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. 2005. The role of crop rotations in determining soil structure and crop growth conditions. Can. J. Soil Sci. 85: 557-577. Increasing concern about the need to provide high-quality food with minimum environmental impact has led to a new interest in crop rotations as a tool to maintain sustainable crop production. We review the role of rotations in the development and preservation of soil structure. After first introducing the types of rotations in current practice and their impact on yield, we assess how soil and crop management in rotations determines soil structure, and in turn how soil structure influences crop growth and yield. We also briefly consider how soil structure might contribute to other beneficial effects of rotations, namely nutrient cycling and disease suppression. Emphasis is given to the influence of crop choice and, where relevant, interaction with tillage system and avoidance of compaction in the improvement and maintenance of soil structure. Crop rotations profoundly modify the soil environment. The sequence of crops in rotation not only influences the removal of nutrients from a soil, but also the return of crop residues, the development and distribution of biopores and the dynamics of microbial communities. These processes contribute to the development of soil structure. We have identified areas where further research is needed to enable the potential benefits of rotations in the management of soil structure to be fully exploited. These include: improved quantitative linkages between soil structure and crop growth, the consequences to soil structure and nutrient cycling of crop residue incorporation, developing natural disease suppression, amelioration of subsoils by crop roots, the fate of carbon deposited by plant roots in soil and the fate of organic nitrogen in soil. Les auteurs passent en revue le rôle de l'assolement dans l'évolution et la préser-vation de la structure du sol. Après avoir présenté les diverses méthodes d'assolement en usage et leur incidence sur le rendement, ils expliquent comment la gestion du sol et des productions végétales durant la rotation affecte la structure du sol et comment cette dernière agit sur la croissance et le rendement des plantes. Les auteurs examinent aussi brièvement la manière dont la structure du sol pourrait contribuer autrement aux effets bénéfiques de l'assolement, notamment dans le cycle des éléments nutritifs et la lutte contre la maladie. Ils insistent sur l'incidence du choix de la culture et, quand la chose est pertinente, sur l'interaction avec le système de travail du sol et la nécessité d'éviter le compactage si l'on veut améliorer ou préserver la structure du sol. L'assolement modifie considérablement l'environnement tellurique. Les cultures qui se succèdent lors de la rotation n'influent pas seulement sur le retrait des éléments nutritifs dans le sol, mais aussi sur la réintégration des débris végétaux, sur le développement et la réparti-tion des biopores et sur la dynamique de la microflore. Ces processus concourent au développe...

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It is widely recognized that the application of organic materials is one of the most effective ways of increasing soil organic carbon (SOC) levels and improving soil quality, but do all forms of organic matter input have the same impact on soil properties A network of seven experimental sites investigated the effects on soil quality of annual applications over a minimum of 3 years of compost and food-based digestate in comparison with farmyard manure (FYM) and livestock slurry. Two of the sites were existing experimental platforms which had previously benefitted from applications of FYM, livestock slurry and green compost allowing the effects of longer-term applications (6-17 years) on soil properties to be quantified. The application of all organic materials increased soil nutrient supply (total nitrogen, extractable phosphorus, potassium, and magnesium) within a short timescale (<3 years), whereas SOC contents were only increased following the long-term (9 years or more) application of bulky organic materials (compost and FYM). SOC increases were associated with improvements in soil biological (microbial biomass) and physical properties (reduced bulk density), although the level of improvement was dependent on the quality of the organic material applied (as determined by its lignin content, an indicator of resistance to decomposition). Applications of low dry matter content materials (digestates and livestock slurries) had a limited capacity to improve soil biological and physical functioning, due to their low organic matter loading.

Amaranthus retroflexus L. is a common annual weed worldwide. It can be found in a wide range of habitats and causes substantial yield reduction in many crops mainly through competition. Alternaria spp. are airborne molds that are considered to have potential for the biological control of weeds. The aim of this study was to assess the effect of spore concentration, host-plant growth stage, dew period, and temperature on the pathogenicity of three Alternaria alternata isolates against A. retroflexus. The pathogenicity of A. alternata increased with increasing spore concentration and length of dew period. A spore concentration of 107 spores ml−1 in a rapeseed oil emulsion and given a 24 h dew period caused 100% mortality of A. retroflexus plants at the four-leaf stage. Infection and mortality in older plants (>four-leaf stage) was lower. The highest levels of plant mortality were obtained at post-inoculum temperatures between 20 and 30 C. These experiments confirm the potential of A. alternata as a mycoherbicide under specific environmental conditions.