Changes in soil organic matter content of grassland and maize land in the Netherlands between 1970 and 2009.

Soil organic matter (SOM) content is of prime importance for sustained levels of agricultural production. Reports indicate that SOM levels have decreased in, e.g., England, Wales and Belgium. Farmers in the Netherlands are concerned that introduction of more strict legislation on manure application will decrease SOM content in the Netherlands too. Here, we report on changes in SOM contents of grasslands and maize lands using a database with more than 2.5 million records of SOM determinations from farmers' fields. SOM content of grassland on mineral soils remained stable or tended to increase on average by 0.097 g kg-1 yr-1 C during the period 1984-2000. However, there were considerable differences between soil types and regions. Areas with relatively low SOM contents showed increasing SOM contents, while areas with relatively high SOMC (e.g., peaty clays) showed decreasing SOM contents.