Abstract
In oder to determine the optimum value of soil air capacity on the plant root development, the auther studied the effects of different bulk densities and moisture contents in the soils upon the thickening growth and branching of tobacco tap root. 1. The root thickening growth and branching in artificially compacted soil: The soils used were taken from Hatano, Utsnomiya, Okayama and Shizunami, and seedings of the variety Bright Yellow were grown on the pots with soils of various moisture contents ranging from 40 to 100%. The auther obtained the air capacity for the optimum thickening growth of tap root as follows: Hatano soil, 20%; Utsunomiya and Okayama soil, 10%; Shizunami soil, 10∼15% (fig. 3) and for the branching of tap root: Hatano soil, 20%; Utsunomiya soil, 15%; Okayama soil, 1∼3%; Shizunami soil, 12∼14% (fig. 5). From these facts, the soil air capacity required for the thickening growth of tap roots was supposed to be different from that equired for the branching of the roots. 2. The root growth and branching in the field: It could not be found that air capacity in the soil was related to the thickening growth of the secondary roots (fig. 7). High level of the soil air content seemed to inhibit the thickening growth of roots. The root penetration was not inhibited in the region of tis experiment by high bulk density of the soil, but number of branching per unit root length was accelerated obviously.
