Resistances to penetration of aggregates from loess-derived topsoils at different soil water tensions

In structured soils roots must at least partly penetrate large soil aggregates even of the topsoil layer to obtain sufficiently water and nutrients during drier periods. The development of aggregates starting from a cohesionless structure is governed by many soil processes as well as by men resulting among others in different properties considering the inner and outer parts of aggregates. Therefore, comparison of strengths for aggregates from different soil types should be carried out on soils from the same parent material. To study the effect of different soil types on penetrability, natural air-dried aggregates (10-15 mm) from 9 different topsoils of loess-derived soils (pure loess: Mollic Hapludalf (MH), Aquic Hapludoll (AH), Fluvaquentic Hapludoll (FH); sand-rich loess: Typic Udorthent (TU1), Typic Udorthent (TU2), Aquic Udorthent (AU), Typic Eutrochrept (TE), Calcic Hapludoll (CH), Aquic Hapludoll (AH)) were equilibrated with different soil water tensions (WT) for each soil. After equilibration 10 aggregates per WT were penetrated by a probe of 0.55 mm diam. As expected, resistance to penetration (RP) increases with depth as well as with decreasing WT independently of soil type. At WTs greater than or equal to-200 hPa RP seldom reaches 2 MPa. At WTs<-200 hPa, however, RP exceeds 5 MPa and, in most cases, 10 MPa at WTs of -600 hPa. The deviations between the single RP-depth relations for a given WT and soil type are considerable leading to averaged irregular RP-depth relations for WTs<-300 hPa. Moreover, RPs strongly increase between 0 mm and 2.5 mm depth at these WTs. In spite of the large variation of RPs it is obvious from the data that the aggregates from the pure loess soils are stronger at almost all WTs showing the following ranking: MH>AH>FH, than those from the sand-rich loess soils having the following ranking: TE>AU>AH>CH>TU2>TU1. These results indicate that, on one hand, due to their smaller strength the aggregates of the sand-rich loess soils will alleviate water and nutrient uptake by plants under drier conditions and, on the other hand, the aggregates of the less moist soil types are stronger. There are, however, rather great differences in strength among aggregates from the same soil even at the same WT. These differences are assumed to affect rooting patterns. (C) 1998 Elsevier Science B.V. All rights reserved.