The Effective Removal of Suspended Materials from Pyrophosphate Extracts of Soils from Tropical and Temperate Regions

Abstract Sodium pyrophosphate (0.1 M ) extracts of some soil samples from Canada and East Malaysia were subjected to low‐speed centrifugation both without and with Superfloc, high‐speed centrifugation, and ultrafiltration of the centrifugates prior to determination of their Fe and Al contents. Relative to ultrafiltration, none of the centrifugation methods cleared the extracts completely and thus part of the extracted Fe p and Al p measured in the supernatants was present in suspended solids. Suspended solids, with mineralogy similar to that of the clay of corresponding samples, were particularly abundant in the supernatants of the tropical soil samples after low‐speed centrifugation without Superfloc. Adding Superfloc to these extracts and centrifuging them at low speed sedimented practically all the aluminum silicates but concentrated the goethite in the material that remained suspended. High‐speed centrifugation gave similar results but with less concentration of goethite. Smaller amounts of suspended solids, also with mineralogy similar to that of the clay of corresponding samples, were present in the supernatants of samples from Canadian soils, except for that from a spodic horizon sample in which suspended minerals were absent. Ultrafiltration of the centrifugates through a 0.025‐µm millipore filter cleared the extracts effectively and is recommended for tropical soils and for soils giving doubtful results with centrifugation methods only. High‐speed centrifugation is probably adequate for most temperate‐region soils, as suspended Fe and Al in the extracts tested accounted for 0.1 dag/kg or less of sample weight.