DESORPTION OF PHOSPHATE ON SOME SOILS AND CLAY MINERAL
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.