Soil structure and plant growth: Impact of bulk density and biopores

Compacted soils are not uniformly hard; they usually contain structural cracks and biopores, the continuous large pores that are formed by soil fauna and by roots of previous crops. Roots growing in compacted soils can traverse otherwise impenetrable soil by using biopores and cracks and thus gain access to a larger reservoir of water and nutrients. Experiments were conducted in a growth chamber to determine the plant response to a range of uniform soil densities, and the effect of artificial and naturally-formed biopores. Barley plants grew best at an intermediate bulk density, which presumably represented a compromise between soil which was soft enough to allow good root development but sufficiently compact to give good root-soil contact. Artificial 3.2 mm diameter biopores made in hard soil gave roots access to the full depth of the pot and were occupied by roots more frequently than expected by chance alone. This resulted in increased ~plant growth in experiments where the soil was allowed to dry. Our experiments suggest that large biopores were ~ not a favourable environment for roots in wet soil; barley plants grew better in pots containing a network of narrov~ biopores made by lucerne and ryegrass roots, responded positively to biopores being filled with peat, and some pea radicles died in biopores. A theoretical analysis of water uptake gave little support to the hypothesis that water supply to the leaves was limiting in either very hard or very soft soil. The net effect of biopores to the plant would be the benefits of securing extra water and nutrients from depth, offset by problems related to poor root-soil contact in the biopore and impeded laterals in the compacted biopore walls.