisms previously described as potential biocontrol agents, such as Pseudomonas spp., Burkholderia spp. or Actinobacteria. Based in such data, a model for the microbial-based suppressiveness is proposed and further isolation of representative microorganisms were performed.