Soil health through soil disease suppression: Which strategy from descriptors to indicators?

Soil is a component of primary importance in crop production, even if it is often neglected, or only regarded as a physical support for the growth of plants. However, with the increasing societal concerns for the sustainability of agriculture, soil must be considered as a living system. Its quality results from the multiple interactions among physicochemical and biological components, notably the microbial communities, primordial for soil function. Crops are threatened by soil-borne diseases. These are often difficult to control, because of the “hidden” status of the pathogens and also because of the absence, noxiousness or lack of efficacy of chemical treatments. In this context, there is a renewed interest for cultural practices such as the use of organic amendments. These practices have a strong influence on soil health, which takes into account plant health, through both changes of physicochemical characteristics and influence on soil microbial communities. Cultural practices are used to improve soil health, and can, in some cases, increase soil disease suppression. The effects of these practices on soil properties have been studied, but the relationship between these properties and soil suppressiveness has not been always very clear. Many different soil descriptors, either abiotic or biotic, have been used to describe the soil health and suppressiveness, but there is a lack of identified, reliable and consistent indicators. One aim of this review is to show that, despite the age of the soil health concept and all the studies that have been conducted, there are still no guidelines for assessment of soil quality from the plant health point of view. Obviously, the extreme diversity of situations makes any generalisation from a given case-study difficult. However, based on what has already been done in related fields, a methodology to search for indicators of soil health can be proposed. In this review we will present (i) how a healthy soil can be defined and what are the concepts hidden behind the words “soil health,” (ii) which cultural practices have been used to control soil-borne diseases and their limitations, (iii) which soil parameters have been measured when studying soil suppressiveness, and which relationships have been found between these parameters, and finally (iv) how these descriptors could become indicators of soil health, using appropriate analytical and statistical methods.