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Background to the Long Term Sludge Experiment

Photograph showing a tractor applying sewage sludge
In 1994, three research Institutes along with two private companies set up a number of field experiments at nine field sites across the UK.

In 1994, three research Institutes along with two private companies set up a number of field experiments at nine field sites across the UK (pdf file). The sites were chosen to reflect the different soil types found in agricultural and land uses suitable for application of waste water sludge thereby identifying those soil types more vulnerable to heavy metal contamination.

Host Institutions and field sites of the LTSE

Host Institute Field Site Site Location
The James Hutton Institute Hartwood North Lanarkshire
Rothamsted Research

Watlington
Woburn
 

Oxfordshire
Bedfordshire
Scottish Agriculture College Auchincruive Ayrshire
ADAS Bridgits
Rosemaund
Shirburn
Gleadthorpe
Pwllpeiran
Hampshire
Herefordshire
Oxfordshire
Nottinghamshire
Cereigion

 

Setting up the experiment

The project was divided into three phases: Phase I: 1994 - 1998, Phase II: 1999 - 2001 and Phase III:  2002 - 2006. In Phase 1, sludge cake with high levels of a single heavy metal (Cd, Cu or Zn) was applied annually so as to bring the soil concentrations up to the target level after four years. Subsequently and with the exception of some long-term build up plots, no more sludge was added and the plots were maintained for regular sampling and analysis. Treated waste water sludge was applied at the start and periodically within each phase up to the level of metal determined by the rates as given above.

Soils were sampled periodically throughout the three phases from each site and analysed for heavy metal content, soil chemical properties and biological properties - respiration (measurement of soil microbial activity), microbial biomass (measurement of total microbial population) and rhizobial nodulation (measurement of soil nitrogen fixation).

Soil types and properties used in the LTSE

Site Topsoil texture pH % organic carbon % total nitrogen
Hartwood Sandy clay loam 5.9 4.32 0.25
Auchincruive Sandy clay loam 6.2 3.39 0.20
Watlington Sandy loam 6.8 1.29 0.17
Woburn Loamy sand 6.9 1.12 0.14
Bridgits Silty clay loam 6.3 1.91 0.23
Rosemaund Silty clay loam 6.2 1.69 0.25
Shirburn Calcareous clay loam 8.1 2.23 0.35
Gleadthorpe Sandy loam 6.0 1.42 0.18
Pwllpeiran Clay loam 6.4 3.71 0.41

 

Choice of metals to add to soil

Although waste water sludge could potentially contain a large number of heavy metals it was impossible to test all the metals which may be present and which could be responsible for the toxic effects noted on crops and soil microbial processes. An initial choice had to be made based on previous experimental evidence from sites around Britain. It was unlikely to be chromium, lead and mercury as these are largely insoluble in sludge treated soil of near neutral to neutral pH, and hence are less likely to be bioavailable. However, zinc and copper had been implicated from trials done at Rothamsted Research and at ADAS so were chosen. Cadmium was also chosen as there was some evidence that this might affect the Rhizobium population and hence the capacity to fix nitrogen in soil.

Metals and their concentrations (mg per kg) added to field site

Metal Rate 1 Rate 2 Rate 3 Rate 4 Upper UK permissible limit (at time of experiment)
Zinc 150 250 350 450 300
Copper 50 100 150 200 135
Cadmium 1 2 3 4 3

 

The rates which were chosen reflected on current legislation and the maximum permissible amounts which could be applied to soil.

More details about the waste water sludge and the project are below.

Image of the SAC logoImage of the WRC-NSF logoImage of the Rothamsted logo

Image of the ADAS logoImage of the James Hutton Institute logo


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The James Hutton Research Institute is the result of the merger in April 2011 of MLURI and SCRI. This merger formed a new powerhouse for research into food, land use, and climate change.