MEASUREMENT AND VARIABILITY OF PHYSICAL-PROPERTIES AND SOIL-WATER DISTRIBUTION IN A FOREST PODZOL

Measurements in a forest podzol show that soil physical properties differ predictably between major soil horizons which have been clearly identified in pits. Even in this 'best' case, there is considerable variability in those physical properties. The method by which soil is sampled for measurement of physical properties is shown to introduce uncertainty into the measured variable values. It is shown that soil horizons locally thicken into 'funnels' where the thickness of A and B horizons increases by many times. It is shown that these 'funnels' are related to surface microtopography at the scale of metres, and it is suggested that these soil 'funnels' represent preferential nodes of infiltration. In soil sampling exercises where a time sequence of cores is taken at different locations in order to characterize temporal change in some variable, the structures described introduce a large element of uncertainty, in that the difference between two cores is a combination of temporal and spatial change in unknown relative proportions.