Organic crop management uses traditional farming methods and modern farming techniques without synthetic chemical inputs for crop production. In most cases, the organic production system reduces nutrient in the soil, and causes a surge in population of insect pests, diseases and noxious weeds. Among the many organic practices, mulches (plastic or organic) are increasingly being used to control weeds in organic production, and may provide additional residue C and nutrients to soil.  The objective of this study was to determine the effects of different organic mulches on soil nutrient availability and microbial biomass at 0-20 cm depth in a transitional organic cropping system. The active fractions of soil C used in this study included microbial biomass (MBC), and soil organic C (SOC). Field experiments were conducted in 2009 and 2010 at Alabama A&M University’s Winfred Thomas Agricultural Research Station (WTARS). Three organic mulches [spent mushroom compost (SMC), Sudan x sorghum hybrid (SS), the combination of both (SS+SMC)] were distributed on planting rows tilled for corn and tomato production. There were significant differences in active carbon fractions and soil nutrient levels among treatments. The organic mulches enhanced MBC and SOC, and potential nutrient availability compared to no-mulched soil. Soil nutrients were greater in the spent mushroom compost (SMC) compared to the Sudan x sorghum hybrid (SS) and SS+SMC treatments. Also, it was noticeable that the active carbon fractions were lower in 2010 due to severe drought. From our  results  organic mulch is a suitable alternative to plastic mulch for sustainable soil, microbial biomass and weed management.