Active management of soil microbial communities is a promising approach to suppress soilborne diseases and improve crop productivity. The goal of this approach is to manipulate, alter, or augment the microbial characteristics of the soil through various management practices to provide disease suppression by increasing microbial activity, diversity, and antagonism toward pathogens. However, relatively little is known regarding the specific populations, effects, interactions, and relationships among soil microorganisms that confer stable disease suppressiveness. Because plants are primary drivers of soil microbial dynamics, rotation crops and residue amendments (cover crops, green manures, etc.) dramatically affect soil microbial communities and may be important components in establishing and maintaining soil suppressiveness. Biological amendments of various types, including recognized biological control agents, diversified microbial inoculants (mixtures of beneficial microorganisms, mycorrhizae, compost teas, etc.), and biostimulants (growth-enhancing plant extracts, enzymes, organic acids, and natural compounds), may effectively introduce, augment, or stimulate soil populations of plant-beneficial microorganisms. Previous research established that different crop rotations result in distinct differences in soil microbial community characteristics (SMCC) (1,2) that were associated with suppression of soilborne potato diseases (2), and that some biocontrol agents could effectively reduce soilborne diseases (3,4). In the current research, various biological amendments were evaluated, alone and in conjunction with different cropping systems, for their efficacy in introducing beneficial microorganisms, affecting SMCC, and reducing soilborne diseases of potato in greenhouse and field tests. SMCC were assessed by microbial populations, soil Substrate Utilization (SU), and Fatty Acid Methyl Ester (FAME) profiles. Most amendments successfully delivered microorganisms into the soil in the short term, significantly altering SMCC to various degrees after 2 weeks to 2 months, and some amendments maintained significant effects throughout the field season. Effects on soilborne diseases and tuber yield were variable, with some microbial inoculants and a biostimulant producing no significant effects, whereas arbuscular mycorrhizae, other microbial inoculants, and soil-applied compost tea reduced certain soilborne diseases (black scurf and common scab) and increased yield in some trials. When used in different crop rotations, biological amendments reduced disease and improved yield in some rotations, but not others. Amendments were effective within specific barley rotations, but not in continuous potato. Both crop rotation and amendment treatments significantly affected SMCC, but rotation effects were more extensive. These results indicate that certain rotations were better able to support the added beneficial organisms from amendments and enable more effective biological control, and also that favorable crop rotations may be more effective than amendments in manipulating or altering SMCC. Establishment and persistence of amendment effects may depend on many factors, but an effective and supportive crop rotation is apparently important. 1. Larkin, R. P. 2003. Characterization of soil microbial communities under different potato cropping systems by microbial population dynamics, substrate utilization, and fatty acid profiles. Soil Biol. Biochem. 35:1451-1466. 2. Larkin, R. P., and Honeycutt, C.W. 2006. Effects of different 3-year cropping systems on soil microbial communities and Rhizoctonia disease of potato. Phytopathology 96: (In Press). 3. Larkin, R. P. 2006. Impacts of biological products on soil microbial communities and Rhizoctonia disease of potato. Plant Disease (Submitted). 4. M. T. Brewer and R. P. Larkin. 2005. Efficacy of several potential biocontrol organisms against Rhizoctonia solani on potato. Crop Protection 24:939-950.