The aim of this study was to determine the current, natural and anthropogenic causes of acidification of agricultural soils in Poland. Management of reaction is important in the metabolism of mineral plant nutrients in the soil, especially in their mobilisation/immobilisation, and associated with the circulation of elements in the environment, and thus for the productivity of agricultural ecosystems. After discussing natural causes of acidification, quantitative proton load was evaluated using data from the Central Statistical Office Annals on the average application of NPK fertiliser and agricultural lime and on the emission and deposition of gaseous air pollutants (SO₂, NO_x and NH₃). Based on those data, the load size of proton was calculated and expressed in (kmol H ⁺ ha^-1). The neutralising capacity of calcium fertilisers used in the last two decades was calculated taking into account that 1 mol CaO of lime fertiliser neutralises 2 mol of protons. The load balance of the proton (H⁺) of anthropogenic origin was calculated and used to assess the anthropogenic acidification pressure of agroecosystems over the last 20 years. In recent years, acidification of agricultural soils was primarily associated with a drastic decrease in agricultural lime application: from 182.8 kg CaO ha^-1 in 1990 to 36.8 kg CaO ha^-1 in 2011, an increased share of nitrogen in the NPK fertilisation, where the ratio of N: P: K in 1990 was 1: 0.59: 0.79, and in 2011 1: 0.37: 0.42. Despite a significant decrease in acidic gaseous emissions of air pollutants, especially SO₂ and NO_x and NH₃, these factors continue to increase the pressure of soil acidification.