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Zhao Meizhi (Institute of Soil Science, Academia Sinica, Nanjing)  
Present paper deals with the characteristics of phosphate desorption isotherms of soils and clay mineral and P desorption with various extraction solutions, Isothermal adsorption of samples was first made and then phosphated samples were continuously desorbed by means of the extraction with 0.5N HOAc, 0.5N NH_4F and 0.1N NaOH or with 0.5N NH_4F only. The results obtained are summarized as follows: 1. The amount of P desorbed of the samples increased with the amount of P adsorbed. The amount P desorbed with 0.5 N HOAc and 0.5 N NH_4F was significantly positively correlated with P adsorbed (Fig. 1a, 1b). 2. The desorption equilibrium of yellow brown earth, latosol and kaolin was obtained only in 1 hour, but red earth needed 1 day. Certain amount of P was detected after the equilibrium of phosphated samples with 0.5 N NH_4F solution for 322 days, which indicates the existence of slow desorption. 3. From the comparison of the isotherms of P in soils before and after readsorption, it was found there was a turning point between the adsorption and desorption, which may be used as a theoretical basis for P fertilization. In addition, the P desorbed increased in soils treated with 0.1 N NaOH after readsorption, which provides an evidence of P released from the soils rich in iron oxide.
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