Observations on soil aggregates following measurements of penetration resistance

Abstract

Force-to-depth curves determined using fine probes for measuring resistance to penetration showed small, but intensive scattering and occasionally pronounced force peaks. To study the causes of these effects, 15–20 mm aggregates from surface and subsurface horizons of a Typic Hapludalf, a Mollic Fluvaquent and an Aeric Fluvaquent were equilibrated with water tensions ranging from 0 hPa to 600 hPa. Penetration resistance was then measured using a 0.55 mm-probe without shaft reduction or lubrication and a penetration velocity of 3.24 mm/min. Stereo-microscopic and SEM examination of the penetration paths revealed that the surfaces were more or less rough. The roughness was influenced by soil texture and soil type, with a tendency for clay and/or organic matter content to decrease the roughness. The roughness decreased with increasing soil water tension within the same soil material except for the Ap horizons of the Fluvaquents. This decreasing roughness, which is in contrast to observations during tillage, is explained. Due to the necessary sensitivity of the force transducer when using fine probes, small obstacles within the soil aggregates cause great changes in the penetration resistance. Obstacles which were penetrated, displaced, or touched were medium- and coarse-sized sand grains, fine gravel grains as well as plant residues, concretions and coatings of iron oxides.