OR/15/030 Summary

Nicole Archer, Barry Rawlins, Stephen Grebby, Ben Marchant and Bridget Emmett. 2015. Identify the opportunities provided by developments in earth observation and remote sensing for national scale monitoring of soil quality. British Geological Survey Internal Report, OR/15/030.

Defra wish to establish to what extent national-scale soil monitoring (both state and change) of a series of soil indicators might be undertaken by the application of remote sensing methods. Current soil monitoring activities rely on the field-based collection and laboratory analysis of soil samples from across the landscape according to different sampling designs. The use of remote sensing offers the potential to encompass a larger proportion of the landscape, but the signal detected by the remote sensor has to be converted into a meaningful soil measurement which may have considerable uncertainty associated with it. The eleven soil indicators which were considered in this report are pH, organic carbon, bulk density, phosphorus (Olsen P), nitrogen (total N), magnesium (extractable), potassium (extractable), copper (aqua regia extractable), cadmium (aqua regia extractable), zinc (aqua regia extractable) and nickel (aqua regia extractable). However, we also comment on the potential use of remote sensing for monitoring of soil depth and (in particular) peat depth, plus soil erosion and compaction.

In assessing the potential of remote sensing methods for soil monitoring of state and change, we addressed the following questions:

1. When will these be ready for use and what level of further development is required?
2. Could remote sensing of any of these indicators replace and/or complement traditional field based national scale soil monitoring?
3. Can meaningful measures of change be derived?
4. How could remote soil monitoring of individual indicators be incorporated into national scale soil monitoring schemes?

To address these questions, we undertook a comprehensive literature and internet search and also wrote to a range of international experts in remote sensing. It is important to note that the monitoring of the status of soil indicators, and the monitoring of their change, are two quite different challenges; they are different variables and their variability is likely to differ. There are particular challenges to the application of remote sensing of soil in northern temperate regions (such as England and Wales), including the presence of year-round vegetation cover which means that soil spectral reflectance cannot be captured by airborne or satellite observations, and long-periods of cloud cover which limits the application of satellite-based spectroscopy.

We summarise the potential for each of the indicators, grouped where appropriate. Unless otherwise stated, the remote sensing methods would need to be combined with ground-based sampling and analysis to make a contribution to detection of state or change in soil indicators.

Soil metals (copper (Cu), cadmium (Cd), zinc (Zn), nickel (Ni)): there is no technical basis for applying current remote sensing approaches to monitor either state or change of these indicators and there are no published studies which have shown how this might be achieved.

Soil nutrients: the most promising remote sensing technique to improve estimates of the status of extractable potassium (K) is the collection and application of airborne radiometric survey (detection of gamma radiation by low-flying aircraft) but this should be investigated further. This is unlikely to assist in monitoring change. Based on published literature, it may be possible to enhance mapping the state of extractable magnesium (Mg), but not to monitor change, using hyperspectral (satellite or airborne) remote sensing in cultivated areas. This needs to be investigated further. There are no current remote sensing methods for detecting state or change of Olsen (extractable) phosphorus (P).

Organic carbon and total nitrogen: Based on published literature, it may be possible to enhance mapping the state of organic carbon and total nitrogen (but not to monitor change), using hyperspectral (satellite or airborne) remote sensing in cultivated areas only. In applying this approach the satellite data are applied using a statistical model which is trained using ground-based sampling and analysis of soil.

Soil pH: Based on published literature, it may be possible to enhance mapping the state of soil pH (but not to monitor change), using hyperspectral (satellite or airborne) remote sensing in cultivated areas only. In applying this approach the satellite data are applied using a statistical model which is trained using ground-based sampling and analysis of soil.

Soil (peat) depth, erosion and compaction: based on some recent research, it may be possible using a new processing technique for InSAR (interferometric synthetic aperture radar) satellite data to monitor the change in surface elevation of peat soils (and overlying vegetation) as a proxy for change in depth. Further research is required to determine whether this approach could be applied to other (non-peat) soils. This approach is based on radar reflection from the Earth’s surface and it cannot be used to quantify soil depth (status). However, this approach does not require field-based measurement of elevation to ground-truth the remotely sensed data. This technique can detect changes in surface elevation of as little as 1 millimetre over areas as small as one hundred square metres, so it may be possible to use it to monitor soil erosion in cultivated areas. This approach needs to be tested further and applied over large scales. Soil compaction also leads to small reductions in soil surface elevation so it may also be possible to detect compaction-induced changes through remote monitoring. Monitoring based on application of InSAR data needs thorough testing.

Bulk density: there are currently no remote sensing technologies which can contribute to improving measurement of status or change of soil bulk density.