The process of organic materials increasing soil pH has not yet been fully understood. This study examined the role of cations and organic anions in regulating soil pH using organic compounds. Calcareous soil, acid soil, and paddy soil were incubated with different simple organic compounds, pH was determined periodically and CO2 emission was also measured. Mixing organic acids with the soil caused an instant decrease of soil pH. The magnitude of pH decrease depended on the initial soil acidity and dissociation degree of the acids. Decomposition of organic acids could only recover the soil pH to about its original level. Mixing organic salts with soil caused an instant increase of soil pH. Decomposition of organic salts of sodium resulted in a steady increase of soil pH, with final soil pH being about 2.7-3.2 pH units over the control. Organic salts with the same anions （citrate） but different cations led to different magnitudes of pH increase, while those having the same cations but different anions led to very similar pH increases. Organic salts of sodium and sodium carbonate caused very similar pH increases of soil when they were added to the acid soil at equimolar concentrations of Na^＋. The results suggested that cations played a central role in regulating soil pH. Decarboxylation might only consume a limited number of protons. Conversion of organic salts into inorganic salts （carbonate） was possibly responsible for pH increase during their decomposition, suggesting that only those plant residues containing high excess base cations could actually increase soil pH.
LI Zhi-An ZOU Bi XIA Han-Ping DING Yong-Zhen TAN Wan-Neng FU Sheng-Lei
A pot experiment was conducted in artificially Cd-contaminated （5 mg Cd kg 1） soils to investigate the feasibility of using lime （3 g kg-1） or phosphate （80 mg P kg-1） to mitigate uptake of Cd by vegetables. Five common vegetables in South China, including lettuce （Lactuca sativa L. var. ramosa Hort.）, Chinese cabbage [Brassica rapa L. subsp. Chinensis （L.） var. parachinensis （L. H. Bailey） Hanect], Chinese broccoli （Brassica oleracea L. vat. albiflora Kuntze）, white amaranth （Amaranthus tricolor L.） and purslane （Amaranthus viridis L.）, were grown in the soils and harvested after 60 d. The results showed that liming significantly reduced Cd uptake by most vegetables by 40%-50% （or a maximum of 70%）, mainly due to immobilization of soil Cd. Increased availability of Ca in the soil might also contribute to the Cd uptake reduction as a result of absorption competition between Ca and Cd. Liming caused biomass reduction in white amaranth and purslane, but did not influence growth of the other vegetables. Phosphate decreased Cd uptake by vegetables by 12% 23%. Compared with lime, phosphate decreased, to a smaller extent, the bioavailability of Cd in most cases. Phosphate markedly promoted growth of vegetables. Changes in soil chemistry by adding lime or phosphate did not markedly influence nutrient uptake of vegetables except that lime increased Ca content and phosphate increased P content in shoots of the vegetables. The results suggested that a proper application of lime could be effective in reducing Cd uptake of vegetables, and phosphate could promote growth of the vegetables as well as alleviate the toxicity of Cd.
TAN Wan-Neng LI Zhi-An QIU Jing ZOU Bi LI Ning-Yu ZHUANG Ping WANG Gang
