Effects of land application of farm dairy effluent on soil properties: a literature review.

Land application of wastes is becoming an increasingly popular practice. The application of farm dairy effluent (FDE) to land, as opposed to direct discharge to waterways, is the preferred mechanism for disposal in New Zealand as regulatory authorities move to protect and enhance water quality and meet Maori spiritual and cultural values. For example, in the Waikato the percentage of dairy farmers who apply FDE to land has risen from 35% in 1993 to effectively 100% in 2004. Land application recognises the nutrient value of FDE; however, it is not without risks. At present the management of land application of FDE is primarily concerned with potential nitrate contamination of groundwater. There are other, considerable, potential risks such as microbial contamination and impacts on soil properties. Despite soil properties being spatially extremely variable and the effects of land application of FDE being long-term, the majority of studies on the impacts of FDE to soil properties are limited in their temporal and spatial extent. The evidence of effects of FDE on soil properties is limited; however, it is reviewed and linked to the effects of land application of other effluents in general and their effect on specific soil properties. The effects of FDE application on soil physical properties are highly variable (e.g., both increases and decreases in soil hydraulic conductivity have been observed). This is in part because of measurement issues and the difficulty in separating out the effects of the FDE application from the effects of stock management. Overall, most changes due to FDE application appear to improve the soil's long-term fertility, i.e., increased concentrations of total nitrogen, total phosphorus, and plant available nutrients. FDE application has been observed to result in a greater and more diverse microbial biomass. However, optimal environmental management and the continued use of the soil for FDE management require ongoing monitoring. [ABSTRACT FROM AUTHOR]