Improving both soil health and production resilience through complementary fertilization strategies is essential for sustainable agriculture. We used data from a 33-year proso millet-potato rotation on the Loess Plateau to compare five fertilization regimes: no fertilizer (CK), nitrogen only (N120), organic fertilizer only (OF6900), and two integrated organic-inorganic treatments (OF1725N90 and OF3450N60). We developed a Soil Quality Index (SQI) and analyzed the relationship between yield variability (coefficient of variation) and the Sustainability Index (SYI). The results show that combining organic and inorganic fertilizers is the most effective strategy for achieving both high, stable yields and improved soil fertility. The combined treatments (OF1725N90 and OF3450N60) produced the highest and statistically similar yields for proso millet (4.51-4.55 t ha-1) and potato (6.35-6.48 t ha-1). However, the relationship between SQI and the SYI was crop specific, indicating a decoupling between soil quality and yield sustainability. For proso millet, OF1725N90 delivered the highest SYI (0.77) despite a moderate SQI (0.63), which reflects a "pathway substitution" effect where yield sustainability improved without maximizing SQI. By contrast, for potato the very high SQI (0.98) under OF3450N60 produced the best outcomes overall, but SYI gains diminished at this highest fertility level, consistent with a "diminishing returns" pattern. These observations indicate that maximizing soil quality alone does not uniformly maximize yield sustainability. Instead, crop-specific optimal soil states-achieved through tailored nutrient management-are essential for improving the resilience and productivity of dryland rotation systems.