Objectives pH is an important index for soil fertility and health. We analyzed the change of soil pH in the farmland across China to provide a scientific basis for making scientific policies and measurements for the protection and improvement of farmland.

Methods The variation of farmland pH value and grading were made by comparing the data between the “Soil Testing and Formulated Fertilization Project” from 2005 to 2014 and “the Second National Soil Census” from 1979 to 1985. The data included the 8.46 million data published in book ‘Basic Soil Nutrient Data Set from Soil Test and Formulated Fertilization’. The variation trend and classification of farmland pH during the past 30 years were compared and analyzed. According to GB/T 33469−2016 farmland Quality Grade, the study area was divided into 9 areas to analyze the spatial distribution and change rate of pH in each region.

Results The results showed that the average pH value of the farmland soil in 25 provinces had a decreasing trend in the past 30 years. In 1979–1985, the proportion of farmland with pH <6.5, 6.5−7.5, 7.5−8.5, and 8.5−9.0 was 35.79%, 25.62%, 36.22%, and 2.22%, respectively, while in 2005–2014, the proportions in the four pH grades were 42.92%, 18.49%, 35.95%, and 2.62%, respectively. Comparing to 1979–1985, the acidification of topsoil in regional scale was intensified in Northeast China, Huang-Huai-Hai, middle and lower reaches of Yangtze River and South China. The proportion of cultivated land with pH below 5.5 increased by 2.61%, 2.24%, 14.93% and 1.16%, respectively. And the soil areas in acid and weak acid grades were all increased. The acidification rate in the neutral and weakly alkaline soil in northeast China and weakly alkaline soil in Huang-Huai-Hai region was higher than that in acid soil, so reasonable measures should be taken to reduce the acidification rate.

Conclusions In the past 30 years, the farmland pH in most area of China tends to decline, the proportion of <6.5 grade is increased by 7.13 percentage points around China. The acidification trend is even worse in the provinces of the middle and lower reaches of the Yangtze River and South China, including Fujian, Hainan, Jiangxi, Guangdong and Zhejiang Provinces, where the soils with pH≤5.5 account for the largest proportion of farmland.