Plant Phosphorous Requirements Determined by the Sorption Isotherm Models in the Calcareous Soils

Soil P dynamics, as a function of soil buffering capacity, affecting P availability for plant use in different Iranian agricultural areas were investigated. The P buffering capacity and requirement of four different calcareous soils were investigated to determine: (1) P surface sorption using the isotherms models, and (2) the indices of buffering capacity and soil P requirement (SPR). Soil samples (0–30 cm) were collected from 26 different regions and following soil physicochemical analyses, 13 soils were selected for further investigation. The sorption isotherm models of Langmuir, Freundlich, Temkin and Van Huay were fitted to the selected soils to determine the P buffering indices including maximum buffering capacity (MBC, ranging from 7 to 127, average = 42 L kg⁻¹), P buffering capacity (PBC, ranging from 11 to 78, average = 31 L kg⁻¹), equilibrium buffering capacity (EBC, ranging from 17 to 80, average = 33 L kg⁻¹), and soil buffering capacity (SBC, ranging from 7 to 120 L kg⁻¹). The slopes of Temkin isotherm model (K₂) and Van Huay (B_VH) were used as the indices of buffering capacity (32 to 168, average = 87 L kg⁻¹) and 26.85–211.05 (average = 93 L kg⁻¹), respectively. The van Huay isotherm model was used to determine SPR (ranging from 42 to 225 L kg⁻¹, average = 120 L kg⁻¹). Soil indices were significantly and positively correlated with soil organic carbon and CaCO₃. It is possible to predict plant P requirement in the experimental regions, and the similar regions, worldwide, as functions of soil colloidal properties (P buffering capacity), using the tested models.