D26 - Organic matter content of agricultural topsoils
Description
Related Indicators
Notes
Source: SSLRC, National Soil
Inventory, MAFF project
Coverage: England and Wales
Soil organic matter is derived from plants (mainly crop residues in arable soils), organic manures and the microbial biomass in the soil. It also comes from root, microbial and fungal exudates and fragments, and other organic materials added to soils. Organic matter plays a key role in maintaining soil attributes such as fertility, structural stability, and water holding and buffering capacity, although the mechanisms by which it does so are not completely understood.
Soil management has a major impact on organic matter content levels and characteristics. Draining wet soils and cultivations raises mineralisation rates, lowering organic matter levels and increasing the rate of organic carbon oxidation to carbon dioxide, which is then lost to the atmosphere. Crop systems and manure handling determine the rate and quality of organic material returning to the soil. The beneficial effects of soil organic matter are derived from the young active fraction, while the older more humified material is largely inert. Mineral soils may therefore not be damaged by declining total bulk organic matter, provided the soil receives sufficient returns of fresh, active organic matter. This will not be the case for organic or peaty soils, where declining organic matter levels represents a loss of the soil itself.
Significant changes in organic matter concentrations are likely to be detectable only over long periods. Furthermore, national trends are not easy to identify as organic matter content changes at different rates in different soils, depending on soil type and management. Soil organic matter content tends to develop an equilibrium for a particular land-use and soil type.
Over the past 15 years, organic matter content levels have generally decreased by an average of 0.49% in the 904 arable or ley-arable soils resampled in the National Soil Inventory. Soils that have been under long-term arable management have generally been stable or only lose organic matter content very slowly. The largest decreases have been on grasslands ploughed up for arable use, and on cultivated peaty or organic soils. Some of the decline in organic matter may have been caused by dilution following deeper ploughing over the last 15 years.
Targets are not yet considered appropriate in relation to soil protection (except at the field or part-field scale) because of the imperfect state of knowledge of the highly diverse nature of soils and soil processes.
Adoption
of alternative farm management systems
Area converted to organic farming
Nitrate and phosphorus losses from agriculture
Phosphorus levels in agricultural topsoils
Manure management
Ammonia emissions from agriculture
Emissions of methane and nitrous oxide from agriculture
Accumulation of heavy metals in agricultural topsoils
Changes in the organic matter level, expressed here as organic carbon percentage of non-humose mineral arable and shortterm grassland topsoils, have been assessed. A total of 900 National Soil Inventory sites, located on arable or ley-arable land were resampled in 1994–96 and the organic matter contents compared with the original samples from 1978–81. The resampling is being extended to another 782 permanent grassland sites and will be reported in future indicators.
In order to develop the indicator further, MAFF has commissioned a three-year project which started in April 1997 to establish whether there are thresholds of organic matter below which soils are at risk of irreversible damage.