The physical properties of a Luvisol derived from loess near Bonn, Germany, under different long-term fertilization treatments were examined. For the investigation of the impact of farmyard manure (FYM) on soil strength at the mesoscale (100 to 300 cm 3 soil cores), undisturbed samples were taken from two different depths (10 and 40 cm), either with no fertilization at all, with full mineral fertilization, with FYM only, and with both mineral and organic fertilization. We investigated hydraulic and mechanical parameters, namely precompression stress, pore-size distribution, saturated hydraulic and air conductivity, and calculated pore connectivity. Long-term organic fertilization resulted in significantly more and coarser pores which in addition were more conductant and mechanically stronger by trend. Mineral fertilization also increased pore volume by trend but not pore functionality. Mechanical strength generally increased with fertilization by trend, however, was reduced again when organic and mineral fertilization were combined. Nonetheless, FYM led to relatively higher soil strength as the FYM-treated plots with lower bulk density attained similar soil strength as the unfertilized but denser plots and thus supported the soilimproving impact of organic amendments. The subsoil physical properties were rather unaffected by fertilization, but were dominated by texture.
We determined the impact of different fertilization, namely organic vs. mineral fertilization, on the mesoscale parameter cyclic compressibility as well as on rheology of soil samples as a microscale parameter and how these parameters are related. Therefore, undisturbed samples were taken from a long-term fertilization trial at the Dikop farm near Bonn (Germany) and tested for their mechanical and hydraulic properties. This paper examines the sensitivity of the soil towards cyclic loading (mesoscale) and oscillatory shearing at the microscale by means of an amplitude sweep test and the resulting parameter maximum shear stress. Fertilization increased cyclic compressibility and thus revealed structural weakness of fertilized soil samples, so did shear stress at the microscale. The main reason for this was a decrease in bulk density in the wake of fertilization. However, within the range of fertilized soil samples, the soil structure became less susceptible towards cyclic loading and oscillatory shearing, respectively, the more organic matter the soil contained (equivalent to the fertilization level). This was assumedly caused by enhanced cementation due to organic substances that could partly substitute the direct grain-grain contacts generally contributing to soil strength. The similar behavior of cyclic compressibility and maximum shear stress enabled a first approach to relate soil mechanical parameters at the microscale to those at the mesoscale.
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