The effect of K fertilization on microstructural soil stability is rarely analyzed until now although the ambiguous impact on bulk soil structure was reported quite often, e.g., with regard to higher erodibility on the one hand and higher water storage on the other. Soil material from different long-term fertilization trials in Germany was examined rheologically by means of an amplitude sweep test where the samples were subjected to oscillating shearing with increasing deflection. The resulting shear stress was recorded, and the maximum stress denoted the maximum shear strength of the sample. Results showed an ambiguous influence of K which depends strongly on the soil properties. On the one hand, an increased ion concentration in the soil solution leads to increasing attractive forces as defined by the DLVO theory and therefore higher shear resistance. With increasing desiccation, K+ like other salts can precipitate at the contact areas between particles and lead to cementation. On the other hand, K+ as a monovalent ion impedes covalent and ionic bonding between clay minerals which holds true for most of the examined soil types while only sandy soils showed an increase in soil strength due to K fertilization. Potassium depletion further resulted in increased interaction of fertilization with other impact factors, e.g., climate and soil properties. Thus, the destabilizing effect of K+ was more pronounced under liming as without liming. Subsequent modeling with selected soil parameters confirmed the high influence of matric potential. The modeling also revealed the interactions with other soil parameters, e.g., pH, oxides, texture, exchangeable cations as well as lack or surplus of K in relation to recommended K content. In conclusion, microstructural stability of soil depends on several soil parameters and requires the inclusion of many chemical and physical soil properties.