From 2006 to 2010 the soil quality of the Netherlands was analyzed during the third cycle of the National Soil Quality Monitoring Network (LMB). For this purpose, 200 locations were sampled: 10 combinations of soil type and land use (=category), and 20 locations per category.
Current soil quality
In accordance with the first goal, a survey was made of the soil quality of the ten categories and the categories were compared. Sandy soils under forest have the lowest acidity and the highest aluminium concentration of all categories. Sandy soils under agriculture have a higher pH, probably due to liming. As expected, clay soils contain a larger portion of particles smaller than two micrometers, and peat soils have a higher organic matter content than sandy soils. Clay and peat soils have significantly higher iron, manganese and heavy metal contents than sandy soils. Insecticides such as lindane and dieldrin were mainly found in soils under arable land. Even though these crop protection products are no longer for sale, remnants can still be found.
Changes in soil quality between the first and third cycle
In accordance with the second goal, for each category the changes between the first (1993-1997) and third cycles (2006-2010) were documented. Amongst other things, it was found that especially where sandy soils under forest are concerned, soil and groundwater quality has changed. These soils have become significantly less acidic. Also nitrate, sulphate, chlorine, aluminium, calcium, magnesium, sodium and strontium in groundwater have decreased. This has probably been caused by less acidifying and eutrophicating substances being deposited through the air, a positive consequence of emission policy.
For each category long term analyses show several significant increases and decreases of substances in the soil. Changes in the sampling locations and in procedures used may have interfered with these trend analyses. For example, between the cycles ten to twenty percent of the locations at which samples were taken changed. Similarly, over the years procedures in laboratories sometimes changed, especially for heavy metals. Because of this, it was not possible to realize the original LMB strategy which was to monitor at fixed times and locations, following fixed sampling and analysis procedures. The monitoring program turned out to be less sensitive to observing changes in soil quality than was calculated at the start.
The advice here is to reconsider the goals of the LMB, also because thanks to policy measures, heavy metal concentrations show fewer large changes than at the start of the LMB. It is recommended to apply the monitoring network mainly to determine the current soil quality and furthermore to study the effect of land use, using soil pressure data from the Agricultural Economics Institute (LEI).