No abstract

Of the 13 fly ashes from Australian power stations, all were dominated by both amorphous and crystalline aluminosilicates and quartz, and these were associated with minor amounts of Fe oxides, lesser amounts of Ca, Mg, Na, K, Ti and P oxides, and variable levels of incompletely combusted C. The ashes consisted of particles predominantly in the silt plus fine sand fraction (67-98%), and electron microscopy revealed that ash matrices consisted of glassy spherical particles and less regularly shaped spongy particles. The available water capacity was high and varied from 27 to 105% with 11 of the 13 samples having values > 40%. Twelve of the 13 ashes were alkaline to strongly alkaline (pH range 8.0-12.8). Soluble salt levels were related to the coal source, and electrical conductivity of the saturation extracts (ECse) varied from 0.63 to 7.0 mS cm -1 for 11 of the samples; two ashes produced from brown coal had ECse values of 46.0 and 55.0 mS cm-1. The nutritional status of the ashes was assessed on samples leached with water to reduce the soluble salts to equilibrium levels. The ashes contained negligible amounts of N, but their P status was variable. The amounts of NH4OAc-extractable Ca and Mg were generally high in most samples and were considered more than adequate for most plant species; however, NH4OAc-extractable K values were generally low. Adequate levels of sulfate-S existed in 12 of the 13 samples, and levels of DTPAextractable Cu, Zn, Mn and Fe were high when compared with published critical values. Most of the unleached samples contained potentially toxic levels of B. The results are discussed with reference to the establishment of vegetation on ash dumps and to the use of fly ash in ameliorating the properties of soils.

Solutions obtained from six soils in tropical North Queensland after incubation at a moisture tension of 0.1 bar were analysed to obtain data on their ionic strengths. Soil extracts, at soil: solution ratios of 1:1, 1:2.5, 1:5, and 1:10 were also examined. Determinations on the aqueous phase included electrical conductivity, pH, ammonium, calcium, magnesium, potassium, sodium, bicarbonate, chloride, sulphate, and nitrate. Ionic concentrations of the soil solutions were found to be low when compared with many of the values reported in the literature. The upper limit for the ionic strength was about 0.005. Ionic strength was well correlated with the electrolytic conductivity of the soil solution itself, and also of the soil extracts. Relationships found between the soil solution and soil extracts in respect of total cation (and anion) content and also cation ratios, allow predictions about the soil solution to be made from soil extract data. Consideration of the ionic ratio of calcium to total cations in these soils suggests that the soils may have suboptimal levels of calcium for the growth of many plant species.

In the course of three experiments, soybean (Glycerine max (L.) Merr.) cv. Forrest was grown in 21 soils (four surface soils and 17 subsoils) amended with liming materials (CaCO3 and Mg CO3) and soluble Ca salts (CaSO4.2H20 and CaCl2.2H2O). In most soils, the soluble salts increased concentrations and activities of Al species in solution to levels that restricted root growth, and MgCO3, induced a Ca limitation to root growth. Root lengths after three days were related to so11 and soil solution attributes.Suitable diagnostic indices for the prediction of Ca limitations to root growth were either Ca saturation of the effective cation exchange capacity or Ca activity ratio of the soil solution, which was defined as the ratio of the activity of Ca to the sum of the activities of Ca, Mg, Na, and K. Values corresponding to 90% relative root length (RRL) of soybean were 0.05 for the Ca activity ratio and 11% for Ca saturation. Calcium activity and Ca concentration in the soil solution and exchangeable Ca were less useful for this purpose.Soil Al saturation was not a good predictor of Al toxicity, but soil solution measurements were. The activities of Al3+ and AlOH2+ gave the best associations with RRL, and values corresponding to 90% RRL were 4 8M and 0.5 8M respectively. The results suggested that Al(OH)3� , Al(OH)2+, and AlSO4+, were not toxic species. Soil solution pH and soil pH measured in water were more sensitive indicators of root growth than soil pH measured in 0.01 M CaCl2.Using a Ca activity ratio of 0.05 and an Al3+ activity of 4 8M as diagnostic indices, none of the 20 soils in two experiments were toxic in Al, while 13 (all subsoils) were deficient in Ca. Thus the first limitation on root growth was Ca deficiency and not Al toxicity, in spite of high Al saturations and relatively low pH in these soils. However, Al toxicity could be induced by increasing the ionic strengths of soil solutions.

A soil incubation and short-term root growth experiment was conducted to investigate the effects of organic matter application on AI toxicity alleviation in a highly weathered acid soil. Ground leaves of a tree legume (Calliandra calothyrsus Meissn.), ground barley (Hordeum vulgare L.) straw, or CaCO 3 were mixed at various rates with A-horizon soil of a red podzolic soil (Epiaquic Haplustult) and incubated at 90% of field capacity for 4 or 10 weeks. After the incubation, a short term (48 h) root growth test was conducted using mung bean (Vigna radiata (L.) Wilczek), followed by the analysis of the solution and solid phases of the post-harvest soil.Adding either CaCO 3 or organic matter increased root length in mung bean largely by decreasing the activity of monomeric AI in the soil solution. With organic matter, the major mechanisms of this decrease were presumed to be precipitation of soluble AI and the formation of Al-organic matter complexes. The former effect was predicted from the pH increase accompanying the organic matter addition, the increase being larger with legume leaves which had the higher exchangeable and soluble Ca and Mg contents. The concentration of A1 complexed with soluble organic matter also was shown to increase with increasing rate of organic matter addition, the effect again being larger with legume leaves.The sum of monomeric AI species activity and A13+ ~ctivity was negatively correlated with relative root length for the organic matter and CaCO 3 treatments. However, indices which took into account the possible alleviation effects of basic cations in soil solution on A1 toxicity provided an improvement in correlation with relative root length.The efficiency of the two organic amendments relative to CaCO 3 in decreasing A1 toxicity was assessed by comparing the rates required to reduce A13+ activity below 10/xM, the value found to be associated with 90% relative root length for mung bean. The rates of CaCO3, legume leaf and barley straw required to reach this critical value were 0.75, 14, and 42 t ha -1 respectively.