The contribution of geophysical methods to the characterisation of soil fertility

In order to tailor agricultural practices as closely as possible to the plant’s needs whilst minimising the environmental impact of fertilisation, it is essential to have as detailed a understanding as possible of the characteristics of the soil – the growing medium – as these are likely to vary significantly over time or across different locations. Here we outline the use of a number of subsurface geophysical tools, in particular the electrical resistivity method, to characterise soil fertility. We begin by outlining the basic principles of electrical resistivity measurement, as well as survey methods using boreholes, electrical panels and towed arrays. We then show that soil structure, a key factor in physical fertility, can be characterised in simple field situations using electrical resistivity. In particular, it is possible to distinguish areas that are heavily compacted by the passage of agricultural machinery from areas that are more porous or more fissured. However, the actual geometry of the compacted areas and the degree of compaction cannot be deduced from electrical prospecting. Secondly, we show that the nitrate content of the soil – provided it remains within the typical range observed in agricultural soils – does not alter the electrical resistivity signal; consequently, this signal cannot serve as a diagnostic method for assessing the soil’s chemical fertility.