THE VOLUME AND POROSITY OF SOIL CRUMBS

A simple and inexpensive apparatus (a test-tube, burette, and pin) is described for measuring volumes by liquid displacement to an accuracy of greater than 0.5 per cent. This has been adapted to measure soil crumb porosities, ε_c, by saturating 3–4 g samples of crumbs with kerosene, measuring the weight of kerosene retained internally, then measuring their volume by displacement. Three estimates of crumb porosity from these measurements are compared. Experimental values range from ε_c= 0.205 for the headland of an arable field to ε_c= 0.351 for a permanent pasture. Crumb porosity is proposed as a measure of structural status for soils because it assesses the degree to which soil management has succeeded in holding the constituent primary particles apart from the positions of inherent closest packing that they would ultimately assume in an unstable soil. By comparison, the inter-crumb porosity, ε_v, can be used as a measure of cultivation status. In the form expressed, these two porosities are related to the more frequently encountered total porosity ε_t by the relation