<p>Pampa biome soils have higher Ca and Mg than tropical soils. However, they also present low soil pH and high Al saturation, which can be cheaply amended with local dolomitic limestone. This study aims to monitor the long-term (18 yrs) changes in the soil acidity-related properties and the exchangeable Ca/Mg ratio and their impact on crop yields as a result of applying lime with different Ca/Mg ratios. In 2004, a field trial established in an Ultisol from Southern Brazil’s Pampa biome tested five lime Ca/Mg ratios (35.7, 10.7, 5.9, 3.5, and 2.7) to raise soil pH to 6.0, plus a control with no lime. The treatments were applied by broadcasting and then half of the plot area was incorporated into the soil. After this single incorporation operation, the entire experimental area was kept in no-tillage. Soil samples (0–20&#xa0;cm) were collected at 0, 12, 44, 141, and 204 months to assess exchangeable Ca, Mg, and Al. Data from 15 crops were used to correlate soil Ca/Mg ratios with crop yields and to estimate critical Al saturation levels for wheat, canola, soybean, and forage mix. The exchangeable Ca/Mg ratios ranged from 1.0 to 6.3, and Ca saturation ranged from 26 to 85%, depending on the type of lime, application method, and diagnostic soil layer. There was no statistical relationship between exchangeable Ca/Mg ratios and Ca saturation values with crop yield. The value of the critical level of Al saturation increases as soil from deeper layers is sampled (14 to 49%), with a higher gradient when broadcast. Canola + wheat crops produced only 47% of their relative yield in soil without acidity correction, while soybeans + winter cover crops reached 86%. Ca/Mg ratios do not affect annual crop yields, but in two of the three wheat crops the highest yields occurred with the use of limestone with a Ca: Mg ratio of 10.7. Dolomitic limestone is recommended for its abundance, low cost, and adequate Ca and Mg supply.</p>