Effects of liquid organic fertilisers on selected soil quality indicators

Anaerobic digestion of organic household waste as a means of energy production leaves considerable amounts of organic residues. These biogas digestates may represent a valuable source of nutrients and organic material, especially in areas with little application of animal manure to arable land. However, due to a typically high amount of available nitrogen and a low dry matter content, only small amounts of organic material are added with untreated digestates, also compared to other liquid organic fertilisers such as cattle slurry. The main objective of the study was to determine how the quality of liquid organic fertilisers affects soil properties. In soil samples from three field experiments in south-eastern Norway, aggregate stability, water retention characteristics, and selected chemical properties were determined. After three to seven years of application, the organic fertiliser treatments showed a positive effect on aggregate stability and pH compared to inorganic or no fertiliser, as well as a trend to higher carbon concentrations especially in soils with relatively low organic matter content. Effects on water retention characteristics depended on the quality of the organic fertilisers: Cattle slurry addition increased the number of medium-sized pores and thus plant-available water, whereas digestate addition instead increased the number of small pores in the soil.