The pH-dependent release of cadmium, copper, and lead from soil materials was studied by use of a stirred flow cell to quantify their release and release rates, and to evaluate the method as a test for the bonding strength and potential mobility of heavy metals in soils. Soil materials from sludge-amended and nonamended A horizons from a Thai coarse-textured Kandiustult and a Danish loamy Hapludalf were characterized and tested. For each soil sample, release experiments with steady state pH values in the range 2.9 to 7.1 and duration of 7 d were performed. The effluent was continuously collected and analyzed. Release rates and total releases were higher for the Hapludalf than the Kandiustult and higher for the sludge-amended soils than the nonamended soils. With two exceptions the relative release rates (release rate/total content of metal in soil) plotted vs. steady state pH followed the same curves for each metal, indicating similar bonding strengths. These curves could be described by a rate expression of the form: relative release rate = k[H+]a, with specific a (empirical constant) and k (rate constant) parameters for each metal demonstrating that metal release in these systems can be explained by proton-induced desorption and dissolution reactions. With decreasing pH, pronounced increases in release rates were observed in the sequence cadmium > lead > copper, which express the order of metal lability in the soils. The flow cell system is useful for comparison of metal releases as a function of soil properties, and can be used as a test to rank soils with respect to heavy metal leaching.

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Sugar mill by-products compost may be a good soil amendment to promote tomato (Lycopersicon esculentum L.) growth. In addition, the compost may further promote plant growth by inoculation with N 2 -fixing bacteria. Compost from sugar-mill waste was prepared with and without the N 2 -fixing bacteria, Azotobacter vinelandii, Beijerinckia derxii and Azospirillum sp. and incubated for 50 days. Each compost type was added to 10 kg of soil in pots at rates of 0, 15, and 45 g with and without fertilizer N at rates of 0, 0.75, and 1.54 g. A blanket application of P and K was applied to all pots. Shoot and root dry weights and N content of the whole plant was measured at 55 days. Dry weight of tomato shoots was increased by 40% by addition of fertilizer N and root weight was increased by 66%. Without fertilizer N the high rate of inoculated compost increased shoot growth 180% and uninoculated compost increased shoot growth 112%. For most treatments with and without fertilizer N, inoculated compost enhanced shoot growth and nitrogen content more than uninoculated compost. Root weights were nearly doubled by addition of either compost in comparison to the 0 N treatment. At the low rate of compost addition without fertilizer N, root weight was the same for uninoculated and inoculated compost but at the high rate of compost addition root weight was 32% higher for inoculated compost. The N 2 -fixing bacteria colonized roots when inoculated compost was used. Sugar mill by-products compost proved to be an effective soil amendment for promoting the growth of tomato plants.

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Inoculation of sugar mill by-products compost with N 2 -fixing bacteria may improve its quality by increasing total N and available P. Compost was inoculated with Azotobacter vinelandii (ATCC 478), Beijerinckia derxii (ATCC 49361), and Azospirillum sp. TS8, each alone and all three together. Numbers of all N 2 -fixing bacteria in compost declined from an initial population of 5 · 10 5 cells g )1 during incubation. The population of Azotobacter declined to approximately 2 · 10 2 cells g )1 and the population of Beijerinckia and Azospirillum declined to approximately 9 · 10 3 and 3.5 · 10 4 cells g )1 respectively, at day 50. Inoculation with N 2 -fixing bacteria increased acetylene reduction, total N by 6-16% and available P by 25-30% in comparison to the uninoculated control. Increasing the N content and P availability of compost increases its value and there may be additional benefit from providing N 2 fixing bacteria.