Phosphorus Fractions of Selected Calcareous Soils of Qazvin Province and Their Relationships with Soil Characteristics

Information about phosphorus fractions is important for evaluation of their status in soil and understanding of soil chemistry that influence soil fertility. To obtain such information, amount and distribution of P in different fractions of 20 soil samples of Qazvin province, Iran were determined by sequential extraction methods and their relationships with each other and with soil characteristics were investigated. Total P ranged from 700 to 1040 mg kg-1. The clay ranged from 18-52%, CEC from 14.5-33.5 c mol(c) kg-1 and the active CaCO3 from 2.9-19.4%. The amount of different P forms i.e. dicalcium phosphates (Ca2-P), octacalcium phosphates (Ca8-P), Al-phosphates (Al-P), Fe- phosphates (Fe-P), occluded-P (O-P), and P as apatite (Ca10-P) were found to range from; 1.6-42.3, 72-314, 14.5-54.8, 8.4-34.8, 5.9-33.4 and 262-697 mg kg-1, respectively. Simple correlation coefficients showed that Olsen-P had a significant correlation with soluble-P, Ca2-P, Fe-P and O-P. A significant correlation was also observed among different forms of P, which presumably reflects the existence of a dynamic relation between the chemical forms of an element in soil. Clay with Fe-P and O-P; total P with Ca8-P, Al-P and Ca10-P; and CEC with Al-P and O-P had positive and significant correlations. However, a significantly negative correlation was found for electrical conductivity (EC) with Ca8-P and Al-P; organic carbon (OC) with Ca10-P, and active CaCO3 with Ca10-P.