Abstract: Organic dairy (OD) production is increasing in the Northeastern United States due to consumer demand. Some physicochemical properties of OD manure differ from conventional dairy (CD) manure, which could influence nutrient cycling and soil fertility differently when OD manure is applied as fertilizer to meet plant N demands. Effects of OD manure on activities of acid phosphomonoesterase (ACP), alkaline phosphomonoesterase (ALP), phosphodiesterase (PDE), available soil phosphorus (P), and plant growth were investigated in a greenhouse study, where sorghum-sudangrass (Sorghum bicolor subsp. drummondii) was fertilized with manures from 13 organic dairies in Maine, CD manure, or NH4NO3. Soil phosphatase activities and modified Morgan P were determined at planting and after 16 weeks of plant growth. Plant growth did not differ (P > 0.05) when fertilized with OD and CD manures or inorganic fertilizer. However, there was a wide range in growth with OD manure, which was negatively correlated to manure C:N and C:P (P < 0.05) ratios. After 16 weeks, OD manure amended soils had higher modified Morgan P than soils with inorganic fertilizer (P < 0.05), but there was no difference between OD and CD manured soils (P > 0.05). Of the three soil phosphatases, ACP activity was highest and increased with OD manure similarly to CD manure. There was a negative correlation (P < 0.01) between ACP activity and manure C:P ratio, suggesting that manure C content influences P cycling and may reduce P availability in soils amended with OD manure, in a manner similar to CD manure.