Comparison of different methods for determination of phosphorus in calcium chloride extracts for prediction of availability to plants

Abstract

Soil samples, dried and in field moisture condition, respectively, were extracted by 0.01M calcium chloride (CaCl2) at two different soil:extractant ratios (1:2 and 1:10), and analysed by inductively coupled plasma emission spectrometry (ICP), by ion Chromatograph, and by two different molybdenum blue methods for content of phosphorus (P). There was a good relationship between the methods, although the different methods gave quite different P values. Phosphorus detected by the molybdenum blue methods included not only orthophosphates, and the values were influenced by method‐dependent factors like effect of temperature. The difference is important when studying the complex of P cycle in soil and its plant availability and stresses the need for a reliable method of detecting low amounts of orthophosphates in soil extracts. Plant samples, collected at the same times as the soil samples, were digested in concentrated nitric acid (HNO₃) and the P content determined by ICP. Linear regressions were performed with plant P as dependent on extractable P according to different methods. The degree of explanation is generally between 0.6 and 0.9, without indicating any method as superior in predicting plant availability of P.