Influence of soil heterogeneity and water tension on the penetration resistance of aggregates from loess-derived soils

Even though structured soils are mostly derived from homogeneous substrates, at least on the horizon level, they are more or less heterogeneous on the aggregate level. This is more pronounced for tilled topsoils. Studies on loess-derived topsoils from 3 different areas in Upper Bavaria, Germany, one of which was affected by recultivation activities, have shown that aggregates (10-15mm) have higher bulk densities than core samples. These aggregate bulk densities vary more than those of core samples, too. As these differing bulk densities do not reflect the behaviour of aggregates against external forces, produced e.g. by roots, forces to penetration were measured on 10 single aggregates (10-15mm) at 6 different water tensions from 17 topsoil samples in total. Shaft friction could not be considered when calculating resistances to penetration (R-P). Thus, the resulting RP-depth-relations usually increased with depth which was much more pronounced at water tension (WT) less than or equal to -200hPa. Each aggregate at the same WT, however, produced its own specific RP-depth-relation resulting in a great scatter of these relations for a defined WT. Moreover, small obstacles and/or fine macropores within these aggregates produced a scattering of the single relation indicated by an r(2) of 0.7, 0.5 or even lower as compared to an r(2) of 0.85-0.99 for relatively smooth relations. Although these scatterings are large the coefficients of regression of the single RP-depth-relations for each soil sample were related to WT. The best regression fits for each soil can be expressed by the general equation y=a(.)e((b.x)) with r(2)-values ranging from 0.3597-0.8258. The lower the r(2)-value for a single aggregate as well as for all aggregates and WTs (same soil), the more heterogeneous is the matrix of that aggregate and are the structural units of that topsoil, respectively. The results also indicate that soils derived from pure loess are stronger at each WT than soils derived from sand-rich loess. The wetter the soil moisture regime the less strong seem to be the aggregates. This means-the preconsolidation by generally lower WTs increased the strength of the aggregates. Moreover, the aggregates from the wettest pure-loess soil behave like those from sand-rich loess. The results for aggregates from the soils affected by recultivation activities indicate that treating gently the soil during recultivation. produce similar strength relations than unaffected soils. Soils treated not gently produced different strength relations which were also weaker. The reason for this latter finding with respect to soil water regime (see above) may be a higher water content at the same WT for these aggregates as compared to those of unaffected or hardly affected soils, although kneading of the soil material during transportation and leveling should increase the number of contacts between particles and thereby the soil strength. The results in total indicate that resistances to penetration and thus soil strength strongly increase At WTs less than or equal to -400hPa reaching in some cases even greater than or equal to 30MPa at -600hPa. At WTs greater than or equal to -300hPa it is hardly possible to differentiate the topsoils.