Effects of Organic Dairy Manure Amendment On Soil Phosphatase Activities.

Organic dairy (OD) production is increasing in the Northeastern U.S. due to consumer demand. Some physico-chemical properties of OD manure differ from conventional dairy (CD) manure, which could influence nutrient cycling and soil fertility differently when OD manure is applied to soil. Effects of OD manure on soil phosphatase activities [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 Maine organic dairies, inorganic fertilizer, or CD manure. Soil phosphatase activities and Modified Morgan P were determined at planting and after 16 weeks of plant growth. Plant biomass production did not differ (p > 0.05) between OD and CD manures, or inorganic fertilization. However, there was a wide range in biomass production 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 that received inorganic fertilizer (p < 0.05), but there was no difference between OD and CD manured soils (p > 0.05). Of the three major soil phosphatases, ACP activity was highest, and increased with OD manure in a manner similar to CD manure. There was a negative correlation (p < 0.01) between ACP activity and OD 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.