This study utilized high-throughput DNA pyrosequencing to assess bacterial diversity in four soils across a large transect of the western hemisphere. The number of 16S rRNA sequences obtained from each site ranged from 26,140 to 53,533. The most abundant bacterial groups were Bacteroidetes, Betaproteobacteria, and Alphaproteobacteria. Using three diversity estimators, the maximum number of unique sequences (operational taxonomic units, OTUs) never exceeded 52,000. The forest soil showed higher phylum richness compared to agricultural soils, which were species-rich but phylum-poor. The forest site also had significantly less diversity of Archaea, with only 0.009% of sequences from this group, compared to 4%–12% in the agricultural sites. This comprehensive examination suggests that agricultural management can significantly influence bacterial and archaeal diversity in soil.This study utilized high-throughput DNA pyrosequencing to assess bacterial diversity in four soils across a large transect of the western hemisphere. The number of 16S rRNA sequences obtained from each site ranged from 26,140 to 53,533. The most abundant bacterial groups were Bacteroidetes, Betaproteobacteria, and Alphaproteobacteria. Using three diversity estimators, the maximum number of unique sequences (operational taxonomic units, OTUs) never exceeded 52,000. The forest soil showed higher phylum richness compared to agricultural soils, which were species-rich but phylum-poor. The forest site also had significantly less diversity of Archaea, with only 0.009% of sequences from this group, compared to 4%–12% in the agricultural sites. This comprehensive examination suggests that agricultural management can significantly influence bacterial and archaeal diversity in soil.