Effect of centrifuge conditions on water and total dissolved phosphorus extraction from soil

Centrifugation of soils to extract soil water is widely used to investigate the chemical properties of soil water. However, no single set of standard operating conditions in terms of centrifuge time and speed have been defined. This study investigated centrifuge conditions on the effect of volume of soil water and phosphorus concentrations extracted to determine a set of standard operating conditions that could be used for soil water extraction in the future. Two contrasting soils that had been dried and rewet, a Ferrosol and a Dermosol, were used to investigate centrifuge speed ( 1000 - 3000 r. p. m.) and centrifuge time ( 5 - 30 min) on the extraction of soil water. Centrifuge speeds of 1000, 2000, and 3000 r. p. m. were trialed in this study giving relative centrifugal forces of 220, 881, and 1982g respectively. Increasing centrifuge time significantly increased (P < 0.001) the volume of soil water extracted. Almost twice as much soil water was extracted when the centrifuge time was increased from 5 to 30 min. Centrifuge speed was also a highly significant factor ( P < 0.001) in increasing the volume of soil water extracted. The amount of soil water extracted increased by up to 15% when the centrifuge speed was increased from 1000 to 3000 r. p. m. The concentration of total dissolved phosphorus (TDP) did not change significantly with either centrifuge speed or time. The most important factor in determining the concentration of TDP was the soil type. The conditions that gave an adequate volume of soil water for analyses from field-moist soils in the shortest amount of time were a centrifuge force of 1982g and a time of 10 min.