A study of the effects of soil bulk density on root and shoot growth of different ryegrass lines

A glasshouse pot trial was designed to investigate the effects of bulk density on the root and shoot growth of two pipeline perennial ryegrass (Lolium perenne) lines, NZA1 and NZA3, and a commercial cultivar Yatsyn 1, in a soil derived from volcanic tephra in New Zealand. Bulk densities of 0.9, 1.0, 1.1, and 1.2 Mg/m(3) were achieved in the subsoil growing medium as a subsurface layer in pots that were arranged in a complete factorial design, and caused penetration resistances of 2.2, 2.9, 3.6, and 4.4 MPa, respectively. Five harvests of herbage were made over 24 weeks from planting. At the final harvest, roots in the surface, subsurface, and lower pot strata were separated from the soil. Estimates of root length and dry weight were made. Increasing subsurface soil bulk density reduced root length from 20.4 to 8.4 m per pot stratum (P < 0.05). This decrease in root development was associated with decreased (P < 0.05) herbage yield (4.72 to 2.38 g/pot) and tillering, showing that the increased mechanical impedance encountered by roots affected overall plant growth. The three ryegrasses produced similar total root length, herbage yield, and tiller numbers. However, the root length, herbage yield, and tillering of Yatsyn 1 were not significantly reduced as bulk density increased from 0.9 to 1.2 Mg/m(3), although few significant interactions were detected. This was in contrast to the responses of NZA1 and NZA3 and shows that Yatsyn 1 was the least sensitive to the effects of soil compaction. Adverse affects on ryegrass root and shoot growth were shown at lower soil bulk densities (<1.3 Mg/m(3)) than are commonly reported. It is suggested that reduced plant growth in response to compaction may be occurring at lower bulk density in soils derived from volcanic parent material than those usually reported.