Abstract: The physical and chemical properties and silicon releasing pattern and phosphate adsorption-desorption of fly ashes derived from six power stations in Hunan province were studied. The results showed that fly ashes could be used as soil amendments because the fly ashes were mostly composed by sand（1-0.05 mm）and coarse silt particle（0.05-0.01mm）accounting for 16.13 %- 85.68 % and 13.03 %-49.64 %, respectively. The bulk density of fly ashes was 2- to 3-fold lower than that of soils. The fly ashes generally have high alkalinity (pH 10.3-12.1, 1︰2.5 in H2O extract) except that the fly ashes of Changsha power factory derived from high sulfur coal have low acidity（pH 7.5, 1︰2.5 in H2O extract）. No obvious difference in contents of water-soluble and available silicon between fly ashes and soils was observed, so fly ashes can not be used directly as Si fertilizer. The contents of total potassium in fly ashes were approximately the same as in soils, but the contents of free iron oxide in fly ashes were 3- to 4-fold lower than those in soils. The releasing processes of Si in fly ashes and soils could be well described with Elovich equation and Freundlich equation ,in which the parameter b value and k value could express the releasing capacity of Si in fly ashes and soils .The larger the b and k values, the more the Si will be released from the fly ashes. The b value and k value in 5 fly ashes derived from various locations in order of magnitude is Xiangtan power factory (1539.3，2139.3)and Yueyang paper factory (1602.4，1274.0)＞Huaneng power factory (1325.7，1181.6)＞Changsha power factory(1250.6，915.9)＞Dongting nitrogenous fertilizer factory (751.9，565.6)＞Zhuzhou power factory (555.5，454.4). The fly ashes can supply certain Si nutrition for crop growth because the amount of silicon released in fly ashes were 3.0-25.0 times higher within 100 hours than those in the soils, thus the fly ashes can be a kind of Si fertilizer additive when manufacturing Si fertilizer. Furthermore, the fly ashes derived from Xiangtan power factory and Yueyang paper factory are fairly good Si fertilizer additives. The contents of total P and available P in fly ashes were generally higher than those in soils. The adsorption capacity of P in fly ashes increased with increasing P added，but the rates of P adsorbed in fly ashes decreased with increasing P added. The rates of P adsorbed in fly ashes were higher than those in soils, but the rates of P desorbed in fly ashes were lower than those in soils. The sorption processes of P in fly ashes could be also well described with Langmuir equation and Freundlich equation and Temkin equation, in which parameter Xm value and a value and k2 value can be used as a comprehensive index to characterize the potential phosphate sorptivity of fly ashes. The phosphate adsorption in fly ashes were mainly specific adsorption and chemical precipitation, thus it is suggested that the fixation of P must be considered when fly ash is used as soil amendments or filling substances of compound fertilizers.

Key words: Fly ashes, Soil amendants, The amount of silicon releasing, The rates of P adsorbed, Agrochemical behaviors