The parameters a and b were estimated by regression of the logarithmic form of the selleck compound data obtained from adsorption isotherms. Theoretical doses of PA and DAP fertilizers to develop a desired soil solution P level, that is, 0.20mgL?1, were calculated. A regression between calculated quantities of P fertilizer and CaCO3 levels was developed to estimate requirement of P fertilizer for any level of soil CaCO3. 3. Results and Discussion3.1. Freundlich Adsorption Isotherms for CaCO3 Amended SoilsThe physical and chemical properties of the soils are presented in Table 1. Both the soils were nonsaline, silty clay loam in texture, and slightly alkaline in reaction. After constructing the P adsorption isotherms, the data were subjected to examine the fitness of modified Freundlich equation.

Linear plot of the modified Freundlich equation presented in Figure 1 and parameters of the equation (amount adsorbed (a), buffer capacity (b) mLg?1, and correlation coefficient (r2)) are presented in Table 2. The goodness of the fit of the model was ascertained from r2 values (��0.84) which indicated high conformity of the adsorption data with the Freundlich model. These findings are in agreement with those of Chaudhry et al. [17] and Sarfraz et al. [18] who also reported dependence of the exponent of Freundlich equation on solution P concentration instead of time and temperature. A good fit of the P adsorption data to the Freundlich adsorption model over the Langmuir and Tempkin was also reported by Khan et al. [19].Figure 1Freundlich isotherms for P adsorption: (a), (b), and (c) represent native, 10%, and 20% CaCO3, respectively.

Table 1Physiochemical properties of the soils used in adsorption studies.Table 2Fitted Freundlich adsorption isotherms.3.2. Calcium Carbonate and P AdsorptionIn adsorption equation, b represents the buffer power of the soil for P. The more the value of b is the more the P adsorption capacity of soil would be. The soils differed slightly in buffer capacities despite a large difference in native CaCO3 that might be due to similar proportion of active CaCO3 and its specific surface area in the soils which mainly govern P behavior. With the addition of CaCO3 in soils, the buffer capacity of the soils was increased (Table 3). Similarly, Samadi and Gilkes [20] and Samadi [21] reported that P adsorption in calcareous soil was related to CaCO3 contents.

Castro and Torrent [22] found an increase in differences among P fertilizers for P adsorption with the increase in carbonate contents of the soil and attributed the fact to the precipitation of Ca-phosphate. However, Samadi [23] reported that both total and active CaCO3 were less important factors for P adsorption. This discrepancy in results has been answered by Pe?a and Torrent [24] as the inability of the standard methods used for the determination of total and active GSK-3 CaCO3. Table 3Buffer capacities of CaCO3 enriched soils as determined from Freundlich adsorption isotherms.3.3.