Long-term application of compost versus other organic fertilizers: effects on phosphorus leaching

Many agricultural soils in north-west Europe contain an amount of phosphorus that not only exceeds the crop's needs, but is also an important source of diffusive P losses to the environment. The restriction of P fertilization also implies a restricted application of organic fertilizers and soil improvers as sources of carbon. Our objective was to compare organic fertilizers in their ability to increase the soil organic carbon level, and their influence on P leaching. Three long-term field trials were conducted where fertilization with compost, farmyard manure, digestates, cattle slurry and mineral fertilizers was compared, to determine soil P availability, soil organic carbon level and the soil P content. Soil samples of the tillage layer (0-30 cm) were used in a leaching experiment in controlled unsaturated conditions. This experiment revealed differences in susceptibility to P leaching between the fertilizer types. We observed that compost based on green waste materials is a better option than cattle farmyard manure to increase the soil organic carbon level, without further increasing P losses in soils with highly elevated P levels. Although farmyard manure has a potential to increase the soil organic carbon level comparable to that of compost, the use of farmyard manure stimulates increased soil P availability and P leaching. The solid fraction of mechanically separated digestate also stimulated increases in soil P availability and P leaching. Stopping P fertilization had an immediate decreasing effect on soil P availability. However, continued zero P fertilization over 4 years did not further decrease soil P availability.