The effect of SOM improving strategies, on the behaviour of soil phosphorus availability and leaching potential in Flanders and the Netherlands

In response to nutrient leaching to the environment, the European Commission introduced the nitrate directive in 1991. The nutrient leaching was caused by over-fertilisation, which led the Flemish and Dutch government to regulate farmers’ fertiliser and manure use. The subsequent restrictions on the N and P-supply has decreased the leaching of nutrients, but it is still substantial. Further restriction is needed to protect the environment from eutrophication. In Flanders, the total organic carbon content of the soil has declined over the last twenty years. The proportion of soils with an excessively low organic carbon content increased by 23% to 35% depending on the region. This decrease is due to several factors: the conversion of grassland into arable land, the increasing plough depth, the use of slurry instead of farm yard manure, and the imposed restrictions in application of animal manure. If manure application is further reduced, the soil’s organic carbon content might fall even faster. The nutrient supply to the soil has to be lowered, but the supply of organic matter to the soil needs to be increased at the same time. The nutrient supply has to be decoupled from the organic matter supply, while manure contains both. N can be separated from organic matter by manure treatment, but this is not possible for P.
The objective of this research is to study the effects of different types of animal manure, compost, mineral fertilizers, soil tillage and green manures on the P-fractions of the soil, the P-leaching and the soil organic matter content (SOM). This will provide us an understanding about the types of manures or combination of practices that the farmer can use to maintain the SOM at a sufficient level, while at the same time meeting crop P-requirements and decreasing P-leaching.
During several multi-annual field experiments in Flanders and the Netherlands, we will study the evolution of SOM and the differences in soil P-fractions under different conditions of fertilization. These field experiments are designed to compare different fertilization types. The P-leaching will be tested on soils with different textures in incubation-percolation-experiments under laboratory conditions. The measured parameters in the different soil and percolation water samples will be: TOC, HWC, PAL, Olsen-P, organic P, P (CaCl2), P (DGT), ortho-P and total P. Special attention will be given to compost and green manures. Compost is typically used to improve SOM and it releases its nutrients only gradually. Green manures are part of Good Agricultural Practices, and can influence the P-availability for the succeeding crop.