Soil water potential decreased exponentially and soil strength increased logarithmically as the volumetric water content decreased in soils of contrasting texture at a range of bulk densities. Soil air-filled porosity was a linear function of volumetric water content.
At constant soil strength and non-limiting soil air-filled porosity (≥ 0.16 cm³ cm-³ the root elongation rate of radiata pine seedlings decreased linearly with soil water potential in the range - 0.01 to - 0.20 MPa. The rate of decrease in elongation rate was reduced with increasing soil strength. When soil water potential was < - 0.20 MPa, the root elongation rate was linearly related to the log of negative soil water potential. The root elongation rate decreased exponentially with increase of soil strength at constant soil matric potential and non-limiting air-filled porosity. This relationship was independent of both soil water potential and soil texture. However the diameters of both root and root tip increased, but root biomass decreased with increase of soil strength.
There was no significant osmotic regulation at water potentials in the rooting media of ≥
- 0.2 MPa. Osmotic regulation commenced at < - 0.20 MPa and this partially compensated for the turgor loss from water stress. The wall yielding coefficient decreased with loss of turgor. The reduction in the root elongation rate with decreased water potential was an integrated effect of both decreased turgor pressure and reduced wall yielding coefficient. Roots osmotically regulated against increasing soil strength. No significant relationship between yield turgor pressure and both water potential and soil strength was observed.
When soil air-filled porosity was non-limiting, root elongation rate in soil (∆R) was determined by soil matric potential (Ψm) and soil strength (Q) and was best described by a non-linear model: ∆R = α e-βQ + γѰm. The effect of water potential in decreasing root growth was most pronounced at low soil strength. Roots of radiata pine are able to penetrate higher soil strength at higher soil matric potential, and root growth of radiate pine seedlings ceased at higher soil matric potential in compacted soil than in loose soil
