SOIL-STRUCTURE AND PLANT-GROWTH - IMPACT OF BULK-DENSITY AND BIOPORES

Compacted soils are not uniformly hard; they usually contain structura l cracks and biopores, the continuous large pores that are formed by s oil fauna and by roots of previous crops; Roots growing in compacted s oils can traverse otherwise impenetrable soil by using biopores and cr acks and thus gain access to a larger reservoir of water and nutrients . Experiments were conducted ina growth chamber to determine the plant response to a range of uniform soil densities, and the effect of arti ficial and naturally-formed biopores. Barley plants grew best at an in termediate bulk density, which presumably represented a compromise bet ween soil which was soft enough to allow good root development but suf ficiently compact to give good root-soil contact. Artificial 3.2 mm di ameter 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; barl ey plants grew better in pots containing a network of narrow biopores made by lucerne and ryegrass roots, responded positively to biopores b eing filled with peat, and some pea radicles died in biopores. A theor etical analysis of water uptake gave little support to the hypothesis that water supply to the leaves was limiting in either very hard or ve ry soft soil. The net effect of biopores to the plant would be the ben efits of securing extra water and nutrients from depth, offset by prob lems related to poor root-soil contact in the biopore and impeded late rals in the compacted biopore walls.