Soil treatments and field site

Studying the long term impact of wastewater sludge on soil fertility

The LTSE field site at Hartwood is a dedicated site and is fenced off to prevent disturbance by grazing livestock. The topsoil, typical of that found in the agricultural land in Scotland is a medium textured sandy clay loam consisting of 21% clay and 4.7% organic carbon. This amount of soil organic carbon is typical of Scottish arable or ley-arable agricultural soils. Soil pH is maintained at about 5.8, the recommended practice for grassland in Scotland.

The site was set up with three replicate blocks of 23, 8x 4.5. m plots with a permament 2-3 m grass buffer strip between plots to prevent soil movement during cultivations. The treatments within each replicate block were arranged randomly with the control and digested blank treatments duplicated within each each block.

In the first four years, sludge was applied to the soil surface mannually and then mixed into the soil to a depth of 20-23 cm using a “Celli” spading machine. Subsequently, the plots were cultivated annually using a rotorvator and reseeded using Italian ryegrass.

Soil samples are taken using a screw auger to a depth of around 15 cm, with 20 "cores" removed from each replicate plot in a "W" pattern. The cores are mixed in a bag and passed through a 2 mm sieve to provide a homogeneous soil sample for analysis. Soil not used immediately in the analysis is either stored frozen at -80^oC for DNA analysis or air-dried as part of the National Soil Archive.

Metals and the target concentrations (mg/kg)

Level of heavy metals added to soil plots in the long term build up treatments