Soil microorganisms play an important role in nutrient cycling and can indicate soil quality. A field study was initiated in August 2008 at Perkins and Lake Carl Blackwell, OK switchgrass production sites. This study assessed soil microbial communities in dual purpose and sole feedstock “Alamo” switchgrass following applications of cattle manure, poultry litter, inter-seeded legumes, or inorganic fertilizers. The experimental design was randomized complete block with four replications.  Microbial communities were determined using the phospholipid fatty acid analysis (PLFA) and percent arbuscular mycorrhizal colonization was determined using the grid intersection method. Percent arbuscular mycorrhizal (AM) fungal  colonization in switchgrass roots did not significantly differ across treatments. Therefore, fertilizer source did not alter the abundance of AM fungi in this switchgrass cultivar. In the Perkins site, soil treated with cow manure and poultry manure resulted in the greatest overall microbial biomass, as determined by PLFA. Soils applied with organic fertilizers (cow manure and poultry manure) had significantly greater amounts of gram positive bacteria, gram negative bacteria, AM and saprophytic fungi based on PLFA. This suggests that organic fertilizers contribute to the quantity of several functional groups of soil microorganisms.  In the Lake Carl Blackwell site, sole purpose feedstock grass production enhanced AM and saprophytic fungal neutral lipid fatty acids (NLFA; indicators of storage compounds), as compared to dual purpose switchgrass. Dual purpose switchgrass is cut twice a year whereas sole feedstock is cut only once each year.  Year-round growth for the sole feedstock switchgrass allows continuous photosynthesis and thus continuous carbon allocated to the root system, which in turn may increase root exudates and increase microbial communities.