Soil sickness represents a condition in which the long-term use of non-sustainable agricultural practices causes changes in the physical, chemical and biological properties of soils that, in turn, negatively affects plant vegetative and reproductive performances. By an extensive analysis of literature, we found that soil sickness is pervasive in agro-ecosystems, occurring in 111 cultivated plants belonging to 41 taxonomic families. To explain the phenomenon of soil sickness, three main hypotheses have been proposed, including soil nutrient depletion or imbalance, build-up of soilborne pathogens coupled with shift in the composition of soil microbial community composition, and presence of phytotoxic and autotoxic compounds. Starting from a detailed analysis of mechanisms it was previously suggested that all proposed hypotheses have as common origin, i.e. the alteration of organic matter cycle caused by intensive agricultural practices.
Based on this consideration, in the present thesis different organic management strategies, in terms of organic matter type and application frequency, were used in order to recover a soil affected by soil sickness. Soil was subjected for two years to 11 different treatments including two ordinary soil managements, eight organic amendment treatments and one untreated soil as the control. At the end of each year, cumulated crop production of Eruca sativa, soil properties and soil microbiota were evaluated. Compared to the use of ordinary managements, the beneficial effects on soil properties and microbial community derived by the use of organic amendments were evident already after one year of conditioning. In detail, pH values near the neutrality, high soil organic carbon content and good level of soil aggregation, as well as an improvement in soil microbial functionality, richness and diversity were observed in soil treated with organic amendments, especially when easily decomposable materials rich in labile carbon and organic nitrogen (i.e., alfalfa plus glucose) were applied at high rate once a year. In contrast, cumulated crop production at the end of the first year was higher in soil with ordinary managements than in soil with application of organic materials. However, during the second year of soil conditioning, an increase in productivity and quality of the crop was observed in soil treated with organic materials as compared to the soil subjected to conventional management. Finally, soil conditioned for two years was used to evaluate the effects that ordinary and organic management strategies had in the disease suppression of soilborne phytopathogenic fungi and viruses. Application of organic amendments, by positively affecting soil properties and soil microbiota, showed a restoration of natural soil suppressivines against soilborne pathogens (i.e., Sclerotinia sclerotiorum and Fusarium oxysporum f. sp. raphani). Surprisingly, this study reports for the first time that the use of organic matter reduces the incidence of Tomato spotted wilt virus infection, as well as the mortality of infected plants, probably by the induction of systematic resistance.
In conclusion, this study revealed that applications of organic materials have an immediate positive effect on soil fertility as well as on soil microbiota, while the increase of crop productivity are of longer-term nature. In addition, the positive effect that organic amendments have on microbial communities, including their abundance, diversity and richness of the several taxa, results in a recovery of the natural soil suppression against soilborne pathogens and the induction of plant resistance against airborne pathogens like viruses. However, the effects on crop production, soil fertility and disease suppression varied depending on quality, amount and frequency of application of organic matter. Therefore, future studies that include different combinations of organic amendment types and application frequencies, as well as different soil types, crop species and pathosystems, are needed to better understand the role of organic matter as a means to recover of soils affected by soil sickness
