Optimization of the soil structure in arable crop production by means of lime (CaCO3) application
Mechanical interventions in the form of soil cultivation on agriculturally used land lead to changes in mechanical, hydraulic and biological soil properties that affect the soil structure. As a result of the applied stress, the pore functions are impaired, and the water and air balance are negatively affected. In order to establish a more stable and at the same time looser soil structure, an optimal supply of lime to the soils is essential. Important chemical (pH value, nutrient sorption (CEC)), biological (microbial activity) and physical (water and gas transport) properties on soils of different textures can be changed in such a way that this leads to an increase in nutrient storage and availability for plants and makes an important contribution to soil protection. The present study therefore deals with the effect of different levels of lime (CaCO3) on various soil properties. Seven (six of them in this thesis) field trials were established with three different rates of lime in various soil regions in Germany, with a wide range of different textures, clay contents and soil organic matter. Disturbed and undisturbed soil material was taken from the topsoil (0-30 cm) during the annual sampling (2017-2020) to quantify the stability and structural changes of the soil on the mesoscale (precompression stress and frame shear tests as well as crushing tests). Furthermore, the bulk density, pore size distribution, air- and saturated water conductivity were measured, and the chemical properties of the equilibrium soil solution were determined. The stability but also the structural change of a soil depends on the complex interaction and a wide range of soil properties and mechanisms. Depending on the clay and organic matter content, the soils show different reactions to lime additions and thus to the release of Ca2+ and carbonates regarding particle interaction. In the stability measurements on the mesoscale, there are already overlaps of a possible lime effect by external factors, such as the annual tillage, which complicates transferability of the Ca2+ effect. Factors such as aggregation and time of sampling as well as associated swelling and shrinkage processes also influence the effect or accessibility of Ca2+ to the exchange places. Nevertheless, a (time-dependent) correlation between changes in the pore system and stability as well as structure-changing processes can be observed during frame shear tests. While an increased addition of lime to sandy soils can lead to both a (short-term) improvement in water retention and saturated water conductivity and shear strength, such an effect can be seen with a delay for soils containing considerable amounts of clay. The rearrangement, interlinkage and connection of soil particles improve the mechanical stability of soil aggregates, shear resistance and the stability of the pore system. Liming reduces the swelling capacity of clay-rich soils and can thus mitigate structural weaknesses of these soils. Therefore, the trafficability of such soils can be improved by liming.