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Critical levels of soil organic matter - SP0306

When soil drops below threshold levels of soil organic matter (SOM), it becomes susceptible to permanent damage and agricultural production may be compromised. This study will aim to investigate whether thresholds of SOM can be related to any soil parameters (including soil type, soil physical stability, land use practices and crop yield). Published literature will be reviewed to establish the relationships between SOM, soil physical properties (structural stability and strength), soil nutrient reserves and crop production including pedo-transfer functions; usefulness of modelling will also be evaluated. The Soil Survey and Land Research Centre (SSLRC) and ADAS will bring together datasets of soil properties in relation to soil type and, where appropriate, management history at the national and local scale. Data from the datasets will be evaluated critically and numerical relationships will be selected that are judged sufficiently robust to be used in the further development of the project. Relationships between properties such as SOM, land-use, soil particle size distribution, cation exchange capacity, bulk density and porosity will be examined in order to group soils into classes related to soil type and SOM. Results of these analyses will be used to select a number of sites within the same soil type classes under different land-uses, and with differing SOM. The selected sites will be sampled and the aggregate stability of the soil will be determined from measurements of dispersion ratio; in addition, the lower plastic limit (LPL) will be measured as an indicator of soil susceptibility to structural damage. Aggregate stability and LPL will be related to SOM and other soil properties. Soil samples from a number of textural classes, with contrasting SOM levels, will be analysed to assess their N mineralization potential, so that relationships between SOM, mineralization potential, and land use and cropping history can be identified. The datasets provided by SSLRC and ADAS will be used to estimate returns of SOM to soils under different land-use and management practices; crop residue returns over a range of fertiliser N additions will be calculated with particular attention being paid to sandy soils. These data will also provide an estimate of the N input required to produce significant return of crop residues to balance mineralization of SOM. The ability of current organic carbon models (e.g. CENTURY) to predict current SOM contents will be evaluated using sites for which historical data have been already assembled. Use of organic carbon models will be extended by constructing scenarios for larger geographical areas, ranges of soil types and cropping patterns. The effect of climate change will be incorporated by using GCM predictions such as those available through the Climate Research Unit (University of East Anglia). Results of the data analysis, the determination of soil structural and strength properties and the SOM/nutrient studies will be assembled into a risk matrix based on soil/cropping combinations under current and future climate. The risk matrix will be used to both identify thresholds of SOM above or below which significant changes in soil behaviour can be identified and also to produce maps showing those areas of soil/cropping combinations where particular care needs to be taken to maintain (or enhance) SOM concentrations. Findings of the research will be summarised for use by MAFF to give additional guidance through the Codes of Good Agricultural Practice for the protection of soil.
Project Documents
• Final Report : Critical levels of soil organic matter   (190k)
• Final Report : Critical levels of soil organic matter - Word Document   (140k)
• Final Report - Annex : Critical levels of soil organic matter   (147k)
• Final Report - Annex : Critical levels of soil organic matter   (890k)
Time-Scale and Cost
From: 1997

To: 2000

Cost: £179,741
Contractor / Funded Organisations
Soil Survey and Land Research Centre, ADAS UK Ltd.
Fields of Study
Soil Protection