The study looked at how purple soil in Chongqing has become more acidic over the past two decades due to acid rain. By comparing soil data from the 1980s to recent samples, researchers found that alkaline soil has decreased while acidic soil has increased. This means the pH of the soil has gone down, and there is more acidity and aluminum in the soil now. The area affected by soil acidification is growing, and the problem is getting worse over time.