Effects of soil air-filled porosity, soil matric potential and soil strength on primary root growth of radiata pine seedlings

The effects of soil air-filled porosity, soil matric potential and soil str ength on primary root growth of radiata pine ( Pinus radiata D. Don) seedli ngs were examined in four soil textures ranging from coarse to fine. At low penetrometer resistance (< 0.5 MPa) and high soil matric potential ( greater than or equal to - 0.01 MPa), root elongation rate was close to zer o when air-filled porosity was < 0.05 m(3) m(-)3, and it increased sharply to 90% of its maximum value at 0.15 m(3)m(-3). This relationship was indepe ndent of soil texture. The diameter of the root tip increased as air-filled porosity decreased, particularly below 0.10 m(3) m(-3). Root elongation rate decreased linearly with decreasing soil matric potenti al over the range - 0.01 to -0.35 MPa at both 0.5 MPa and 1.5 MPa soil stre ngth. This relationship was independent of soil texture. The rate of root e longation at 0.5 MPa was about twice that at 1.5 MPa and the rate of decrea se in root elongation with decreasing soil matric potential was 1.35 times greater at the lower (0.5 MPa) than the higher (1.5 MPa) soil strength. The effect of water potential (over the range -0.01 to -1.5 MPa) on root elong ation at zero soil strength was simulated using PEG 4000 solutions as rooti ng media. Root elongation declined exponentially over the range of water po tentials established in the rooting medium. Root elongation rate decreased exponentially with increasing soil strength when soil matric potential was constant and air-filled porosity was > 0.20 m(3) m(-3). This relationship was independent of soil texture. Root elongat ion rate was half its maximum at a penetrometer resistance of 1.3 MPa. Incr easing bulk density has a greater effect of increasing soil strength in coa rse soil than in fine soil but decreasing soil water content has a greater effect on increasing soil strength in fine soil than in coarse soil.