Farmers used to sniff their soil to gauge how healthy it was. According to ecologist Rosa Boone, you can deduce a great deal from its smell: ‘The fungi and bacteria present in the soil are important for soil health and are responsible for the soil’s smell.’ Boone discovered that the smell of soil can be captured and analysed in the lab, providing insights into these microbial communities. ‘Monitoring soil life is becoming increasingly important, particularly in view of the forthcoming soil monitoring guidelines. My research shows that there is great potential for developing a sensor – an ‘electronic nose’ – that could be used to analyse microbial soil life directly, based on how it smells.’
Tubes
For her research, the ecologist installed tubes in 18 grasslands in the Ooijpolder, near Nijmegen. The grasslands were all used by farmers, but under different management practices: some farmers used more artificial fertiliser (intensive management), while others farmed entirely naturally (extensive management) – and everything in between. The large tubes had holes through which odours could enter. Inside each large tube, Boone suspended a smaller tube connected to a pump. ‘The pump drew air from the soil into the smaller tube, where the odours were captured because they ‘stuck’ to it. We could then analyse those odours in the lab.’
Odour analysis
There are plenty of farmers who want to experiment with more extensive farming methods, such as using less artificial fertiliser, more organic manure and greater plant diversity. At the same time, they want to be able to monitor the effect this has on soil life, as Boone learnt from interviews. ‘Farmers need an analysis of the communities living in their soil, but they don’t want to have to carry out a complicated DNA analysis every month. A sensor that captures odours would be much simpler.’
The results showed that odours do indeed reveal information about the fungi and bacteria living in the soil and about the overall state of the microbial community – in other words, which species are present, how many there are and what they do.
‘We were able to identify 75 odours, 16 of which were very good at indicating whether a farmer was managing their land differently; 9 increased as management became more extensive, while 7 actually decreased. It was striking that aldehydes in particular – compounds largely produced by fungi – proved to be sensitive to changes in soil management. If management practices change, this can affect the fungal and bacterial communities, which in turn alters the odour profiles.’
The importance of soil life
Soil life, including the fungi and bacteria in the soil, is the driving force behind healthy soil, and management practices can be used to try to steer that soil life. Boone says: ‘We saw this in our other study too: grasslands under more extensive management had a specific fungal community, there was more carbon in the soil, and soil life was more active. Healthier soil can also be more resilient during extreme weather conditions, such as a dry summer. By stimulating soil life, you need fewer external inputs, helping to build more sustainable and future-proof soil.’
The electronic nose isn’t here yet, and there are still a few steps to be taken before it can be developed, but Boone’s research shows that it has great potential. ‘We’ve made significant progress, and I can certainly envisage the electronic nose becoming a reality in the future.’
Boone’s research was made possible in part by imec, via the OnePlanet Research Centre.