Amount of P required to bring its desired concentration ABT-888 in soil solution could be better determined by P sorption isotherms [5, 6] instead of conventional soil P test; those do not take into consideration the physicochemical properties of soil. Although both the Freundlich and Langmuir isotherms describe the adsorption phenomena satisfactorily [7], the former is preferred because it is capable of rigorous derivation and correlates well with soil properties [8]. Moreover, it is based on assumptions more realistic than some other cases; that is, an adsorption maximum is not obtainable from the isotherm that seems compatible with most of the observed P sorption by soils, at least, under normal laboratory conditions.

Keeping in view the above facts, a laboratory study was conducted using Freundlich adsorption isotherm to assess the P adsorption capacity of two soils when treated with PA and DAP at varying levels of CaCO3.2. Materials and Methods2.1. Soil Preparation and AnalysesSurface soil samples were collected from Faisalabad and T. T. Singh districts (hereafter referred to as S-I and S-II, resp.), air-dried, passed through a 2mm sieve, mixed thoroughly, and stored in labeled plastic bottles. Samples were analyzed for various physiochemical properties like texture [9], pH of saturated paste (pHs), electrical conductivity of saturation extract (ECe) [10], available K [11], Olsen P [12], organic matter [13], and calcium carbonate [14].2.2. Development of CaCO3 Levels in SoilsOne kg of each soil was taken in plastic buckets, and three levels of CaCO3 (native, 10%, and 20%) were developed by mixing reagent grade salts with soils.

The soils were wetted with distilled water to attain field capacity and equilibrated for 15 days at room temperature. At the termination of incubation, soils were mixed, dried, and passed through a 2mm sieve and stored in plastic bottles for use in adsorption studies.2.3. Adsorption IsothermsAdsorption isotherms were constructed using a series of solutions with P concentrations (2.5, 5, 7.5, 10, 20, 40, and 80ppm) prepared from each of DAP and PA in 0.01MCaCl2. To 2.5g samples of the soils, 25mL of the above-said P solutions was added and shaken for 24h on a mechanical shaker. After equilibration, the samples were centrifuged for 15min. at 4000rpm and filtered through Whatman number 42 filter paper. Phosphorus concentration in the final solutions Anacetrapib was determined following the method of Murphy and Riley [15]. The difference in P concentration of solutions before and after equilibrium was taken as the amount of P adsorbed.