The Effect of Soil Parameters on Phosphorous Adsorption Coefficients in Heavy Soils of Different Areas of Qazvin Plain

Over application of phosphorous-containing fertilizers is common among the farmers. Excess amounts of phosphorus can potentially cause more phosphorous losses through water flow on the soil surface or leaching into the soil profile. The ability of highly phosphorus-fertilized soils to maintain excessive amounts of phosphorus and prevent losses largely depends on the phosphorus adsorption capacity of the soil. The purpose of this study was to investigate and compare phosphorous adsorption isotherms in soil samples of four agricultural areas located in Qazvin plain and determine the most appropriate equation to describe the equilibrium adsorption in the studied samples. Identification of the most accurate model of adsorption kinetics using the investigated kinetics equations in one of the soil samples was another objective of this study. The linear regression analysis and correlation between physical and chemical properties of different soils with adsorption coefficients of Langmuir equation was also investigated. Based on mentioned points, the results of this study can help to increase the availability of applied phosphorous for plants, reduce phosphorous losses and proper management of phosphate fertilizers consumption in the study areas.

In order to study the soil properties and phosphorous adsorption, soil samples of four villages included Zaaferan (A), Koochar (B), Mehdi Abad (C) and Kamal Abad (D) were taken from 0 to 30 cm depth and stored in plastic bags after air drying. Batch experiments using a standard method recommended by the SERA-IEG17 group were used to determine the amount of phosphorous adsorbed to soil particles. The steps to perform the equilibrium were as follows: 1- Dry soil samples were crushed and passed through a 2 mm sieve. 2- One gram of the soil sample was placed in a 60 ml container. 3- 0.01 M CaCl2 solution was prepared and different concentrations of phosphorous including 0, 5, 10, 15, 20, 30 and 80 mg/l were created by adding certain amounts of KH2PO4 to this solution. 25 ml of these solutions were added to each soli sample to give a 1:25 soil to solution ratio and three drops of chloroform were added to each container to prevent microbial activity. 4- The suspension samples were placed in a shaker machine (250 rpm) at 25°C for 24 hours. 5- Then, the samples were removed from the shaker and allowed to settle for one hour and then passes through a fine filter (Mesh 42). 6- Phosphorous concentration was measured by the molybdate-vanadate method followed by spectrophotometric determination at 470 nm. 7- The amount of phosphorous adsorbed in each soil sample was calculated from the difference of the initial and secondary concentration values. The adsorption kinetics experiment was similarly performed, with the exception that one soil sample with average adsorption value (sample C) was selected and the phosphorous solution at a concentration of 20 mg/l added to the soil samples. Phosphorous contact times with soil were considered as 0.17, 0.5, 1, 2, 4, 8, 16, 24, 48 and 72 hours. In this study, using CurveExpert 1.4 software and by matching Pseudo-first-order, Pseudo-second-order, Intra-particle diffusion, Kuo and Lotse (1974), Barrow and Shaw (1975) and Panda et al. (1978), equations on the data obtained from kinetics adsorption experiments, and the coefficients were estimated in these equations (adsorption parameters). Furthermore, by calculating the coefficient of determination (R2) of these equations and the standard error of the estimate (s), the most appropriate and accurate model of phosphorous adsorption kinetics for the soil sample was determined. Similarly, from Langmuir, Freundlich, Linear and Van Huay equations, the most appropriate isotherm was determined for estimating phosphorous equilibrium adsorption in the studied areas. Also, correlation and linear regression analysis were performed to determine the relationship between the physical and chemical parameters of the soils and the coefficients of Langmuir isotherm using Minitab software.

According to the results, the highest coefficient of determination (R2) and the lowest standard error of the estimate (s) for all four samples were related to Langmuir, Freundlich, Van Huay and Linear equations, respectively. Therefore, in this study, Langmuir isotherm was the most accurate model for estimating equilibrium adsorption of the phosphorus to the soils of the study areas. However, the Freundlich and Van Huay equations also showed a good correlation with the laboratory data. Comparison of the results of various studies in these fields showed that the type of isotherm corresponds to phosphorous adsorption data in each experiment is related to the physical and chemical properties of soil and adsorption sites. The amounts of maximum phosphorous adsorption capacity (qm coefficient in Langmuir equation) for the soil samples A, B, C and D were 0.49, 0.31, 0.42 and 0.4 mg/g, respectively. In kinetic study, Although, Kuo and Lotse, Barrow and Shaw and Panda et al. equations had a coefficient of determination (R2) above 0.95 ; the highest accuracy was related to the Kuo and Lotse equation with R2 of 0.974. The coefficients of this model included k (reaction rate) and m (constant coefficient) were 0.007 l/gr.min and 13.2, respectively. Based on the results of this study and other adsorption studies, soil physical and chemical properties including EC, PH, soil calcium content, clay content and porosity were among the parameters affecting adsorption rate and the type of the most accurate equation of adsorption estimation. Considering the soil properties that were most correlated with adsorption coefficients, it can be concluded that the high percentage of clay and low levels of organic matter and soluble calcium are the main causes of the high phosphorous adsorption in soil. The correlation coefficients (r) of these three soil parameters with the maximum adsorption capacity (qm) were 0.61, -0.97 and -0.92, respectively.

According to the results of this study, Langmuir was the most accurate isotherm model and the soil sample of Zaaferan area has the most adsorption capacity with qm of 0.49 mg/g, which is related to low levels of soil organic matter. Therefore adding organic matter to the soils can be used as a solution to increase cultivated plants access to applied phosphorous and reduce phosphorous adsorption into the soil and thus reduce losses and leaching of excess phosphorous in the profile or soil surface.