Acidity regime of the Red Soils in a subtropical region of southern China under field conditions

The acidity regimes of Red Soils in Yingtan, Jiangxi Province were examined by determining pH and pCa of the soil paste as well as determining pH in-situ. The results show that for upland soil profiles, the pH decreases gradually from the upper surface layer to the lower layer at a depth of 20 cm by 0.3 units, then decreases slightly until it does not change. For soil profiles under tea trees, the pH decreases gradually from the upper surface layer to the lower layer at a certain depth, then increases slightly with the increase in depth until it reaches a constant value. The distribution pattern of pH of the soil profiles under natural vegetation is similar to that of the soil profiles under tea trees. For paddy soil profiles, the pH increases from the upper layer to the lower layers within the depth of 0-40 cm by 1.37 units, then decreases gradually with increasing depth. For soil profiles under upland crops, tea trees, and natural vegetation, the pCa increases gradually from the surface layer to lower layers. However, in the paddy soil profiles, the pCa decreases from the upper surface downwards to a depth of 40 cm, then increases gradually with increasing depth. The lime potential (pH-0.5pCa) shows a similar trend as the pH. For a given soil, the measured pH value of the soil paste is lower by about 0.5 units than the value determined by the conventional method with a water-to-soil ratio of 5:1 or 10:1. The pH determined in-situ is even lower. The soil acidity status is principally determined by the balance between the leaching loss of base ions, especially calcium ions, and enrichment of these cations from the litter and agricultural measures. (C) 2003 Elsevier Science B.V. All rights reserved.