pH and the exchangeable aluminum content in acid soils as affected by parent materials and cropping systems

As a result of high annual average temperature and plentiful rainfall, acidity of soils in south China was intensive and a large number of exchangeable aluminum (Al) was accumulated in the soils. Impacts of exchangeable Al on soil properties, soil fertility, and Al toxicity to living organisms have received ample attention. Limestone fertilization has long been regarded as an effective measure to alleviate the toxicity of soluble Al to crops planted in acid soils. Acquirement of the status of pH as well as the amount of exchangeable Al in these soils was essential to resolve the problems above. This paper presented the influence of parent materials and cropping systems on the pH and exchangeable Al content in acid soils. Three types of natural soil samples, namely red soil, latosolic red soil, and latosol were collected from north Guangdong Province, its central, and Leizhou peninsula respectively, which were all representative of local soil type. Soil samples from corresponding cultivated land were also collected. As the results shown, the average pH value of these surface natural soils (0～20cm) decreased as the following sequence: 5.29 for red soil originated from limestone4.77 for latosolic red soil originated from granite≥4.70 for latosol originated from basalt. As for the value of exchangeable Al content in the same soils, the sequence turned into 5.0 cmol/kg(1/3Al~(3+)) for red soil20.7 cmol/kg(1/3Al~(3+)) for latosol 45.6 cmol/kg(1/3Al~(3+)) for latosolic red soil. Both pH and exchangeable Al content in the soils, however, changed to some extent after planting crops. pH of red soil became the lowest among these three soils after planting tea though the soil was inherited from limestone with the highest pH value of natural soils. At the same time, the average exchangeable Al content in surface red soil (0～20cm) was significantly increased from 5.0 to 47.4 cmol/kg(1/3Al~(3+)), which mainly resulted from use of physiological acidic fertilizer such as KCl and (NH_4)_2SO_4 for a long period in the tea plantation. In addition, the average pH value of soils was increased after planting paddy rice, which could be attributed to the depletion of hydrogen ions (H~+) during the reduction of Fe or Mn oxides in soils by reductive substances produced during the decomposition of organic matter under flooding condition. Exchangeable Al content in these paddy soils was also larger than that of corresponding natural soils, which might result from the dissolution of Al oxides by organic acid. Finally, the average pH value of soils was increased after rotation of peanut, while the average exchangeable Al content decreased because some parts of them were exchanged by Ca~(2+) which was introduced to soils after fertilizing limestone. Then these Al were precipitated and turned to be nonexchangeable. As a conclusion, pH value and exchangeable Al content in soils were jointly determined by their parent materials and the crepping systems as well, which should be taken into account in the amelioration of acid soils.