Phosphorus adsorption characteristics of soft rock modified sandy soil

【Objectives】Appropriate amount of soft rock addition to sandy soil could improve water absorbing and holding capacities, but little information about the nutrient availability was published. The adsorption of phosphorus after added into such soft rock modified sandy soil was studied, aiming to understand phosphorus adsorption characteristics and mechanism of soil improvement effect, and to guide reasonable fertilization of phosphorus.【Methods】 Soft rock was mixed with sandy soil in seven ratios: 0∶100(L), 10∶90(LS1), 25∶75(LS2), 50∶50(LS3), 75∶25(LS4), 90∶10(LS5)and 100∶0(S). The adsorption kinetic process and adsorption isotherms of phosphorus were measured at 25℃, the adsorption kinetics and isothermal adsorption model was used to reveal phosphorus adsorption mechanism of improved soils. The relation between proportion of soft rock and maximum amount of phosphorus adsorption in improved soil has been known.【Results】 1)Under the same initial concentration, the phosphorus adsorption capacity of improved soil showed a trend of increase with the extension of reaction times and gradually achieved balance after 24 h. 2)In the case of adsorption time was determined, the phosphorus adsorption quantity ncreased gradually with initial concentration increased in improved soil and until to close or reach the maximum amount of phosphorus adsorption. 3)Adsorption capacity of phosphorus was larger in sandy soil than in soft rock. With the increase of the soft rock proportion, phosphorus adsorption of improved soil decreased. 4)The pseudo-second order kinetic modell was the best mode to fit phosphorus absorptive dynamics processes. Within the concentration range in this study, the phosphorus adsorption isotherms were well represented by Langmuir isotherm model. 5)The studies on the kinetics and isothermal absorption showed that the phosphorus adsorption of improved soil was homogeneous monolayer adsorption, and both membrane diffusion and intra-particle diffusion were the rate-controlling process in the adsorption system tested. The adsorption mechanism was mainly ion exchange and chemical adsorption in the processes. 6)The maximum adsorption of phosphorus decreased about linearly with the increased of soft rock proportion in improved soil. In the production practice,we can estimate the maximum adsorption of phosphorus in modified soil through measure the maximum adsorption of phosphorus in soft rock and sandy soil and the proportion of soft rock.【Conclusions】 Soft rock can significantly reduce the phosphorus adsorption of sandy soil and increase the effectiveness of phosphorus fertilizer. So when re-vegetation, the more soft rock proportion in sandy soil, the better effect of fertilizer. However, as plants growth, further understanding of desorption and continuity of phosphorus in improved soil is needed.