Historical evolution of soil organic matter concepts and their relationships with the fertility and sustainability of cropping systems

Soil organic matter (SOM) is understood today as the non-living product of the decomposition of plant and animal substances. Because it is now recognised that SOM tightly controls many soil properties and major biogeochemical cycles its status is often taken as a strong indicator of fertility and land degradation. Nonetheless the building of the SOM concept has not been easy. A reason for this is that the SOM concept is the product of interdisciplinary cognitive production as well as of a cultural moving context. Historically, three periods involving SOM in relation to cropping sustainability can be distinguished. (1) Until 1840, some still believed that plant dry matter was mainly derived from uptake of matter supplied by SOM, which was termed humus at that time. Agriculturists who believed this based the management of cropping systems fertility on the management of humus, i.e. through organic inputs. In 1809 Thaër proposed a “Humus Theory” that remained very influential for 30 years, as well as a quantified assessment of the agro-ecological and economic sustainability of farming systems. (2) From the 1840s to the 1940s, Liebig's “mineral nutrition theory”, progressive abandonment of recycling of nutrients between cities and country, and breakthroughs in the processes of fertilizer industry paved the way for intensive mineral fertilization as a substitute for organic practices. Although understanding of SOM and soil biological functioning was improving it had little impact on the rise of new mineral-based cropping patterns. (3) Since the 1940s, SOM has been gaining recognition as a complex bio-organo-mineral system, and as a pivotal indicator for soil quality and agro-ecosystems fertility. This has resulted from: (a) methodological and conceptual breakthroughs in its study, leading to significant scientific developments in characterising the role of humus as an ecosystem component; (b) a growing societal demand for the assessment of the environmental cost of intensification in modern agricultural practices, which has led to growing interest in organic farming, agroforestry, conservation tillage, and the use of plant cover; (c) investigation of the potential of SOM as a sink for greenhouse gas carbon in response to concerns about global climate change. In summary the interest in SOM over time, both from the viewpoint of scientific concept and that of field practices, can be described by a sine curve. Its definition and the recognition of its functions have gained both much from the combination of holistic and reductionist approaches and from the progressive amplification of the scale at which it has been considered.