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Disturbance of Water-Extractable Phosphorus Determination by Colloidal Particles in a Heavy Clay Soil from the Netherlands

^a Alterra, Wageningen University and Research Centre (WUR), P.O. Box 47, 6700 AA, Wageningen, the Netherlands
^b Department of Soil Quality, Wageningen University, WUR, P.O. Box 85, 6700 EC, Wageningen, the Netherlands

^* Corresponding author (gerwin.koopmans{at}wur.nl)

Received for publication January 25, 2005. Water extraction methods are widely used to extract phosphorus (P) from soils for both agronomic and environmental purposes. Both the presence of soil colloids in soil water filtrates, and the contribution of colloidal P to the molybdate-reactive phosphorus (MRP) concentration measured in these filtrates, are well documented. However, relatively little attention has been given to the direct disturbance by colloids of MRP measurement. The objective of this paper is to show this influence found for water extracts with a soil to solution ratio of 1:60 (v/v) (P_w), obtained from a heavy clay soil in the Netherlands. Colloidal particles, which passed a 0.45-µm filter, caused a large overestimation of MRP. The low ionic strength of the P_w filtrates (on average 0.64 mmol_c L^–1) probably caused soil dispersion and increased detachment of colloids from soil during extraction. After NaCl addition, followed by 0.45-µm filtration, MRP was on average 93% lower. This can be ascribed to flocculation of colloids and removal by filtration. A low ionic strength can thus lead to the direct disturbance by colloidal particles of MRP measurement in waters from soils sensitive to release of colloids.

Abbreviations: MRP, molybdate-reactive phosphorus • P_w, water-extractable phosphorus at a soil to solution ratio of 1:60 (v/v)