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.

No abstract

The difference in hydrostatic pressure between the xylem of the leaf and the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway. Many studies have shown that the sum of the resistances in the plant and the soil is too small to account for the fall in water pressure between the leaf xylem and the soil, especially when plants are growing in sandy soils, which are prone to dry rapidly. A resistance at the root–soil interface, caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy. We explored the resistance in the pathway from soil to leaf using a technique that allows precise and continuous non‐destructive measurement of the hydrostatic pressure in the leaf xylem. When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial resistance. However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70kPa, the hydrostatic pressure in the leaf xylem fell several times faster than that in the soil. We suggest that solutes accumulated either in the root or just outside it, creating large osmotic pressures, which gave the appearance of an interfacial resistance.