Rapid redistribution of P to deeper soil layers in P saturated acid sandy soils

Problems of excessive soil phosphorus (P) levels as a result of intensive agriculture are found in many regions in Western Europe, USA, Canada and New Zealand. This may lead to phosphorus leaching in soils with low P binding capacity. However, little is known about the changes in P saturation degree (PSD) of such soils with time. Between 1995 and 2005, an intensive inventory of the PSD status of acid sandy soils in Flanders was conducted, the results of which were used to enforce strict rules on P fertilizer inputs on P saturated soils. A new smaller survey on a selection of these fields was undertaken in 2009 and 2010. Comparison of the survey results shows that the mean PSD increased significantly from 46 to 59% over this period. We found evidence for a strong shift of the PSD from the upper to lower layers. The PSD level in the top layer (030cm) generally increased significantly (P<0.01) from 83 to 91%. The average increase in the PSD level of the 3060 and 6090cm layers was even greater, from 33 to 55% and from 14 to 25%, respectively (P<0.01). Current limits on P fertilizer application have not yet resulted in P mining in these soils and will thus need to be further restricted. The very clear increase in PSD movement in deeper layers from both increased Pox, and reduced Feox concentrations show that these high PSD soils pose a very serious and direct threat to groundwater quality.