This study uses high-throughput DNA pyrosequencing and statistical analysis to assess bacterial diversity in four soils across the western hemisphere. The soils sampled included three agricultural soils from Brazil, Florida, and Illinois, and a boreal forest soil from Canada. The number of bacterial 16S rRNA sequences obtained from each site ranged from 26,140 to 53,533. The most abundant bacterial groups in all four soils were Bacteroidetes, Betaproteobacteria, and Alphaproteobacteria. Using three diversity estimators, the maximum number of unique sequences (operational taxonomic units, OTUs) never exceeded 52,000 at the lowest level of dissimilarity. The forest soil was phylum rich compared to the agricultural soils, which were species rich but phylum poor. The forest site also showed far less diversity of Archaea, with only 0.009% of sequences from that site being from this group, compared to 4-12% in the agricultural soils. The study suggests that agricultural management significantly influences bacterial and archaeal diversity. The results indicate that the number of bacterial species per gram of soil is likely between 2000 and 8.3 million, but the methods used here can estimate the number of OTUs with high accuracy. The study also highlights the importance of soil management in shaping microbial diversity and the need for further research to understand the factors influencing soil microbial communities. The findings suggest that forest soils may be more diverse than agricultural soils, and that nitrogen cycling in agricultural soils may play a key role in archaeal diversity. The study provides a comprehensive examination of bacterial diversity in soil and highlights the potential of pyrosequencing for assessing microbial diversity in various environments.This study uses high-throughput DNA pyrosequencing and statistical analysis to assess bacterial diversity in four soils across the western hemisphere. The soils sampled included three agricultural soils from Brazil, Florida, and Illinois, and a boreal forest soil from Canada. The number of bacterial 16S rRNA sequences obtained from each site ranged from 26,140 to 53,533. The most abundant bacterial groups in all four soils were Bacteroidetes, Betaproteobacteria, and Alphaproteobacteria. Using three diversity estimators, the maximum number of unique sequences (operational taxonomic units, OTUs) never exceeded 52,000 at the lowest level of dissimilarity. The forest soil was phylum rich compared to the agricultural soils, which were species rich but phylum poor. The forest site also showed far less diversity of Archaea, with only 0.009% of sequences from that site being from this group, compared to 4-12% in the agricultural soils. The study suggests that agricultural management significantly influences bacterial and archaeal diversity. The results indicate that the number of bacterial species per gram of soil is likely between 2000 and 8.3 million, but the methods used here can estimate the number of OTUs with high accuracy. The study also highlights the importance of soil management in shaping microbial diversity and the need for further research to understand the factors influencing soil microbial communities. The findings suggest that forest soils may be more diverse than agricultural soils, and that nitrogen cycling in agricultural soils may play a key role in archaeal diversity. The study provides a comprehensive examination of bacterial diversity in soil and highlights the potential of pyrosequencing for assessing microbial diversity in various environments.